Use our heat pump size calculator for a quick estimate of the required heat pump capacity for your home. However, you should not choose a heat pump size based on this calculator alone. More detailed calculations must be done by a qualified installer before fitting, as accurate heat pump sizing is very important.
How to calculate heat pump size
You can use the heat pump size calculator above to quickly estimate the heat pump size for your home. The calculator is quick and convenient – great for initial estimates. But it’s not accurate enough to be used for the final design of a heat pump system.
The final design of a heat pump is much more complicated. So consult a qualified heat pump installer who can calculate these values precisely before getting a heat pump installed. Specifically, the S.R-50 standard can be used for exact heat pump sizing.
What does heat pump size/capacity mean?
Heat pump capacity refers to the rate at which a heat pump can output heat. Typically, this is advertised in the product name. But be careful! A heat pump’s capacity depends on the specific temperature conditions that it’s used in. So check the datasheet for the capacity of the heat pump under the design conditions that you’ll be using. This may be different to the nameplate capacity.
What is my heat loss indicator (HLI)?
Your heat loss indicator measures the rate of heat loss from your house. This number will be high for a poorly-insulated house, and low for a well-insulated house. It’s calculated as part of the Building Energy Rating (BER) process, and also during a heat pump technical assessment.
| Date Home Built | Heat Loss Indicator (HLI) |
| Pre-2005 | Over 2 |
| 2005-2011 | ~ 2 |
| Post-2011 | Under 2 |
It’s generally not recommend to install a heat pump with a HLI above 2. Indeed, you can only claim heat pump grants for homes with a HLI of 2 or lower. So most Irish homes will need insulation upgrades before they can get a heat pump.
Why use a heat pump design temperature of -3 °C ?
Temperatures in Ireland occasionally drop well below -3 °C. So you might worry that a heat pump won’t keep your home warm on particularly cold nights. However, this is not the case. Heat pumps have an electric boost heater will kick in once the temperature drops below -3.
In Ireland, -3 °C is low enough that the boost heater will run only rarely. This saves energy as the boost heater is more energy-hungry than the heat pump. On the other hand, using a lower design temperature would result in an oversized heat pump. So -3 °C is a sensible compromise which reduces the use of the boost heather without oversizing the heat pump.
How the heat pump size calculator works
Our heat pump size calculator ignores some finer details to give a quick and approximate estimate.
In particular, it doesn’t account for the following factors:
- Hot water required for taps and showers
- Waste heat coming from household appliances
These factors approximately balance each other out – giving a reasonable first estimate of heat pump size requirement.
What happens if a heat pump is undersized?
There are two main problems with an undersized heat pump. First, an undersized heat pump will have to run its defrost cycle too frequently. This increases electricity consumption. Second, an undersized heat pump will have to fall back on the boost heater too often. This likewise increases electricity consumption and cost. Moreover, an undersized heat pump will wear more quickly than a correctly-sized heat pump.
What happens is a heat pump is oversized?
An oversized heat pump will cycle frequently. This increases both energy consumption and wear and tear. Cycling can be reduced somewhat with a large buffer tank. Variable-speed heat pumps also minimize the cycling problem. But even then, the heat pump will run very frequently at part-load, reducing efficiency.
Heat pump radiator under-sizing and oversizing
Under-sizing heat pump radiators is a bad idea. This is because a small radiator has to be hotter for the same heat output as a large radiator. In the best case scenario, as heat pump with undersized radiators will be more expensive to run. In the worst case, it won’t be able to keep your home warm.
On the other hand, oversizing heat pump radiators is generally harmless, or even beneficial. Bigger radiators give more heat at low temperatures. And these lower temperatures are great for heat pump efficiency. Just make sure the plumbing is adequate – heat pumps generally work with much larger diameter piping than oil or gas boilers.
Heat pump sizing table for houses in Ireland
The below table gives a general indication of the heat pump size for homes in Ireland. It assumes that Pre-2005 houses have retrofitted insulation to give them a heat loss indicator around 2.2. Meanwhile, it assumes that 2005-2011 houses have a heat loss indicator of 2, and post-2011 houses have a heat loss indicator of 1.5.
| Built Pre-2005 (retrofitted) | Built 2005-2011 | Post-2011 | |
| Small (~50 m2 / 1-bed) | 2.6 kW | 2.4 kW | 1.8 kW |
| Medium (~110 m2 / 3-bed) | 5.8 kW | 5.3 kW | 4 kW |
| Large (~180 m2 / 5-bed) | 9.5 kW | 8.6 kW | 6.5 kW |
| X-Large (~230 m2 / Large 5-bed) | 12.1 kW | 11 kW | 8.3 kW |