Heat Loss Calculator: What Size Heat Pump Does My Home Need?
Estimate how much heat your whole home loses on a cold day, the heat pump size to ask installers about, how much heat you need a year and what it costs with a heat pump, and which insulation would cut it most. For the UK, Ireland and mainland Europe.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Work out your heat loss
Heat Loss Calculator
Estimate how much heat your whole home loses on a cold day, the heat pump size to ask installers about, what heating it costs a year, and which insulation would cut it most.
1 Your home
On your energy certificate (EPC or BER), or add up the rooms. A 3-bedroom semi is often 80–95 m².
2 How it's built
Quick fill by ageFills in typical answers for a home of that age; change any that have been improved.
3 Where you live and your heating
Heat loss on a cold day
0 kW
Per m² of floor
0 W
Heat Loss Indicator
0
Heat lost per m² for each degree of difference. Lower is better.
Heat needed a year
0 kWh
Where the heat goes
Heating costs a year
Insulation that helps most
Each on its own, against your home as it is now. A smaller heat loss also means a smaller, cheaper heat pump.
Heat pump grants where you live
Grants change: check the current rules before you sign anything.
What to ask installers for
Get at least three quotes, each with its own heat loss survey.
What's next?
How it's worked out: the home's shape is estimated from its type and floor area (windows about 20% of the floor area). Heat loss = U-value × area × (inside − design outdoor temperature) for the walls, windows and roof; the ground floor uses the yearly mean outdoor temperature instead; air leaks = 0,33 × air changes an hour × volume × the same difference (BS EN 12831 method, U-values from RdSAP 10, design temperatures from CIBSE Guide A and ASHRAE). Yearly heat = heat loss ÷ the temperature difference × degree days × 24 (MCS method), plus 900 kWh of hot water a person. Degree days: MCS figures in the UK; elsewhere Eurostat country averages brought to the same 15,5 °C base. Heat pump running cost = heating ÷ SCOP + hot water ÷ 2,6, × electricity price; boilers 85% efficient. Standing charges not included.
Estimates only. This is a quick estimate from typical figures, not a heat loss survey. Before fitting a heat pump, an installer must work out the heat loss room by room. Real homes often use less heat than this, because not every room is kept warm all day.
How it works
From heat loss to heat pump size
A home loses heat through its walls, windows, roof and floor, and through air leaking in and out. The colder it is outside, the faster it goes. The heat pump has to replace it on the coldest days. The sources and assumptions behind it are on our How we calculate page.
Heat loss on a cold day
For each part of the home, heat loss = its area × its U-value (how easily heat passes through) × the difference between inside and the local design temperature, a cold day that only happens a few times a year. Air leaks add more. The total, in kW, is the heat loss.
Heat pump size
A heat pump is sized to cover the heat loss on that cold day: a 6.7 kW heat loss means about a 7 kW heat pump. Bigger isn’t better: an oversized heat pump switches on and off more and runs less efficiently.
Heat a year
Most days are far milder than the design day. The yearly heat uses degree days (how cold the whole year is where you live), the same method installers use, plus hot water. Divide by the heat pump’s efficiency for the electricity it needs.
Examples
How much heat does a house lose?
The same calculator for a few typical homes. Insulation makes a bigger difference than the size of the house.
Illustrative only, from the calculator above: 2 storeys, 2.4 m ceilings, 20 °C inside, the age’s typical windows, loft, floor and draughts, and 3 people’s hot water. Heat loss on a cold day at the local design temperature; heat a year by the MCS degree-day method. A survey of your own home will differ.
Tips
Before you get a heat pump
A heat pump works best in a home that keeps its heat, with radiators big enough to run on cooler water.
Insulate first
Cavity wall and loft insulation are the cheapest ways to cut heat loss, and a smaller heat loss means a smaller, cheaper heat pump that runs on cooler water. In Ireland, the SEAI grant needs a Heat Loss Indicator of 2 or less.
Check your bills
Compare the estimate with your gas or oil use over a year. If you use much less, your home loses less heat than typical figures suggest (many homes don’t heat every room all day), and a smaller heat pump may do.
Get a proper survey
Before fitting a heat pump, an installer must work out the heat loss room by room (BS EN 12831; MCS in the UK). Use this estimate to check the quotes, and the Radiator & BTU Calculator to check each radiator at the heat pump’s water temperature.
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Common questions
Frequently asked questions about heat loss and heat pump size
Common questions about working out your home’s heat loss and the heat pump it needs.
How do I calculate the heat loss of my house?
Add up the heat lost through each part of the home: area × U-value × the temperature difference between inside and a cold day outside, for the walls, windows, roof and floor, plus the air that leaks in and out (0.33 × air changes an hour × volume × temperature difference). The calculator does it from your home’s type, size and age.
What size heat pump do I need?
One that covers your home’s heat loss on a cold day. A typical 1930s semi of 90 m² loses about 6.7 kW at −3 °C outside, so it needs about a 7 kW heat pump; with the cavity walls filled it loses about 5 kW, and a 5 or 6 kW heat pump will do. A newer, well-insulated home of the same size may need only 4 kW.
What is a good heat loss per m²?
Under 50 W per m² of floor is good, 50–100 W is typical for older homes, and over 100 W is high. Ireland’s SEAI uses the Heat Loss Indicator instead: heat loss per degree per m², which should be 2 or less for the heat pump grant.
How much heat does a house need a year?
A typical older 3-bedroom semi in the UK needs about 14,000–15,000 kWh of heat a year for heating, plus 2,000–3,000 kWh for hot water. A heat pump turns 1 kWh of electricity into about 3 to 4 kWh of heat, so it uses roughly 4,500–5,500 kWh of electricity.
Is a heat pump cheaper to run than a gas boiler?
It depends on the price of electricity against gas. In the UK and Ireland, a heat pump running on 45 °C water usually costs a little less than gas; in Spain, where gas is cheap, often a little more. The calculator compares both with your own prices.
Do I need new radiators for a heat pump?
Sometimes. Heat pumps run on cooler water (35–50 °C) than boilers, so radiators give out less heat. Some rooms need bigger radiators: check each room with the Radiator & BTU Calculator at the heat pump’s water temperature.
What grants are there for heat pumps?
In England and Wales, the Boiler Upgrade Scheme pays £7,500; Scotland has Home Energy Scotland grants; Ireland’s SEAI pays up to €6,500 for a house; Germany’s KfW pays from 30% up to 70% of the cost. Other countries have their own schemes, and the rules change, so check before you sign.
Are the results exact?
No. They are an estimate from typical U-values and an estimated shape for your home, not a heat loss survey. Before fitting a heat pump, an installer must work out the heat loss room by room.