Calculate the energy and time needed to heat your actual hot-water cylinder, then compare continuous heat loss with a timer schedule.
Tank-heating physics · Economy 7 ready · Timer comparison
Energy is litres of water × 4.186 kJ/kg°C × temperature rise, divided by 3,600 to kWh and adjusted for element-to-water efficiency. We then add the standing loss you enter.
Actual usable tank volume, inlet temperature, thermostat cycling, pipe losses, stratification and cylinder insulation vary. Timer savings are a scenario, not a guarantee.
The useful heat is based on the water volume and temperature rise. Heater power changes heating time, while the tariff changes cost; power alone does not change the energy a full tank needs.
Standing heat loss is separate from the full-tank heating cycle. Use the cylinder product label or manual when it publishes a kWh-per-day figure.
Set a timer-loss reduction to model how less time at full temperature could reduce standing losses. The calculator does not assume a timer eliminates losses or changes the hot water you consume.
If you use a dual-rate tariff, schedule the element within the confirmed off-peak window and enter that rate.
Cost depends on the litres heated, temperature rise and tariff. A 150-litre tank heated from 15°C to 60°C needs roughly 8.3 kWh before standing losses, so enter your own p/kWh rate for the current cost.
Divide the required tank energy by 3 kW. A full 150-litre tank with a 45°C rise is around 2.8 hours in this model; reheating a partly warm tank takes less.
It can be if the heating cycle happens inside your tariff’s off-peak window. Economy 7 times and rates vary, so use the off-peak rate from your bill and confirm the timer matches the supplier window.
A thermostat stops continuous element heating, but the cylinder still loses heat and reheats. A timer can reduce avoidable standing losses when hot water is not needed; the result depends on insulation and usage.
