How this calculator works
Both options are mostly heated water, so the comparison has two parts. The water itself costs the meter price per cubic metre. Heating it costs energy: raising a litre by a temperature difference takes 4.186 kilojoules per kelvin, and a heater is not perfectly efficient, so the useful energy is divided by efficiency. A shower uses flow rate times duration times how often you shower. A bath uses the water volume times how often you bathe. Adding the water and energy costs gives a weekly figure for each, and the difference over a year shows what the cheaper habit is worth.
Formula
Shower water per week = flow rate x shower minutes x showers per week. Bath water per week = bath volume x baths per week. Energy per litre = 4.186 x (hot temperature - cold temperature) / 3600, divided by heater efficiency. Cost = litres / 1000 x water price per cubic metre + litres x energy per litre x energy price per kWh. Annual saving = difference in weekly cost x 52.
Worked example
A nine litre a minute shower lasting eight minutes, seven times a week, uses 504 litres. Two baths of 150 litres use 300 litres. With water heated from 10 to 40 degrees at 90 percent efficiency, the shower costs about 5.14 a week and the bath about 3.06, so the bath is cheaper by 2.08 a week, or about 108 a year.
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Frequently asked questions
Is a shower always cheaper than a bath?
No. A long shower with a high flow rate can easily exceed a full bath. The crossover depends on flow rate, duration and bath size. A nine litre a minute shower needs more than about 17 minutes to match a 150 litre bath, before heating differences are counted.
Why does the calculator need the cold water temperature?
The energy cost is for raising cold water to the temperature you use. Warmer incoming water in summer reduces the heating energy, so the same shower costs less in energy than in winter. Using your local seasonal average gives a better estimate.
How much water does a bath really hold?
A standard tub holds around 150 to 200 litres when filled, but people rarely fill it to the overflow. Estimate the depth you use and multiply by the tub interior, or use a meter reading if you want an exact figure.
Does the calculator include the energy used by the shower pump?
No. It covers heating energy and the water charge. A power shower or booster pump adds a small amount of electricity, which depends on the pump and is usually much smaller than the heating load.