Heat pump size calculator: what output does your house need?
A 150 m² house insulated after 2000 in climate zone III (-20°C) needs about 8.0 kW for heating plus about 0.4 kW for hot water for four people, about 8.4 kW in total: a 10 kW class heat pump. Enter the floor area, insulation and zone to get the output, the size class and the yearly electricity bill.
Example: 150 m² house, insulated, zone III
Heat pump output needed at −20°C: about 8.4 kW
heating about 8.0 kWhot water about 0.4 kW
Heat pump size for comparing quotes
10 kW class
- 4
- 6
- 8
- 10
- 12
- 14
- 16
The number in the model name is the output at one operating point, for example +7°C. In frost the pump gives less: the Buderus Logatherm WPLS8.2 delivers 9.34 kW at +7°C and 7.82 kW at -7°C. Check in the datasheet that the pump gives at least 8.4 kW at −20°C.
Electricity for heating and hot water
about 5,314 kWh a year, about 5,527 PLN
about 461 PLN a month on average
Prices from 11 October 2026, estimate
| Item | Value |
|---|---|
| Heat for space heating | 15,000 kWh/yr |
| Heat for hot water | 3,600 kWh/yr |
| Full-load hours | 1,871 h |
| House heat loss per degree | 0.20 kW/K |
| Heating load at −20°C | 8.0 kW |
| Hot water allowance | 0.4 kW |
| Design load | 8.4 kW |
| Heat pump size class | 10 kW class |
| Electricity per year | 5,314 kWh/yr |
| Electricity cost per year | 5,527 PLN |
Assumptions and formulaPrices from 11 October 2026
t = 3,118.9 × 24 h ÷ (20 - (−20)) = 1,871 h P = 15,000 kWh ÷ 1,871 h = 8.0 kW Pdhw = 4 people × 900 kWh ÷ 8760 h = 0.4 kW E = 18,600 kWh ÷ 3.5 = 5,314 kWh, cost = 5,314 kWh × 1.04 PLN = 5,527 PLN
The hot water allowance is the yearly hot water heat spread over the hours of the year: the minimum the pump must add to heating on a frosty day. Yearly use comes from SCOP. A datasheet SCOP (EN 14825) already includes the heater's work in a typical season, so switching the heater on does not change the kWh result here.
- Degree days per year
- 3,118.9Eurostat, heating degree days (nrg_chdd_m), Poland, 2015-2025 average
- Design temperature, zone III
- −20 °CPN-EN 12831 and PN-B-02403, via Gramwzielone.pl (25 Oct 2024)
- Room temperature
- 20 °C
- Hot water per person
- 900 kWh/yr
- Heat pump size classes
- 4-16 kW in 2 kW stepsrounded up, manufacturers' ranges differ
- Heat pump SCOP
- 3.5
- Electricity price
- 1.04 PLN/kWhURE G11 tariffs 2026 (PGE, Tauron, Enea, Energa), average with distribution
An estimate based on yearly heat demand. The output to buy is set by a designer or installer from a heat load calculation to PN-EN 12831. The cost excludes fixed charges.
How electricity, gas, pellet and coal prices have changed, month by month. Price history
Ready-made calculations
How to use the calculator
- Set the heated floor area and pick the insulation standard from the list.
- Choose your climate zone. Its design temperature appears under the switch.
- Decide whether the heat pump also heats water and whether a backup heater helps on the coldest days. If you know your yearly heat use or have an installer's quote, enter them in the extra fields.
- Click Size the heat pump. After that the result updates with every change.
How we calculate the output
We start from the yearly heat demand and divide it by the number of hours the heat pump would need to run at full output. Degree days give that number: Poland averages about 3,119 a year according to Eurostat. For rooms at 20°C and -20°C outside that is about 1,871 h.
Example: a 150 m² house insulated after 2000 uses about 15,000 kWh a year for heating. 15,000 kWh ÷ 1,871 h = about 8.0 kW, or about 53 W/m². Hot water for four people is 3,600 kWh a year, about 0.4 kW on average around the clock. Together that is about 8.4 kW, so the calculator points to a 10 kW class heat pump.
Each 2-degree step between zones changes the result by a few percent. The same house needs about 7.6 kW in zone I (-16°C) and about 9.2 kW in zone V (-24°C).
Size class and output in frost
The size class is the output rounded up in 2 kW steps, from 4 to 16 kW. The number in a model name does not say at what outdoor temperature the pump reaches it. For example the Buderus WPLS8.2 gives 9.34 kW at +7°C and 7.82 kW at -7°C (35°C flow). Compare quotes using the datasheet output at your zone's design temperature.
With a backup heater for the coldest days the heat pump can be smaller. At a bivalent temperature of, for example, -9°C the example house needs about 6.2 kW from the pump and the heater adds about 2.2 kW at -20°C. An 8 kW class is then enough.
What the calculator leaves out
This is an estimate, not a design. The calculator does not know your walls, windows, ventilation or the flow temperature of your heating system. The output to buy is set by a designer or installer from a heat load calculation to PN-EN 12831. Use the result to check whether a quote is in a sensible range.
Zone temperatures come from the standard. An analysis initiated by PORT PC, based on 1991-2020 data, gave -15.3°C for Warsaw instead of -20°C, so a result based on the standard is more likely to have a margin than a shortfall.
Yearly electricity use is heat divided by SCOP, whatever the heat pump's output. The example house needs about 18,600 kWh of heat: at SCOP 3.5 that is about 5,314 kWh of electricity and about 5,527 PLN a year, before fixed charges.
Frequently asked questions
What size house does an 8 kW heat pump suit?
In zone III (-20°C), a heat pump that gives 8 kW at that temperature is enough for roughly 150 m² of a house insulated after 2000, about 250 m² of a new house built to WT 2021, or about 75 m² of an uninsulated house. If it also heats water for four people, that drops to about 140 m², 235 m² and 70 m².
What heat pump output does a 100 m² house need?
A 100 m² house insulated after 2000 needs about 5.8 kW in zone III including hot water for four people, so a 6 kW class heat pump. A new house built to WT 2021 needs about 3.6 kW and an uninsulated house about 11.1 kW.
Should hot water be added to the heat pump output?
Yes, if the heat pump also heats water. The calculator spreads the yearly hot water heat over all hours of the year: about 0.1 kW per person, about 0.4 kW for four people. That is the minimum. If the pump should heat water quickly in frost too, ask the installer about a larger margin or heating water outside the coldest hours.
Does a bigger heat pump use less electricity?
No. Use depends on yearly heat and SCOP. A 150 m² house insulated after 2000 with hot water for four people uses about 5,314 kWh of electricity a year at SCOP 3.5, whether the pump is 10 or 12 kW. An oversized pump switches on and off more often, which lowers its efficiency.
What if the result is over 16 kW?
That usually happens in a large uninsulated house: a 150 m² house needs about 16.4 kW. Work out insulation first, because after it the same house needs about 8.4 kW. If insulating is not an option, two heat pumps in a cascade or a pump with a backup heater give that output. Have a designer do the sizing.