How this calculator works
The calculator counts the showers the household takes in a year, then multiplies by the water each one uses: flow rate times duration. The water charge is the volume in cubic metres times the local price. Heating energy comes from the specific heat of water: raising a litre by a temperature difference takes about 4.186 kilojoules per kelvin, or 0.001163 kilowatt hours. Dividing by the water heater efficiency accounts for heat lost in the tank or exhausted with the flue. Adding the two costs gives the yearly total and, divided by the number of showers, the cost of one.
Formula
Showers per year = people x showers per person per week x 52. Litres per shower = flow rate x duration. Water cost = litres / 1000 x price per cubic metre. Energy = litres x 4.186 x (hot temperature - cold temperature) / 3600 / heater efficiency. Energy cost = energy x price per kWh. Total cost = water cost + energy cost. Cost per shower = total / showers.
Worked example
Three people showering seven times a week is 1,092 showers a year. At nine litres a minute for eight minutes, each shower uses 72 litres, for 78,624 litres a year. Water at 4 per cubic metre costs 314.50 and heating from 10 to 40 degrees at 90 percent efficiency uses about 3,047 kWh, costing about 487.58. The total is about 802.08 a year, or 0.73 a shower.
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Frequently asked questions
Why is energy usually the larger part of the bill?
Heating water takes a lot of energy compared with the cost of the water itself in most places. Your energy price and heater efficiency decide the balance, so a household with cheap electricity or an efficient heat pump water heater may see a smaller energy share.
What flow rate should I use?
Check the showerhead or time how long a measured container takes to fill. Standard showerheads are often around nine litres a minute, while low-flow models can be under six. A power shower can exceed fifteen, which changes the result substantially.
Does the calculator include the energy used by a pump?
No. It covers heating energy and the water charge. A booster pump adds a small amount of electricity, usually far less than the heating load, but you can add it separately if you know the pump power and run time.
Why does the hot water temperature matter?
Energy is proportional to the temperature rise. Setting the tank to 60 degrees and mixing cold water at the shower still costs the same energy for the hot water actually used, because the rise from the incoming temperature is larger. Lowering the tank temperature without increasing the hot share is what reduces energy.