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Home > Resource Conservation > Heat > Water Heaters  

Resource Conservation: Heat: Water Heaters

water heating
It maybe a simple process to heat water but it requires a certain amount of energy. In this section, we look at ways to make sure that energy does not go to waste. We have a special page devoted to explain the heating technologies.



Water Temperature: The "cold" water that comes into your home (business) is a certain temperature. We assume that it is about 50F. If you have seasons, it will fluctuate but probably average out to 50F throughout the year. The goals is to bring in that 50F water and deliver it to different areas of your house (business) at a temperature right for the job.
Hot Water Jobs: In a home, the basic jobs requiring hot water are for sinks, shower/bath, washing clothes and washing dishes. For businesses, it is about the same but sometimes your state government may place certain requirements on those jobs. We build most all our homes (and businesses) in such a way that we can have our hot water do these jobs whenever we want (on-demand).

Ways to heat the water: In general, there are 5 technologies that can do all or part of our water heating job.
  • Conventional storage water heater: fills a tank of water to a temperature to be used on demand.

  • Tankless (on demand) water heaters: Only heats water as it is used.

  • Heat pump water heater: Uses Heat pump technology to heat the water.

  • Solar water heaters: Uses the sun to heat all or a portion of water heating needs.

  • Tankless coil water heater: Uses homes heating system to heat the water.

Want to see a more complete explanation of these technologies?
How much do we want? Some people may say, infinity. What they are really saying is, "I should never run out of hot water if I am taking a one hour shower, while doing dishes and filling the hot tub." A more pragmatic person would want an amount that would fulfill peak usage comfortably (enough for 3 showers and a couple of shaves in one hour). In the next sections, we will challenge you to truly figure out what your needs are.

How much energy do we waste heating water? A lot. Yes, that is our answer! What we do know is your situation will be different than the average, so we will give you the tools to determine what it right for you. On average 20% of a households energy bill is used to heat water. Many hot water heaters are on 24 hours a day. This way they are always ready to heat new water or to heat the already heated water that became cooler. The waste is heating water that has already been heated and the efficiency by which it heats.

Energy Types: In general you can choose to use electricity, gas and the sun (solar) to heat water. However, some people use geothermal, wood stove, coal, diesel and many other raw energy types to provide heat to water.

How much money can I save (ROI)? This is a great question but complicated to answer. Hopefully, we give you enough information to answer that question throughout this section. We do this though links and references, and through calculators that can help for many peoples (and businesses) situations. One thing that will be common for all technologies discussed is that conserving hot water will go a long way to conserving energy (and money).

Conservation First:

Please, forgive us for breaking our usual standard write up but we feel that a little has to be said about how hot water is used. In order to calculate what your hot water needs are, we feel that an evaluation of the things that use hot water should be the first thing to do.
     You may skip this part if you have done this already.
     The question is, are your sinks, showers, washing machines and dish washers operating with the right amount of hot water? Do they need more? Can they perform with less? In other areas of Greencompletely.com we talk about water conservation and some of that is conservation of hot water. We suggest you check those sections out.
Why? It does the most good. Conserving the water you heat saves more energy than trying to save energy making the hot water.  If you calculate your needs for water heating without optimizing the devices that use hot water, you may buy a bigger system than you need. 
     It is simple, make your conservation changes first. Then, make your water heating optimization last. This could have a much faster return on investment.
    This includes, but is not limited to, evaluation of your shower heads, your sink aerators, your dishwasher type, your washing machine uses and types.

Energy Efficient?

Fuel Efficient: As stated in the "General Comments" there are many choices of fuels to heat our water. In general the choices are electricity, gas and solar. Of course, there are other choices but we will not cover them a lot here.
     Solar: The main type of solar heater is one that gets heat from the sun (not electricity). This is considered the "free" fuel option. There are two general types of heaters: "active" and "passive". There are two types of active: "direct circulation" and "indirect circulation" systems. There are also two types of passive systems: "integral collector" and "thermosyphon". Active systems tend to cost a little more but are more efficient then passive systems. Regardless of where you live, you will probably want to have a back up fuel source when the sun is not out.
     Gas: When we say "gas" in this section we mean Natural gas and Propane gas. Gas water heaters are rated among the cheapest ways to heat water. It is fast, cheap and reasonably efficient. Many of the tankless options only work with gas. However, if gas is not feasible at your location or you have a problem burning a hydrocarbon, gas may not be for you.
       Electricity: A very common way to heat water. This fuel source may not be as cost effective (long term) as solar or gas, but electricity is usually always available.

Money Efficiency: Picking a water heating system to replace an existing system or on a new building can be difficult. It will depend on initial costs, maintenance costs, fuel costs, consumption and a few other factors. One thing we would like to point out, technology in this area is changing, therefore, cost feasibility may not be the same as a few years ago. We will give you the tools to determine that for yourself during this write up. 
Energy Efficiency: There are 5 water heating methods mentioned in the "General Comments" section. They are: Conventional storage, Tankless (on-demand), Heat Pump, Solar and Tankless coil. The reasons to use these technologies is as varied as there are households and businesses out there. We will try to simplify your choices.
     Weather Dependence: Some systems will work better if it is cold out and some will work better if it is warm out (does not freeze very often). Conventional storage and tankless demand-type will generally work well in all weather (indoors). However, for cooler climates Tankless Coil  and a few types of Solar could work  well. For warmer (sunnier) climates, Solar and Heat Pump may be a better choice.
     Usage Dependence: Hot water usage can play a big part in choosing a technology. Big families and big businesses can demand a lot from their hot water system. 
     Helper technologies: We do want to make a distinction here. Some of these technologies can be made to help other water heating technologies. Therefore, they can either heat your water or help heat your water.

Want to see a more complete explanation of these technologies?

What Is The Energy Required?

We will start with the basic definition:

1 Calorie = Amount of energy required to raise the temperature of 1 gram of water 1deg C.

We can show you a fancy derivation but for now we will just say that we put this fact into a spreadsheet and came up with the following numbers.

We get: 2111 Calories to raise 1 gallon of water 1 deg F.

That does not do us any good until we know it in electrical power or gas use (KWHR or Therm).

0.00245 KWHR to heat 1 gallon of water 1 deg F.
0.0000837 Therm to heat 1 gallon of water 1 deg F.

Special Note: Gas companies will charge based on different ways to measure the gas. One common way is CCF and it is almost the same as Therm (about 3% different).
Now if the water is 50 deg F and we want to bring it up to 120 deg F. The raw amount of power needed to heat a gallon of water is:

0.172 KWHR to heat 1 gallon of water 70 deg F
0.0059 Therm to heat 1 gallon of water 70 deg F

This is the raw amounts of energy required. The EF of the water heater will tell you basically how much energy you will expend for that gallon of water.

Assuming you are comparing two water heaters that both have an EF of 0.7 and one is gas and one is electric. Energy spent to heat 1 gallon of water 70 deg F is:

0.245 KWHR
0.0084 Therm

In the "Green Calculators" section we will help you calculate the costs of heating your water.

Other Considerations (Please Read):

 The majority of people do not like to change things in their homes. If the home is newly built, the contractor will usually pick a water heater and install. When water heater breaks, most of us, replace it with the same type and size water heater. Doing this we do not get in trouble. We let the contractor worry about the prevailing codes and since the heater was working for us before, why not replace it with the same thing. That is the easy thing to do.
     However, since you are reading this, we assume you want to find what is right for you and your needs. When doing this research, you should be aware of factors besides cost and efficiency.

Codes: Generally, this is the reason most people do not change out their water heater. A professional contractor (plumber) have the skills and knowledge of all the requirements. If you contaminate the water supply, it can be a big problem. If you are a do-it-yourselfer, you need to understand the local codes well to undertake any project like this. Neighborhood covenants may also need to be consulted.

Conservation First: We said this before and we will say this again. Make sure you have installed your water conserving devices (shower heads, aerators, etc.) before you change out your water heater.

Know Your Energy Rates: All too often people "sell" things because you will "save" tons of money by saving energy. It usually is not a lie that you will save energy, however, what you need to do is determine cost of ownership and any inconvenience that you are not willing to deal with. Do not be "sold", you should buy with confidence. 1/2 hour of research (perhaps on our site) can make a big difference.
Safety: Of course, safety should always be a concern when doing any project. Beside the basic safety concerns during installation/removal of water heating systems, you should be concerned about long term safety. You should ask if this system will hurt your roof (solar collector)? Will the water get too hot and scald my children? If there is a leak, do you have a system in place to take care of it? Will the water be potable (generally drinkable)?

R-Value: The "R-value" of a tank used in the storage of heating water Usually will range from about 5 to 25 and it is usually a measure of insulation a heater has between the tank and the environment outside the tank. In general, for every inch (thickness) of the insulation your R value increases by about 8.3. Therefore, a tank at R 16 would have about 2 inches of insulation. What amount is right? In general, you can not go wrong at about an R-value of 15 or better with conventional tank water heater.

Efficiency Gadgets: People have come up with many things to help make your water heating system more efficient. Some of these things are: insulating blankets, heat traps, pipe insulation and more. These items can save big but under certain conditions. If you are not heating your water in those conditions you may only save a few pennies a month. We address some of these items later.

Warranties: As with most things, the longer the warranty the better the quality. With water heaters this usually means better quality parts, technologies that keep it from breaking and maybe a higher R-Value. Some elements may have coatings or be made out of different materials, tank liners could be better and there could be more energy saving features. Usually the longer warranty heaters will be a better value long term.


Do It Yourselfers, Beware: Hot water heaters, as you probably are discovering can be complicated no matter the technology you choose.
     There is nothing preventing you from installing yourself, but in most areas, if you do not do it right, it can be a big problem.
     Make sure you know exactly what is required  in your area to install whatever you choose.
     If you are looking for rebates on some of the technologies, you may be required to buy systems with the EnergyStar rating and installed by a professional contractor (or solar contractor). These tax credits and rebates may be worth more than the cost of the contractor and the EnergyStar label.
     We did a quick YouTube search and found some do-it-yourself videos (we did not make any of these) that may help you. However if you install yourself it is your responsibility.

Requirements For Installation: If you buy a system, then later you find out that you have to spend $1,000's tearing up your house to install a new system, it may not be worth it. Know what you are getting into.  
Non-Do-it-yourselfers: Do not wait till the heater is broken before you start to make decisions about water heating. When your water heater breaks, all you want is to have running hot water again. The last thing you need to do is let a contractor make the decision for you without knowing the long term costs.
     In general, most contractors are knowledgeable and really want to help. They will ask questions about water use habits that will help him/her make recommendations. However, knowing what you want ahead of time could save a lot of time and money in the long term.
     Use the information here (or elsewhere) to pick a heating technology. Check out contractors with the BBB or Angie's List. When you have the technology picked and the contractor, you can pick sizes and features so that when the time comes, you will make decisions that are right for you.
     Most contractors can install conventional tank, tankless and heat pump type technologies. However, usually you will need to go to a solar contractor to install solar technology.


Disposal is something that we may have to worry about, when replacing our water heaters. If you have a contractor replacing the heater, they might just take it away when they are installing the new one. Well, what does "away" mean?
     Conventional Tank Water Heater: Many contractors will have a recycling procedure.  If you need to dispose of a conventional tank water heater many cities and utility companies have buy back programs. However, many systems contain a lot of recyclable metal. You may be able to take the metal to a scrap yard. The insulation and some of the plastic parts will not be recyclable.
     Tankless Water Heater: These units are brief case sized. That means that they can potentially contribute less to the land fill. They would also contain a lot of recyclable metal. However, it is not the disposal of a unit but the installation that could be a problem. You may need to string new wire, upgrade your utility box. You may even have to pull out entire sections of sheet rock to install one of these units. Every house (business) will be different but it may generate extra garbage.
Solar Water Heater: It seems like a solar system is a conventional tank water heater with some extra stuff to collect solar energy. In most cases, that means you have 3 to 5 times more disposal at the end of life of the system. We also looked all over the SRCC site and could not find any requirements for these parts to be recyclable. Most of the do-it-yourself sites construct units that are mostly non recyclable (PVC pipe, glues, paints, tanks, etc). Even if these systems were completely recyclable you still have a larger disposal problem. For the active systems that require a "non freezing liquid" for their heat exchanger, this would probably be a much bigger disposal problem.
     Heat Pump Water Heater: Most all of these systems we have seen are a conventional tank water heater with extra equipment built on top. So, it is an extra compressor, tubing and some other stuff. Many have Refrigerant 410a as the heat exchange liquid and that tube is immersed directly into the tank of water. So, you have an extra compressor, extra wiring, extra tubing and a refrigerant to deal with.

"Standby" Power Loss:

Everyone talks about it. "Standby power loss". This is the power your conventional tank water heater uses to reheat  water that it has already heated but has fallen below the set temperature. This forms the impetus for creating tankless water heaters and heating systems.
     But how much is it for typical water heaters? We have looked everywhere for the answer. No one is providing a clear answer. Some sites say 20-30% of your water heating bill, and some place it at a few %. They state average numbers from national averages but we have not found a concrete data set to help you individually.
What We Know: As long as the water temperature in the tank is higher than the temperature of the room, there will always be heat loss.
     The bigger the tank capacity the faster the heat loss. The higher the temperature is set the faster the heat loss. The lower the temperature in the room the faster the heat loss. The higher the R-Value, the slower the heat loss. If your water heater is always on, your heating elements will come on and off within a range of temperatures around your set point.
     We help you figure this out in our "Green Savings Calculator" section below.

Recovery Rate:

If you have a storage tank water heater when hot water leaves the tank new cold water enters. The heating elements turn on automatically to start heating that new water. How quickly can the new water come up to the desired temperature?
     It depends. There are a few factors that you will need to use to calculate it yourself. This "recovery" rate is calculated in Gallons per hour.
     However, what we have discovered is that if you have a tank all one temperature, You can remove quickly about 70% of the water at the set temperature. The rest will be a mixture of the new cold water and the remaining hot water. On a 50 gallon tank that is the first 35 gallons.
As a tank is recovering from loosing hot water our research basically says to also use that 70% factor for heating up the new cold water.
     To make the calculation of Recovery Rate we need to use a few factors. First we use the wattage of the heating element (or BTU/hr if gas). We next use the cold water temperature, the set temperature of the tank and the efficiency factor (EF) of the tank (about 0.93 for electric and about 0.65 for gas). Lastly we apply the 0.7 factor (recovery factor).
     We put a calculator for this in the "Green Savings Calculator" section below.
     Of course, there is no recovery rate for tankless because they make hot water when it is demanded.

Tank Size:

Conventional Wisdom "Buy to Peak": In just about everything we read while researching this area, everyone says "buy to peak".
     For conventional storage tank systems you figure out what is the possible use for any one hour in your home. Then you buy something rated with a "first hour" performance near that number. For a typical house of 4 that might be 2 showers, 10 minutes at the sink and a load in the dishwasher. That would be 40 + 5 + 7 = 52 gallons of hot water. Most people would then buy a 50 gallon water heater (56 gallon first hour rating)
     For tankless heaters it is a little different. The question becomes how many devices are possible to be on at the same time. Let us say that someone is taking a shower, someone washing their hands, and the dishwasher is running. That could be 2 + 0.5 + 1.5 = 4 gallons a minute required by the tankless water heater. So you buy a 4-5 gallon a minute system.
     If you go to other web sites and to home improvement stores you will see other guides. One guide that is popular is based on the number of people in the house. Another guide is based on the number of bathrooms.

   However, for household that have implemented recent conservation strategies (low flow showerheads, HE washer, etc) these old guidelines may deserve rethinking. If you install water conserving devices and apply the same logic, a family of 4 may only need a 30 gallon water heater or a 2 gallon a minute tankless heater. These options would cost less for their type of water heating short and long term.

    Our experience shows that the actual flow for some devices can vary from the rated value. We suggest you measure your flow rates especially for sinks and showers. If a shower head is rated at 2.5 gallons a minute at 80 psi your flow may be as low as 1.2 gallons a minute (at say 30 psi).  A shower is a mix of hot and cold water, so if you use 20 gallons of water for your shower only about 14 gallons are hot water. Similarly, faucets and clothes washing typically do not use 100% hot water.

     An awareness of how much water your piping may hold can help you calculate some of your water heating jobs. We created a table below to help you with that.
Greencompletely.com Wisdom: "Buy to Likely": Instead of counting up what is "possible" use in a 1 or 2 hour peak at your home you start by choosing the lowest capacity water heater (say 10 gallons). Next you look at its recovery rate (say 3 gallons an hour).
     At this point you make a list of uses possible in a day.

Pretend that list looks like this:
  • Shower: 7-12 gallon hot per shower, 4 showers
  • Clothes wash: 5 gallons hot (warm cycle), 1 loads in a day.
  • Dishwasher: 6 gallons hot, 1 time a day.
  • Sink use: 10 gallons hot, 30 minutes of use spread through the day.
This is about 60 gallons of use throughout the day. Then you ask the question, "Can we find a way to have a 10 gallon hot water heater with a 3 gallon an hour recovery, and never run out of water?" If your answer is no, that is fine.
     Pick the next water heater up. Say that is a 15 gallon water heater with a 5 gallon an hour recovery. Ask the same question. If the answer is still "no". Select the next one up (say a 20 gallon with 7 gallon an hour recovery).
     Seriously think about how you can use it and convenient wait times between showers and washing the dishes. These wait times may only need to be 10-20 minutes. A 20 gallon hot water heater may make 30 to 35 gallons of hot water available in a 2 hour span of time. A small bath may be possible with a 20 gallon tank.
     When you get to the 30 gallon tank usually that is a 220V appliance. It will have faster recovery and you have quality choices that you do not have with the 110V 20 gallon and below. Higher R-value, circulation and a brass drain are just a few possible upgrades. A comfortable bath is very possible with a 30 gallon unit. 

Consider using our "sizing" calculator below in the "Green Savings Calculator" section.

For fun we calculated, in a spread sheet, how many feet a gallon of water would take up for different pipe (inner) diameters.

Pipe Diameter (inch) Approx. Feet Containing 1 Gallon (feet)
3/8 175
1/2 100
5/8 65
3/4* 45

* Typical Home

Temperature Setting:

In this industry, there seems to be 3 magic numbers: 120 deg F, 130 deg F and 140 deg F.
    120 deg F is usually the factory default temperature setting. Therefore, when a contractor installs the system it will be set at 120 deg F.
     130 deg F is basically a goal temperature of water to get dishes clean and to kill bacteria. Clothes washed near this temperature will get most all the bacteria and grease out of the clothing just from the water temperature.
     140 deg F is usually the maximum setting of the hot water heater. Some heaters will actually add cold water if it detects water above 140 deg F.
     Over all, the lower the temperature setting the less energy you will use to heat your water. However, there are some jobs you ask of hot water that you will want to think about the temperature a little more.

What temperature is right? Many modern dishwashers have built in heating elements. If the dishwasher "sees" a temperature below the goal temperature (probably 130 deg F) it will turn on the heating element. However, some only have the capability to warm the water only about 10 deg F.
     For clothes washing machines, the "hot" wash is whatever temperature it gets from the hot line. If there is a great distance between the hot water heater and the washing machine the final temperature of the "hot" cycle could be 10 deg cooler than the setting of the hot water heater.
     We suggest a setting of 125 to 130 deg F if you have an older dishwasher. A setting of 115 to 120 deg F should do well with modern appliances.
Expecting Company? Make your hot water heater bigger. If your hot water heater is set to 120 deg F and you are worried that your hot water heater will not keep up with the demands of having visitors (showers especially) we suggest you increase the size of your water heater. You can do this by raising the temperature of the water heater from 120 deg F to 135 deg F, then simply lower it when the company leaves. This will give approximately 20% more initial capacity for showering (it won't speed recovery rate).

What We Have Forgotten:

Of course, we can not write about what we have forgotten.
     However, in our research we have omitted some details. This area is so vast that we would need to make our write up about water heaters twice the size. We do give more details about the water heating technologies in a separate write up.
What we strived to do is to give you enough information so that you can make an informed decision about buying your next water heater. For those that want additional info, we have provided references below. The decision you make can last 8 to 15 years, so it may be worth spending some time to make the best decision. 

Our Picks:

We just have two things to say. "Less Is More" and "Keep It Simple"

Less is more: Less hot water use is much more energy efficient than trying to do something fancy. When buying a water heater we say, the smaller the better. You can not get around how much energy is required to make hot water hot. However, having a smaller volume of water to heat reduces the standby power losses your heater will have. Also, we have found some water heaters (with plastic tanks) that reduce heat loss by a factor of 2 to 4.

Keep It Simple: We can not stop you from changing the fuel source or water heating technology in your house. However, the simplest thing to do is to keep it the same. Any of the changes can cost you $1,000's. You may have to rip down sheet rock and do other invasive things to your house. All the while, you may contribute to the landfill much more than you want in the long term. We encourage you to seriously consider the total cost of changing your system.

If your are getting a tank (conventional tank or heat pump) you may want to locate your water heater on the south side of your house in front of a south facing window. If you open the blinds during the day (in summer) you will increase the temperature of that room making a heat pump more efficient and reduce "standby power loss" for a conventional tank. This should also increase the water temperature of the cold water by a little bit. In the winter, you may want to insolate the window. 
Having said all of that, we are having a hard time finding quality products in this area. We have put a few choices in the store but this does not serve every household or business.
    Our pick from Rheem, is the Marathon series of water heaters. The smaller water heaters are for households between 1 and 3 people (see our sizing calculator below). These water heaters are what is called "Point of Use" heaters and are usually not for heating the water for a whole house. Therefore, heater producers do not have to give you as much information about point of use as they do about the "home sized" water heaters. We have yet to find a quality gas powered 20 gallon heater.
     The Marathon tank (from Rheem) is polybutene which we believe is not recyclable but very durable and can hold heat longer. If you do not take baths or take low water baths the 20 gallon tank should do all your heating jobs. Go to the store to see them.
Rheem water heater

Quick Note On Savings:

As we have said before, conservation first. The way to save the most money, energy and water is to not use the water in the first place. Install water conserving shower heads, aerators on the sinks, HE clothes washer, etc.
     Doing all of that first, will help you determine the size of the water heating system you actually need. When you go to buy a water heater, tell them what you need not the other way around.
See if you can be "water aware" when using hot water. See if you can time different events (dishwashing, showers, etc.) so that you will not run out of water. Once you begin doing that it will get easier and eventually you won't have to think about it.
     The alternative is to spend more money on standby power loss, and bigger costly systems. This can waste $100's of dollars a year. 

Green Savings Calculator:


Calculators For Hot Water Heating:

Useful information for calculators:

Temperature Map
This is a basic map of water temperatures across the nation. Click on it to see an average in your area (EPA Site).
Typical elements for Water heater (electric): 1,500 Watt, 2,000 Watt, 3,000 Watt, 3,800 Watt, 4,500 Watt and 5,500 Watt.

1 Therm is approx equal to 100 CCF of Gas. Slight difference between natural gas and propane.

Most water heaters are set to 120 deg F from the factory (but you can adjust them).

See how we Make the calculations.
Water Heat Cost Calculator:

Water Heat Cost Calculator

Water Inlet Temp (F) Hot Temp Setting (F)
Efficiency Factor (EF) Cost Per KWHR ($)
Shower Water (100 F) $/gal Warm Water Cycle (40% hot water) $/gal
Faucet Water (50% hot water) $/gal Dishwasher (100% hot water) $/gal


Water Inlet Temp (F) Hot Temp Setting (F)
Efficiency Factor (EF) Cost Per Therm ($)
Shower Water (100 F) $/gal Warm Water Cycle (40% hot water) $/gal
Faucet Water (50% hot water) $/gal Dishwasher (100% hot water) $/gal

See how we Make the calculations.

Average Hot Water Recovery Rate:

Average Hot Water Recovery Rate

Water Inlet Temp (F) Hot Temp Setting (F)
Recovery Efficiency Heating Element Wattage
Recovery Rate hotgal/hour This is for your set of conditions (industry standards report based on a 90 deg F temperature rise).


Water Inlet Temp (F) Hot Temp Setting (F)
Recovery Efficiency (0.76 to 0.78 or for high efficiency 0.94) Heating Element BTU/Hour
Recovery Rate hotgal/hour This is for your set of conditions (industry standards report based on a 90 deg F temperature rise).

See how we Make the calculations.

How Many Showers Can We Take Before They Are Cold?

How Many Showers Can We Take Before Running Out Of Hot Water?

To determine back to back showers possible with a small wait time, enter the information below (use winter inlet temperature typically 5 to 15 deg F cooler than your average). We suggest you measure your shower head flow rate before you make this calculation. We assume shower is 100 deg F and no other hot water use during this period.

Water Inlet Temp (F) Hot Temp Setting (F)
Recovery Efficiency Heating Element (Watts)
Showerhead Flow Rate (gal/min) Time for each Shower (min)
Wait time between showers (min) Tank Capacity (gallons)
Recovery Rate hotgal/hour Showers*
* Negative numbers mean infinite showers
For those with gas water heaters, use 6900 Watts in the "Heating Element" box to estimate for a 30,000 BTU/HR gas burner. 9200 Watts for 40,000 BTU/HR. 11500 Watts for 50,000 BTU/HR.

See how we Make the calculations.

Standby Power Cost:

Standby Power Cost

We calculate the power loss based on a simple model. Pipes, valves, connections and other factors cause additional heat loss. We have doubled the theoretical loss to estimate a range of power loss.

Average Room Temperature (F) Hot Temp Setting (F)
Diameter Of Tank (inches) Height Of Tank (inches)
R-Value Of Heater (5 - 25) Efficiency(.99 electric or .78 gas or .94 for high eff. gas)
Electricity Rates ($/KWHR) Gas Rates ($/Therm)
Electricity: Gas:
Heat Loss KWHR/Day to Heat Loss Therm/Day to
Heat Loss KWHR/Year to Heat Loss Therm/Year to
Standby Cost ($/Year) to Standby Cost ($/Year) to

See how we Make the calculations.

Water Heater Size:

Water Heater Size

When figuring out the water heater size, consider all the changes that have happened and may happen in your household. If you have changed out showerheads and faucet aerators, then chances are you may not need as big of a water heater. Please, measure your devices before you make these calculations.
     Conventional wisdom is to look at your peak 1 to 2 hours of hot water usage during a day. Consider how many showers, loads of wash, loads of dishes, and total sink use during that peak time. In addition, we ask that you look at what is realistic and common usage (not the usage you might see once a year). Then, we ask you to consider some wait times between those events. You will be surprised just how big an effect a few minutes can have. To calculate the peak tank size, we assume water heating in the winter months.
For this calculation we assume shower use first. We assume faucet use throughout the two hours. Then, if the washing machine and/or dishwasher are used you will wait a small amount of time after showers to use them. You may also set a wait time after the dishwasher to start the clothes washer. If you set the wait times to 0, that will mean you start the dishwasher immediately after the last shower and start the clothes washer at the same time as the dishwasher. By changing wait times and other entries you can see the effect on your "best fit" water heater.
     Here we assume: Showers are 100 deg F, Sink use is 60% hot water, Dishwasher is 100% hot water, Warm cycle on Clothes washer is 40% hot water, Hot cycle on clothes washer is 100% hot water. This calculator ignores leaks and pipe heat loss.

2 Hour Peak Hot Water Usage
Water Inlet Temp (F) Hot Temp Setting (F) Gas Recovery Efficiency
Showers During 2 hours?
Showerhead Flow Rate (gal/min) # Of Showers Time Per Shower (min) Wait Time Between Showers (min)
Faucet Use During 2 Hours?
Kitchen Faucet Use (min) Kitchen Faucet Flow Rate (gal/min) Other Sink Use (min) Other Sink Flow Rate (gallon/min)
Dishwasher Used During 2 Hours?
Wait Time After Showers (Min)
1 Load Size Mode?
Clothes Washer Used During 2 hours?
Wait Time After Showers And Dishwasher Start (Min) Washer Type
1 Load Size Of Tub (cubic Feet) Cycle Temperature
The numbers represent the number of usable gallons left after each event (showers, dishes, clothes). Red = "undersized". Green = "best fit". Pink = "oversized".
Electricity: Gas:
Shower Dishes Clothes Shower Dishes Clothes
1. 10 Gal. 1,500W Element 1. 30 Gal. 30kBTU/HR
2. 15 Gal. 1,500W Element 2. 40 Gal. 35kBTU/HR
3. 20 Gal. 1,500W Element 3. 40 Gal. 40kBTU/HR
4. 20 Gal. 2,000W Element 4. 50 Gal. 40kBTU/HR
5. 20 Gal. 3,000W Element 5. 50 Gal. 50kBTU/HR
6. 30 Gal. 3,800W Element 6. 65 Gal. 60kBTU/HR
7. 30 Gal. 4,500W Element 7. 75 Gal. 70kBTU/HR
8. 40 Gal. 3,800W Element
9. 40 Gal. 4,500W Element If you own a standard top loading clothes washer and wash in "hot", the minimum water heater is a 50 gallon during non-peak time.

If you take a normal bath, the absolute minimum tank size should be about 30 gallon (for a tank that is completely up to temperature). 40 gallon would probably do a better job.
10. 40 Gal. 5,500W Element
11. 50 Gal. 4,500W Element
12. 50 Gal. 5,500W Element
13. 80 Gal. 4,500W Element
14. 80 Gal. 5,500W Element

See how we Make the calculations.

Glossary Of Terms:


BTU: British Thermal Unit. A unit of measure of heat. Defined as the amount of energy required to heat 1 pound of water 1 deg F going from 39 F to 40 F. About 1055 joules.

Programmable: An attribute of a thermostat that means that you can program in temperatures and times of day to control the heating system.

Resource Conservation and Recovery Act (RCRA): This act classifies certain elements and compounds as hazardous waste in the United States.

e-cycle: The practice of excepting composite electronic devices for recycling. Usually T.V.'s, printers, computers, cell phones and readers (like the kindle).

Standby Energy: This is energy used, usually by conventional storage tank systems, to reheat water that has already been heated.

Dip Tube: The tube leading into the bottom of a conventional water tank from the cold water supply.

Recovery Rate: The rate at which a water heater can generate new hot water when it senses new cold water in the tank. It is based on the wattage of the element (electric) or the btu/hour of the burner (gas). The recovery rate is express in gallons/hour. Usually the printed recovery rate is to raise the temperature 90 deg F.
Thermostat: a device that activates or deactivates the heating system based on the temperature it measures and the temperature that is "set".

LCD: Liquid crystal display. Made of Indium Tin Oxide. Used as a display monitor for thermostats.

Return on Investment (ROI): Usually measured in an amount of time to pay back an investment by saving money that you would otherwise spend.

Energy Factor (EF): This is basically a measure of the efficiency of the water heating system. It is the amount of energy required to heat the water used divided by the actual energy used by the heater. The general range for gas is about 0.5 to 0.8 and electric is between 0.75 to 0.95. This includes "standby energy" loss.

Solar Fraction (SF): The the fraction of heating a solar heating system can do compared to no solar heating. 0 being none and 1 meaning all the water you use is heated by solar. Typical solar fractions run between 0.5 and 0.75.


http://energy.gov/energysaver/articles/selecting-new-water-heater, Selecting a Water Heater, Energy.gov, taken 9/16/12.

http://energy.gov/energysaver/articles/solar-water-heaters, Solar Water Heaters, Energy.gov, taken 9/16/12.

http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=WGS, gas water heaters, energystar.gov, taken 9/16/12.

http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=WHC, Gas Condensing Water heaters, Energystar.gov, taken 9/`6/12.

http://www.consumerenergycenter.org/home/appliances/waterheaters.html, Water Heaters, California Energy Commission, Taken 9/19/12.

http://blog.chilipepperapp.com/2010/03/is-it-safe-to-drink-hot-water-from-my.html, Is it safe to drink hot water from my water heater?, Hot Water Guy, 9/22/12.

http://aceee.org/node/3068, Water Heating, American Council for an Energy-Efficient Economy, taken 9/22/12.

http://www.ehow.com/how_5007598_recycle-hot-water-heater.html, How to recycle a water heater, Taken 9/24/12.

http://www.consumerreports.org/cro/water-heaters/buying-guide.htm, Consumer Reports bulletin on Hot Water heater, taken 9/28.12

http://housekeeping.about.com/od/laundry/a/laundry_h20temp.htm, Laundry and Water Temperature. Taknen 9/30/12.

http://www.unitjuggler.com/convert-energy-from-cal-to-thm.html, Unit juggler conversion. taken 10/4/12.

http://homerepair.about.com/od/plumbingrepair/ss/tankless_hwh_5.htm, Tankless water heaters, taken 10/4/12.

http://www.epa.gov/athens/learn2model/part-two/onsite/ex/jne_henrys_map.html, ground water temperature map. Taken 10/4/12.

http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/22267/how-do-you-calculate-water-heater-recovery-ef, Recovery efficiency, taken 10/7/12.

http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/dishwasher_nopr.pdf, Proposed Rules, Federal Register, from 2002, for dishwasher test procedures, taken 10/12/12.