HEATLOSSPROHeat pump size from your gas or oil usage
Your gas, oil or LPG bill already tells you how much heat your house needs. Enter last year's use and get the heat loss and the heat pump size in a minute.
In short: peak heat load (kW) = (annual fuel kWh × boiler efficiency − hot water kWh) ÷ (heating degree days × 24) × (indoor − design outdoor temperature). A house that used 12,000 kWh of gas last year, with a condensing boiler and three people, needs a heat pump that gives about 3.7 kW at −3 °C.
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The method, step by step
- Fuel in kWh. Use a full year of bills. Gas bills usually show kWh; otherwise multiply m³ or litres by the energy content (UK gas 11.2 kWh/m³, Dutch gas 9.77 kWh/m³, heating oil 10.35 kWh/l, LPG 7.0 kWh/l).
- Heat delivered. Multiply by the boiler efficiency: about 90% for a condensing boiler, 70–75% for an old non-condensing one.
- Take off the hot water. About 900 kWh of heat per person per year, or twelve times a summer month's use. What is left heated the rooms.
- Heat loss per degree. Divide the space heating by the heating degree days × 24 hours. The result, in kW/K, is how much heat the house loses for every degree it is warmer inside than outside.
- Peak heat load. Multiply by the difference between the average indoor temperature and the design outdoor temperature. That is the heat the house needs on the coldest day you design for.
Worked example
| Gas used last year | 12,000 kWh |
| Heat delivered (condensing boiler, 90%) | 12,000 × 0.90 = 10,800 kWh |
| Hot water (3 people) | 3 × 900 = 2,700 kWh |
| Space heating | 10,800 − 2,700 = 8,100 kWh |
| Heat loss per degree (2,100 degree days) | 8,100 ÷ (2,100 × 24) = 0.161 kW/K |
| Peak heat load (20 °C in, −3 °C out) | 0.161 × 23 = 3.7 kW |
When to trust it, and when not
The gas bill method is quick and grounded in the real house, which makes it a good sanity check. It has limits:
- How the house was heated matters. Rooms left cold, a low thermostat or a winter away make the figure too low.
- It gives no room-by-room figures. A heat pump runs at a low flow temperature, so every radiator must be checked against its room's heat loss. The bill can't tell you which rooms need a bigger radiator.
- Planned improvements are not in it. New windows or insulation lower the load, and only a calculation from the building itself shows by how much.
For the full picture, trace the floor plan in the HeatLoss Pro designer. You get each room's heat loss, the radiator and underfloor heating check, and the heat pump size in about 5 minutes, and can compare the result with the figure from your bill. Drawing is free; a full calculation with a PDF report costs €1. Or read how to calculate heat loss by hand, and what the heat pump will cost to run.
Frequently asked questions
How do I estimate heat pump size from my gas bill?
Take last year's gas use in kWh and multiply by the boiler efficiency (about 0.9 for a condensing boiler) to get the heat delivered. Subtract the hot water (roughly 900 kWh per person per year). Divide the rest by the heating degree days × 24 to get the heat loss per degree in kW/K, and multiply that by the difference between the indoor and the design outdoor temperature. That is the peak heat load the heat pump must cover.
Is the gas bill method accurate?
It is a good check on the whole house, because it is based on how the house actually behaves. It can come out low if parts of the house were left unheated or the thermostat was kept low, and high if the boiler was oversized and short-cycling. It gives no figure per room, so radiators and underfloor heating still need a room-by-room heat loss calculation.
What if I heat with oil or LPG?
Convert the fuel to kWh first: heating oil (kerosene) holds about 10.35 kWh per litre and LPG about 7.0 kWh per litre. Natural gas holds about 11.2 kWh per m³ in the UK and about 9.77 kWh per m³ in the Netherlands, though most bills already show kWh. The rest of the method is the same.
How much of my gas is used for hot water?
Roughly 900 kWh of heat per person per year for showers, baths and taps. You can also look at your summer months: with the heating off, the monthly gas use is mostly hot water (and cooking), so twelve times a summer month is a good estimate of the yearly hot water share.
What are heating degree days?
Heating degree days add up, day by day, how far the outdoor temperature stays below a base temperature (15.5 °C in the UK) over a year. A colder location has more. They let you turn a yearly energy use into a heat loss per degree of temperature difference. Typical values are about 1,900 in south-east England, 2,100–2,250 in the Midlands, northern England and the Netherlands, and 2,500 or more in Scotland. Energy companies often quote degree days to a base of 18 °C, which come out several hundred higher; use base 15.5 °C here.
Should I size the heat pump bigger to be safe?
No. A heat pump that is much bigger than the load switches on and off on mild days, which wears it and lowers its efficiency. Choose the smallest model whose datasheet output at your design outdoor temperature covers the peak heat load.
Check it room by room in 5 minutes
Trace your floor plan to see which rooms need bigger radiators, and compare the heat pump size with the figure from your gas bill. Free to try, no sign-up.
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