Estimate annual running cost of your water heater.
Water heating is usually the second largest energy load in a home after space heating, and it comes down to one physical fact: raising a litre of water by one degree takes 4.186 kJ. Bringing 180 litres from 12 C to 60 C therefore needs about 10 kWh of useful heat every day. What you pay depends on how efficiently the appliance delivers that: a resistance element is nearly 100% efficient at the point of use, a gas tank around 60% once flue losses are counted, a modern combi about 85%, and a heat pump water heater over 250% because it moves ambient heat rather than making it. Stored systems also lose roughly a kilowatt-hour a day to standby whether you draw water or not, which is the one loss a tankless unit avoids entirely.
Water heating energy and annual cost
Useful kWh per day = litres x 4.186 x (set temp - inlet temp) / 3600; delivered kWh = useful / efficiency + standby loss; annual cost = delivered kWh x rate x 365
Energy cost estimates only. Stored hot water below about 55 C can allow Legionella bacteria to multiply, while water above 50 C at the tap scalds quickly — especially children and older adults. Follow local plumbing regulations and consider a thermostatic mixing valve before changing any setting.
Only to a point. Cost scales with the temperature rise, so 55 C instead of 65 C saves roughly a fifth — but stored water below about 55 C risks Legionella growth, so do not go lower on a tank system.
Yes, on energy. A coefficient of performance near 2.8 means it uses about a third of the electricity of a resistance element for the same hot water. It is slower to recover and cools the space it sits in, which suits a garage better than a cupboard.
Because it stores no hot water — it heats on demand as water flows. That eliminates the daily standby kilowatt-hour, though it needs a much higher instantaneous power or gas input.