Water Heat-Up Time Calculator

How it works

The energy to heat water is Q = mass × specific heat × temperature rise. Water’s specific heat is 4.186 kJ per kg per °C, and 1 litre ≈ 1 kg. Power is energy over time, so time (minutes) = (litres × 4.186 × ΔT) ÷ (power kW × 60).

This is an ideal whole-volume heat-up: all the entered power reaches the water continuously, with no draw-off or heat loss. Actual reheat times also depend on the starting temperature, heating controls, coil performance and how the hot and cold water are distributed in the cylinder.

Worked example

Heating 120 litres by 45 °C at 3 kW: (120 × 4.186 × 45) ÷ (3 × 60) ≈ 22,604 ÷ 180 ≈ 125.6 minutes — a little over two hours.

How long does it take to heat a 300 litre cylinder?

Starting at 15 °C and heating the whole 300 litres to 60 °C gives a temperature rise of 45 °C. At a constant 3 kW delivered to the water, the calculation is 300 × 4.186 × 45 ÷ (3 × 60) ≈ 314 minutes: about 5 hours 14 minutes. At 6 kW, the same ideal calculation is about 157 minutes.

Enter the temperature rise, not the final temperature. For 15 °C to 60 °C, enter 45. If the water already starts at 40 °C, the rise to 60 °C is only 20 °C, so the ideal 3 kW time falls to about 140 minutes. Starting temperature can change the answer as much as cylinder size.

Use the power reaching the stored water

For an immersion example, use the heater’s rated power and recognise the continuous-heating assumption. For an indirect cylinder, the boiler rating alone does not tell you how much heat the coil transfers to the store. Coil data, primary flow and temperature, controls and any other demand affect that transfer.

Manufacturer reheat times refer to stated test conditions. Check the volume heated, starting and finishing temperatures, and primary conditions before comparing them with this whole-cylinder estimate. Water can be hot at the top before the entire stored volume reaches the target, so first usable hot water and a complete heat-up are different measurements.

Convert the heating calculation into kWh

Divide the heat energy in kilojoules by 3600 to express it in kWh: energy = litres × 4.186 × temperature rise ÷ 3600. The 300 litre, 45 °C-rise example needs about 15.7 kWh of heat in the water. Doubling the delivered power halves the ideal time; it does not halve that heat requirement.

Use the hot-water revision notes to connect energy, power and recovery time. The unvented cylinder diagram helps distinguish the heating arrangement from the temperature controls and safety devices; the calculated time alone does not establish that a heater or thermostat is working correctly.

Method: Level 2 plumbing science

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