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At ImmersionHeaters.UK, (Call Jamie on 07897 246779) we have 2 types of customer.  There are those who tell us what they want as they know their exact requirements.

Then there are others who need guidance and advice as to what they need.  One question which comes up time and again is “How many kW do I need to heat up my tank?”

If we can calculate the volume of water and the required temperature rise, we can answer this question.

The following formula is used to calculate the power of heating element needed to heat a specific volume of water by a given temperature rise in 1 hour.

volume in litres x 4  x temperature rise in degrees centigrade / 3412

(4 being a factor and 3412 being a given constant)

for example 100 litres of water, to be heated from 20ºC to 50ºC, giving a temperature rise of 30ºC would give –

100 x 4 x 30 / 3412 = 3.52

meaning that the water would be heated in 1 hour by 3.5kW of applied heat.

Also we can use this information to extrapolate both ways.  To heat the same water volume in half the time (30 minutes) would need twice the heating power, ie, 7kW.

Converesely, if we only use half the heating power, 1.75kW, it will take twice as long to heat up to desired temperature, ie, 2 hours.

If we only have a 1kW element available, we will expect a heat up time in excess of 3 hours.

Also we can use this formula as the basis of similar calculations for heating oil.  Generally, oil heats up in half the time of water, due to its density.  However, oil needs a much lower watts density element than water, as described in the “How to choose an oil heater” article.