Heat Loss Calculation: How To Size My Heat Pump Or Boiler! With Cheat Sheet

February 8, 2022

Consumer Advice

The Heating Industry has Lost its Way

The heating industry has been left in a right old pickle. 

Years of upgrading insulation in houses without upgrading the rules of thumb have left behind a wave of boilers and heat pumps that have their sizings way off.

Then when boilers are replaced they are replaced with the same power input as the old unit, forgetting that the old scaled unit was only 50% efficient yet worked fine and the new one is 90% efficient. 

Then combination boilers came along, and people started believing that the bigger the better. 

boiler installation on wall

This concept was not helped by manufacturers displaying pictures of larger houses and stating that they would require a 40kW boiler then medium houses a 30kW unit with smaller houses requiring a 25kW unit. 

Ok enough of the ranting…

So What is my Heat Loss and how do I calculate it?

The reality is that the average property requirement is around 6 to 8 kW. But also that it really depends on the size of the property and how well insulated it is. 

To help you as a homeowner or an installer get an idea of what your heat loss is, we've created a little cheat sheet table.

Now bear in mind that this is not to be used instead of heat loss calculations. But more of a guide to show the breadth of heat losses and to give an idea of where accurate heat loss calculations may come out.

If you're getting a heat pump installation it's imperative that you get a proper room by room heat loss calculation done. 

Also remember that although engineers say they have completed heat loss calculations, they very much vary on how in-depth they are. 

If your surveyor has not measured every window, wall and door, as well as looked at the depth of the loft insulation, enquired about cavity insulation and measured every radiator, they have not performed a heat loss survey or calculation. 

Just the details of the heat loss survey alone should take 1 to 2 hours on site to obtain, excluding time talking to the client about any options etc. 

If you get to the installation stage and this information still hasn't been taken, stop the installation and abandon the contract.

The Heat Geek Heat Loss Calculation Cheat Sheet

So, with all that in mind, here's our cheat sheet!

Heat Geek heat loss calculation cheat sheet

So, in reverse order, if you have an older Victorian or before building, with little upgrades, say with single glazing and little to no loft insulation you should expect 95-110w m2

That's the square meterage of every floor, not just the ground floor. 

If the same property has upgraded to double glazing as most of the UK has now, and has a small amount of loft insulation, this drops to 65 to 85W, further upgrades to loft insulation or cavity insulation would decrease to 40 to 65W.

Pre 2006 newer build or recent full renovations would be 30 to 50W

And recent newbuilds 20 to 40W.

Low carbon homes are around 10 to 20W.

And passive haus standard below 10W.

Please bear in mind that hot water may have a small impact on the size of the unit chosen as well as the amount of hot water storage space you have available.

If you want another method or are trying to install a unit under a certain size you can measure the heat loss by inputting a known fixed output heat source in the building such as a fan heater, measuring the internal and external temperatures overnight. 

Or use a service to do this for you such as Veritherm.

If you would like to learn how to do the long hand calculations take a look at our in depth full explanation of heat loss calculations here.

If you want a Heat Geek to complete a proper heat loss calculation on your property then check out our Map to find your local engineer.


To work out an approximate for your total annual kWh demand (the total amount of annual heat required), multiply your heat loss by 1,500 for heating and 500 for hot water.

I.e. A 10kW heat loss would use 20,000kWh pA, 15,000 on heating and 5,000 on hot water.

Click here for our Off Plans Heat Loss Service

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