Immersion Heater: basic calculation

Electricity is measured in kilowatt hours (or kWh). A kWh is a unit of energy equivalent to one kW of power expended for one hour.

In some cases, people heat their water with electricity – normally through the use of electric immersion heaters – that act like huge like kettles. We often get asked whether these are expensive to run, so the following should help you work that out for yourselves!

How many KW of electricity are needed to heat a hot water tank?

So the two bits of information you are going to need to help calculate this are the size of the tank measured in litres and the required temperature rise.

The required temperature rise (measured in centigrade) is calculated by taking the starting temp of your cold water from the hot water temperature required. For example if your water is 100C and you want to heat it up to 600C, then the temperature rise would be 500C.

The following formula can then be used to calculate the power (in kW) needed to heat a specific volume of water by a given temperature in 1 hour.

Volume of tank x 4 x temperature rise / 3412 = Power required

So if the tank is 125 litres and you want to take the water up to 650C from 100C, then it would simply be:

125 x 4 x 55 / 3412 = 8kW

This means that you would need 8kW of electricity to heat this volume of water up in 1 hour. If you wanted to heat it up in 30 mins you would need 16kW of power and so forth.

Immersion heaters are measured in kW

When you buy an immersion heater it is given a kW rating, so for example 3kW or 6kW.

If you take the above example and have a 3kW immersion heater it would take about 2hrs 30min to heat the water (3kW each hour, so in two and a half hours that would give 7.5kW).

Cost of running an immersion heater

The cost of electricity is about 14cts/kWh, which means that it would cost about 45cts to run a 3kW immersion heater for an hour.

Normally though the immersion will turn on and off via the thermostat or a temperature controller so the amount of time they are on fluctuates, depending on the existing temperature of the water in the hot water tank.

Dpstar Group is the largest manufacturer of Heaters and Thermocouples in Malaysia.

For further enquiries on immersion heater please visit:- and our engineer are ready to serve you.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s