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Assumes 100% transfer efficiency — real coil/immersion recovery will be slightly longer.
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This free cylinder reheat calculator works out how long a hot water cylinder takes to heat up, and how much energy it needs, from the cylinder volume, temperature rise and heat input. It is built for UK heating engineers and plumbers comparing immersion versus boiler coil recovery, answering customer questions about reheat times, or checking whether a cylinder and heat source are well matched.
Enter the cylinder volume in litres, the incoming cold water temperature, the target storage temperature and the heat input in kW — 3 kW for a standard immersion heater, or the coil rating for boiler-fed reheat. The calculator uses the specific heat capacity of water (4.18 kJ/kg·°C) to find the energy required in kWh, then divides by the heat input to give the reheat time in minutes.
Energy (kWh) = volume × 4.18 × ΔT ÷ 3600; Reheat time (min) = (kWh ÷ kW) × 60
Volume is the cylinder capacity in litres (1 litre of water is 1 kg), 4.18 is the specific heat capacity of water in kJ/kg·°C, and ΔT is the temperature rise from cold to target in °C. Multiplying them gives the energy in kJ, and dividing by 3600 converts to kWh. Dividing the energy by the heat input in kW gives hours, and multiplying by 60 gives the reheat time in minutes.
Enter the cylinder capacity in litres — the default of 210 litres suits a common domestic indirect cylinder. Use the actual rated capacity from the cylinder label where available.
Enter the incoming cold water temperature (default 10°C) and the target storage temperature (default 60°C). The difference between them is the ΔT that drives the energy calculation.
Enter the heat input in kW: 3 kW for a typical immersion heater, or the coil transfer rating for boiler reheat. A higher input heats the same cylinder proportionally faster.
The calculator shows the energy required in kWh and the reheat time in minutes. Treat the time as a best case — real recovery is slightly longer because no coil or immersion transfers heat with perfect efficiency.
The calculation is straight physics: the energy to raise a mass of water through a temperature difference, using water's specific heat capacity of 4.18 kJ/kg·°C. It assumes 100% of the heat input reaches the water, so real reheat times will be slightly longer once standing losses and heat exchanger performance are accounted for — more so on older cylinders with slow coils.
It models heating the full cylinder volume from cold. In everyday use, cylinders rarely start fully cold: stratification means the top stays hot while cold water enters at the bottom, so partial recovery after a bath or shower is quicker than the full-reheat figure.
The 60°C default target reflects common UK practice for stored hot water storage temperature. Follow the cylinder manufacturer's instructions and current guidance on storage temperatures and any hot water safety devices — this calculator only tells you the energy and time, not the compliance requirements for the installation.
Assumes 100% transfer efficiency — real coil/immersion recovery will be slightly longer.
A 210-litre cylinder heated from 10°C to 60°C needs about 12.2 kWh. On a 3 kW immersion heater that's roughly 4 hours; on a boiler coil transferring 15 kW it drops to around 49 minutes. Real times run slightly longer than calculated because heat transfer is never 100% efficient.
First work out the energy: this calculator gives it in kWh (about 12.2 kWh for a 210-litre cylinder raised 50°C). Multiply that by your electricity unit rate to get the cost per full reheat — then compare against your gas unit rate for boiler reheat, remembering the boiler is not 100% efficient either.
The calculation assumes 100% of the heat input reaches the water. In practice, coil performance, scale on the immersion element or coil, standing heat losses and boiler cycling all stretch the real time. A significantly slower reheat than calculated — especially in a hard water area — often points to scale build-up.
This calculator defaults to a 60°C target, which is common UK practice for stored hot water. Confirm the correct storage temperature and any required safety controls against the cylinder manufacturer's instructions and current guidance for the installation — the right setting balances scald risk, bacterial control and running cost.
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