Heat load and heat demand are two different things
The heat load is the power in kilowatts a heating system must deliver on the coldest design day so that all rooms stay at their target temperature. Heat demand, by contrast, is the amount of energy in kilowatt-hours needed over the year. A house using 12,000 kilowatt-hours a year can have a heat load of 6 or 8 kilowatts, depending on location and use. For choosing a heat pump, the heat load is what counts.
Method 1: rule of thumb by construction year
The quickest way is a typical value per square metre of living area. Commonly quoted values for German homes at 20 °C inside and about −12 °C outside:
| Building standard | Typical value [W/m²] |
|---|---|
| before 1978, unrenovated | approx. 130 |
| before 1978, partly renovated | approx. 100 |
| 1978–1994 | approx. 80 |
| 1995–2001 | approx. 60 |
| 2002–2015 | approx. 45 |
| from 2016, new build | approx. 35 |
| passive house | max. 10 |
A 140 m² house from 1985 would come to 140 × 80 = 11.2 kilowatts. The calculator also scales the value to your design outdoor temperature, because a house in Cologne at −7 °C needs less power than the same house in Hof at −14.6 °C. The rule of thumb is rough, though: two houses from the same year can differ by 30 percent or more, depending on whether windows have been replaced or the roof insulated.
Method 2: estimate via the building envelope
It gets more precise if you know the areas and U-values of the external elements. For each element: heat loss = U-value × area × temperature difference. Add an allowance for thermal bridges, usually a flat 0.10 W/(m²K) on the entire envelope, and ventilation losses. For ventilation, roughly 0.34 × air change rate × air volume applies. At an air change rate of 0.5 per hour, the room air is replaced once every two hours.
Example: 180 m² of wall with U 0.22, 100 m² of roof with 0.25, 90 m² of basement ceiling with 0.40, 30 m² of windows with 1.3 and 350 m³ of air volume give about 7 kilowatts at a 31-degree temperature difference. The calculator also shows how much each element contributes. Windows and ventilation are often surprisingly large items.
Method 3: from past consumption
If you are replacing an old heating system, you can use the annual consumption. Divide it by the full-load hours, roughly 1,800 to 2,100 hours for a detached house depending on location. 20,000 kilowatt-hours of gas thus give about 10 kilowatts. First deduct the hot water share and take the efficiency of the old boiler into account, because not every kilowatt-hour of gas reached the room as heat.
Method 4: room by room to EN 12831-1
The standard calculates every room individually, with its elements, thermal bridges, ventilation and design indoor temperature, such as 24 °C in the bathroom. Only then can radiators or underfloor heating circuits be sized correctly and hydronic balancing be carried out. For a heat pump subsidy and a reliable quote this calculation is usually mandatory.
Why oversizing is expensive
Boilers used to be generously sized, which cost little. With heat pumps oversizing is a real disadvantage: the purchase costs more, the unit cycles on and off frequently in spring and autumn, and efficiency drops. An estimate helps you sanity-check quotes. If a quote is well above your estimate, it is worth asking how the capacity was determined.
Heat pumps often get an allowance for hot water on top of the heat load, as a rule of thumb about 0.2 kilowatts per person. Whether an allowance for utility blocking periods is needed depends on the electricity tariff. You can then work through costs and subsidy in the heat pump calculator. Which outdoor temperature applies to your location is covered in the post on the design outdoor temperature.