Tuesday, 7 June 2022

How big should your heat pump be?

Most people have combi-boilers and the size of the boiler required is often dictated by the need to heat hot water for baths and showers quickly rather than space heating. With heat pumps, you normally have a cylinder for hot water and it is the space heating requirement that dictates the size you need. So how big should it be? Hearsay tells me that even with boilers, sales-people love to sell you systems that are bigger than you need. My heat pump is oversized by at least 50%.


My heat pump is much bigger than it needs to be. I have tried to make it less intrusive with some stick-on leaf patterns. Still, at least I have renewable heating :-)

Oversizing does matter, both for boilers and heat pumps.

  • Large systems cost more - not as much as you might expect, but still more.
  • Running at small fractions of full capacity reduces efficiency (usually).
  • Large systems are physically large and, in the case of heat pumps, have a greater impact on the landscape (see picture above).

In this post I discuss a very simple rule of thumb for heat pump size - thanks to Michael de Podesta - and some reasons it gives an underestimate in my case. Also I show you how to cross check your installer's estimate using the EPC certificate for your home. I wish I had done this!


Podesta's rule, divide your annual gas bill by 2900.

Podesta's rule is based on your annual gas or oil bill. Podesta says, for a rough estimate of the size of your heat pump, divide your annual gas bill (kWh/year) by 2900 (assuming you live in the southern half of England). If you use oil, it is litres/year divided by 290.

My gas bill used to be about 16000 kWh/year. (It is a big house.) That would imply a heat pump requirement of 5.5 kW. However my heat pump installers gave me a 16 kW model. I really should have queried this size at the time. Previous quotes for a heat pump for this house from other companies suggested around 10 kW, but I did not remember and did not check. The Podesta rule hides some important assumptions which contribute to potential underestimates in my case. They would not scale it up to 16 kW but they could bump it up to 7-8 kW.

By the way 16 kW means it can deliver up to 16 kW of heat. This requires much less than 16 kW electricity. 

Our boiler was probably better than average efficiency.

Most boilers are between 80% and 90% efficient. That would mean that our 16,000 kWh/year gas bill delivered between 12,900 and 14,400 kWh heat. It could actually be much worse (with a scaled-up heat exchanger) or better, if it was very well configured. I have no way of knowing but I suspect ours was pretty good because it was well maintained, and it had weather compensation controls for high efficiency. Therefore, our heat demand is higher than you would expect if you assume average efficiency.

Half of our hot water comes from a solar panel.

Some of the heat is for heating hot water but people vary greatly in how much they use. A 5-minute shower with a low flow shower head could use 30 litres or even less – a bath could easily be 110. You also use hot water from basin taps, washing up and so on. 50 litres/person/day is supposedly typical and that translates to about 2 kWh/day (ignoring losses from the cylinder which also count for heating the house though not all of that is useful). 

We are two people in the house so 2 kWh/day means 1500 kWh/year, about 12% of the total heat from gas, could be for hot water. However, as about half of that comes from a solar hot water panel, the proportion is even less. In a smaller/more crowded house it could be much higher - perhaps 30%. In a very well insulated house it could be 50%. I suspect Podesta will have assumed about 25% to 30% so our space heating demand is higher than his estimate.

We use a lot of electricity for appliances – electricity use supplies heat too

Electricity used for lights and appliances in the house also counts as heat so the more electricity you use, the less gas you need. We run a lot of IT equipment in the house including servers. Without this our gas bill would have been bigger and the Podesta estimate correspondingly higher.

Our electricity bill is not very high because we get a lot of free electricity from our solar panels. This counts as free heat too.

On the other hand, there is at least one factor that could work the other way:

We heat our house all day.

Your annual space heating demand depends on your average internal temperature (during the heating season) but your maximum demand depends on the maximum winter time internal temperature. For example, if your thermostat setting is 20°C but you only heat for 4 hours per day and the house cools rapidly – usually because it is not well insulated - you may find that the average is only about 17.0°C. Whereas if you are well insulated and you heat 15 hours/day the average might be 19°C. We heat all day because we are at home all day. This means that the difference between average and maximum temperature is smaller than typical and that would make the Podesta rule overestimate. However I doubt the difference is very much.

Cross checking the installer’s estimate against an EPC.

The installer will estimate your heat demand under a worst case scenario (e.g. -3°C outside, overcast and with hardly any appliances being used), based on the dimensions and constructions or your house. The basic method is similar to that used to generate an EPC. As well as computing the size of heat pump you need they will compute your annual heat demand. This was reported on our radiator survey, You can cross check this against the space heating demand reported on the EPC certificate. 

Space heating demand from our home EPC.


There can be a large difference between the EPC estimate and your actual use, because the EPC certificate is based on standardised heating patterns and thermostat settings while the actual heating demand is based on your personal heating patterns (and other things mentioned above). Also EPC estimates are not reliably accurate. (see SMETER - measuring the energy performance of a house). The EPC is more likely to overestimate than underestimate, but this varies. You should check the space heating demand, not the hot water demand, because this is what governs the size of the heat pump.

In our case we have the following estimates of space heating demand (in kWh/year).

  • 14,400 - based on actual gas bill 16,000, assuming 90% boiler efficiency and 5% for hot water.. As a very rough guide for most homes, assume 85% efficiency (times 0.85) and 25% for hot water (times 0.75).
  • 20,620 – most recent EPC. 
  • 18,600 and 21,800 – previous installer estimates from 2017/2018. These recommended heat pump sizes 9.5 kW and 10.1 kW
  • 32,400 – most recent installer estimate, and 16 kW heat pump.
  • 15,000 (with hindsight) based on metered heat supplied from the heat pump over the last year (see Performance of our heat pump for more information about our setup).

It is not surprising that the EPC estimate is higher than our actual gas bill, partly because we had fairly low thermostat settings and partly because of our high electricity usage for IT equipment and other stuff. Also I am not convinced the RdSAP estimates for heat loss from the fabric are accurate in our case; I think it underestimates our insulation, but that is a separate issue.

Heat pumps are sized for the house, not your personal use

If you want any kind of subsidy you are going to have to accept that the heat pump is sized for your heating demand based on standard thermostat settings rather than your personal use. This is sensible, partly because you may be less miserly in future (my husband needs more heat now for health reasons) and partly because when you sell the house the next owner may have different requirements.

What about the hot water? - another 1-2 kW?

In the worst case scenario, your heat pump not only has to keep the house warm when it is -3°C outside but it also has to heat your hot water. The extra power needed for the hot water depends on the size of your cylinder and how rapidly you want to reheat it. For example, suppose you have a bath and this is 110 litres – quite a big bath but not ridiculous. This requires around 4 kWh of heat. If you want to be able to replenish the cylinder in 1 hour, that means you need 4 kW from your heat source. If you have a good sized cylinder you can probably get away with considerably longer reheat time so less power.

As far as I can tell this is not allowed for in the calculations for the heat pump size, which surprises me. Please can someone correct me if I am wrong. However if it was included my heat pump might be even bigger.

Heating hot water is included in combi-boiler sizing calculations. For example, to run a 4 kWh bath in 10 minutes would need 24 kW. However, this would be much the same for every home so it effectively sets a minimum size for combi boilers.

What about heating up a cold house? - another 1-2 kW?

It is generally recommended that you do not let the temperature in your home drop more than 2°C overnight, if you have a heat pump. On this basis, BS12831.2003 Table D 10b (which is now superceded but has a conveniently simple calculation) suggests that to reheat in 1 hour (which would be fast for a heat pump) requires an additional 22 W/m2. For a typical semi detached, 100m2, that would be 2.2 kW. 

I am pretty sure this is not allowed for in the heat pump sizing calculations either.

Assuming you can avoid reheating both the house and the hot water at the same time, it seems reasonable to allow an extra 1-2 kW in heating capacity for a typical house. 

Another Podesta rule - do I have enough insulation?

In the same blog post Podesta gives another useful rule, for whether you have enough insulation to make a heat pump feasible. Divide your gas bill by your floor area, and it should be not much more than 100 kWh/m2. In our case, it comes to about 70 which is well below 100. If this figure is too high, then you may need excessively large radiators to go with your heat pump.

In short.

Getting the right size heat pump is important.

The Podesta rule of thumb is a start but it incorporates a lot of assumptions. In our case, which is fairly unusual, it probably underestimated space heating demand by 30% or so - because our boiler was efficient, we use little hot water and half of that comes from solar panels, and because we have high gains from using computer equipment in the house.

In any case, heat pumps are sized for the house, not for you and your personal heating patterns and hence bills. They are based on similar methodology to your home EPC. EPCs are a long way from perfect but they do quite well as a cross check for your installer’s heating calculations. It is a good idea to check the installers estimate of space heating demand against the the figure given in your EPC.



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