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Home > Resource Conservation > Water > Shower > Our Calculations

Resource Conservation: Water: Shower: Our Calculations

calculations
We show you how we make the calculations on the green calculator for the "Shower" page here.

 

While it is our intention to give you the best calculations possible, these calculations have many factors that will influence accuracy. Your results may vary based on water temperature, cost to heat, flow rate and other factors. The calculators are for information purposes only.

Cost to heat a gallon of water:
We fill in this box in the calculator but you can change it to whatever you want. When we researched national averages we saw numbers all over the place for heating shower water. Therefore we decided to create a calculator just for estimating costs of heating shower water. That can be found at Water Heat Cost Calculator. This calculator will estimate the cost of heating a gallon shower water to 100oF (gcost). It does this by calculating the energy required to heat water to 100oF from the inlet temperature (Ti). Then based uses the efficiency factor (ef) and price per kilowatt hour (pk) or price per therm (pt).

Here are the formulas used:

For Electricity:
hcost = (((100 - T1) X 0.00245) / ef ) X pk.

For Gas:
hcost = (((100 - T1) X 0.0000837) / ef ) X pt.

    We fill in $0.02 which is based on heating the water with an electric water heater where rates are $0.15 per kilowatt hour and inlet temperature is 50oF.

Cost of water:
Usually you are charge per 100 cubic feet of water that comes into to your home or business. Nationally, the price you will pay is about $2-$3 per 100 cubic feet (748 gallons). Many cities will also base their sewer rate on your consumption of water. You can find the calculator for water at our Water Cost Calculator. In that calculator you you simply put in the total cost for your water (water and sewer) per 100 cubic feet and it will tell you how much you pay for each gallon of water.
     The formula is simple. Input your total water rate (twr) into the calculator (this should be the sum of your water rate plus your sewer rate if applicable). Hit "Calculate". The calculator shows you a number for cost per gallon (gcost).

gcost = twr / 748.

We fill $0.005 into the box on the calculator. This assumes $2 per 100 cubic feet for the water rate and $1.75 per 100 cubic feet for your sewer rate.

 

Shower Calculator formulas:
In this calculator you put in the number of showers per day taken (sn1 and sn2), flow rate of two different shower heads (fl1 and fl2) and the time for each shower (ti1 and ti2). You also put in your costs to heat a gallon of water (hcost) and your cost for a gallon of water (gcost). You also select if you are going to take a cooler shower and how much you are spending to make changes (setcost) by buying a new shower head or Waterpebble or even hiring a plumber.
      We set up the calculator this way so that you can mix and match the three different options we explain in the write up (low flow, shorten shower and lower temperature shower). The calculator will provide before and after water use and cost estimates for the changes you are looking to make.

To calculate the before (g1) and after (g2) annual gallons use we use:

g1 = sn1 X fl1 X ti1 X 365.
g2 = sn2 X fl2 X ti2 X 365.

To calculate the before (c1) and after (c2) annual cost we use:

c1 = g1 X (hcost + gcost)
c2 = g2 X (hcost + gcost)

To get the "return on investment" (roi) estimate in months we convert the annual savings into a monthly savings rate (mr). Next, we take the set up costs (setcost) and find calculate how many months of the savings it would take to make back the set up costs.

mr = (c1 - c2) / 12

roi = setcost / mr

For the option to take a cooler shower, we reduce the heating cost (by 3%) for c2 and make all the other calculations as normal.

c2 = g2 X ((hcost X 0.97) + gcost)

Shower dynamics:
Your shower use habits may change a bit when using a lower flow shower head. It may take a little longer for the shower to get up to the desired temperature because less water is going through the shower head. Therefore, your shower time might increase a little bit (20-30 seconds).
     Some people report that they run the shower a little warmer because it is cooler when standing outside the water stream. Other people report that they need a little more time to rinse off. These factors may contribute a little to the accuracy of the estimates obtained from the calculator.
Water conserving shower products in our Amazon.com store.

  • Showerheads:
    Showerheads ________________
  • Waterpebble:
    Shorter Shower With Waterpebble ________________
  • Outdoor Solar Shower:
    Outdoor Solar Shower ________________

 

 

 

 

Check out WaterSense at the EPA: