Water Heating Calculator
Water's heat capacity is why this costs so much.
Work out Water Heating. Water's heat capacity is why this costs so much. Free, with no account and nothing to install.
Energy to heat the tank
8.72 kWh
174 minutes at 3 kW · costs 2.44
Water has a specific heat capacity of 4.186 kJ per kilogram per degree, which is unusually high — higher than almost any common substance. That single property is why heating water is the second largest energy use in most homes after space heating, and why a long shower costs more than people assume. Standing losses run continuously whether you use the water or not, and over a year they often exceed the energy in the water actually used. Insulation on the tank and the first metre of pipework is the cheapest saving available in a hot water system. The heat-up figure assumes the whole tank goes from cold. In practice water stratifies, so the top of a tank is usable well before the bottom reaches temperature — which is why a partly heated tank still gives a hot shower. Store at 60 °C to control legionella, and use a blending valve to deliver it safely.
How the Water Heating Calculator works
Energy, time and cost to heat a tank of water, with standing losses and showers per tank. Water has an unusually high specific heat capacity, which is why hot water is most homes' second largest energy use.
Also known as: cost to heat a hot water tank · how long does an immersion take · energy to heat 150 litres · hot water cylinder running cost
Frequently asked questions
How much energy does it take to heat water?
4.186 kJ per kilogram per degree — about 0.00116 kWh per litre per degree. Heating 150 litres by 50 degrees takes about 8.7 kWh, which is a substantial share of a household's daily electricity.
How long does an immersion heater take?
Energy divided by element power. A 3 kW element heating 150 litres by 50 degrees needs roughly three hours from fully cold — though stratification means the top is usable much sooner.
What are standing losses?
Heat leaking from the tank whether you use the water or not. At 60 W continuously that is 1.4 kWh a day, and over a year it often exceeds the energy in the water actually used.
What temperature should a cylinder be?
60 °C to control legionella, with a blending valve to deliver it at a safe temperature at the tap. Storing cooler risks bacterial growth; delivering that hot risks scalding, so both parts matter.
Is it cheaper to heat only when needed?
Usually yes, for a well-used tank on a timer. A poorly insulated tank kept hot around the clock loses more to standing losses than the reheat costs — though modern tanks with thick insulation narrow the gap considerably.
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