Hot water cost: shower, water heater and the litres you need
From the litres per minute of your shower head to the real cost of a shower, with electric, gas, heat pump and solar side by side, and the tank size that survives your busy hour.
What one shower costs, water and energy together, and what it adds up to in a year. The maths starts from the litres, not from the watts.
If you do not know the flow, fill a measuring bucket for 30 seconds and double it. The menu next to it writes typical values, and you can still edit the number.
Temperatures and prices
These change the result more than anything else. The prices are on your bill, charges and VAT included.
How to use it
The first tab starts from the shower you actually take (litres per minute and minutes) and tells you what it costs, water and energy split by heating system. The second starts from the heater you own (litres and kW) and works out the energy of one full reheat, how long it takes and how many showers come out of it. The third answers the question you get in the shop, 50 or 80 litres, starting from the back to back showers of the busy hour. Temperatures and prices are shared and live in the panel you can open.
The formula, and there is only one
Warming one litre of water by one degree takes 1.163 watt hours, the specific heat of water (4186 joules per kilo per degree) divided by 3600. So the useful heat is litres times temperature rise times 1.163, in watt hours. Everything else on this page is that multiplication plus the price of energy.
Why turning the thermostat down does not save on the shower
The shower delivers 38 degrees, not 60, because the mixer adds cold water. Mixing conserves energy, so the heat you consume depends only on the litres that came out and on the rise from cold water to outlet temperature, not on how hot the tank is. Turn the thermostat down and you draw more hot litres and less cold, and the energy of the shower stays the same. What does go down are the standing losses of the tank while nobody is using water. Consumption is instead proportional to the litres, which is why a 6 litre per minute head instead of a 12 really does halve the bill.
Efficiencies, prices and the cubic metre of gas
Electric resistance is credited with 95%, the heat pump with the COP you set (a domestic cylinder sits between 2.5 and 3.5 over the year, less in winter if it draws cold air), gas with the efficiency on its plate. One cubic metre of natural gas is 9.59 kWh of net calorific value, the same basis on which those efficiencies are declared. If you used the gross value, around 10.7, you would have to lower the efficiency as well, and mixing the two bases is the most common mistake. Solar thermal has no price per kWh, so the tool works on the share it covers over the year and on the cost of the backup only.
The limits, stated up front
These are estimates, not meter readings. They do not include the standing losses of the tank (1 to 2 kWh a day for an 80 litre model, depending on its class) nor the water that runs cold until the hot reaches the head, which on long pipe runs is far from negligible. Water tariffs are banded and change from town to town, so you enter the figure yourself. The 80% draw off coefficient and the 15% margin for basins and kitchen are reasonable conventions and they are written in the result rather than hidden: professionals size systems with industry standards and the tapping profiles of the energy label. On tank temperature, remember that below roughly 50 degrees legionella multiplies, and that above 45 degrees at the tap water scalds. The classic compromise is storage at 60 degrees with a thermostatic mixing valve downstream.