Does a bigger heat pump use more electricity?
Barely: on a new house in Würzburg, a 16 kW heat pump used about 70 kWh (18 EUR) a year more than the 10 kW unit that the sizing proposed, and started about 40 percent more often. Too small is what costs money: a 4 kW unit cost about 320 EUR a year more, through its electric backup heater.
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Read the transcript
Three installers, three heat pump sizes. Does the bigger one use more electricity?
Barely. On this house, the sixteen kilowatt unit uses about 70 kilowatt hours a year more than the ten. About 18 euros. Too small is what costs money.
This house: a new build from 2025 in Würzburg, two storeys, underfloor heating, ten kilowatts of solar on the roof.
The wizard works out its heat loss and proposes a size: here, the 10 kilowatt class.
To test a quote, pick its size and run again. Sixteen, then four.
The scenario comparison puts the three runs side by side: four, ten and sixteen kilowatts, all on this house.
First the sizing. This house needs 6.2 kilowatts on the coldest design day. At minus 11.5 degrees, the four kilowatt unit delivers 3.4, the ten 8.6, the sixteen 10.7. Only the four falls short.
Then the bill. Ten kilowatts: 1,095 euros a year. Sixteen: 1,112. Four: 1,413, because the backup heater covers 1,296 kilowatt hours at a COP of one.
And what the bigger unit changes: how often it starts. About 40 percent more compressor starts, 2,797 a year against 2,006.
For this house, the ten kilowatt class is the right size: the lowest bill, and fewer starts than sixteen. Your house will give its own answer. Simulate it at EnergyFlow dot tech.
How I got this answer
Three installers quote three heat pump sizes: 4 kW, 10 kW and 16 kW. I ran each size on the same house and changed nothing else.
The house is a new build from 2025 in Würzburg, Germany: two storeys, an unheated garage inside the house, underfloor heating, ventilation with heat recovery and 10 kWp of PV facing south. No cooling, no battery, no electric car. The weather is the typical year the model builds for Würzburg, and the electricity is on a fixed tariff of 0.3214 EUR/kWh bought and 0.077 EUR/kWh sold.
EnergyFlow worked out the heat loss of the house and proposed the 10 kW class: 6.2 kW needed at the design temperature of -11.5 °C, 0.55 kW more for hot water, and 8.59 kW delivered by the 10 kW unit at that temperature. For the other two quotes I picked the 16 kW class, then the 4 kW class, and ran the house again each time. The scenario comparison puts the three runs side by side.
| One year, same house | 4 kW | 10 kW (proposed) | 16 kW |
|---|---|---|---|
| Heat delivered to the house, kWh | 4,306 | 4,304 | 4,306 |
| Heat pump electricity, standby included, kWh | 873 | 1,161 | 1,232 |
| Backup heater electricity, kWh | 1,296 | 15 | 0 |
| Compressor starts a year (estimate) | 1,772 | 2,006 | 2,797 |
| Net electricity cost, EUR | 1,413 | 1,095 | 1,112 |
| Compared with the 10 kW unit, EUR | +318 | 0 | +18 |
All three runs delivered about 4,300 kWh of heat, so the comparison is like for like. Above the proposed size the bill barely moves: the 16 kW unit uses about 70 kWh more electricity over the year, about 18 EUR, and starts its compressor about 40 percent more often, 2,797 times against 2,006 (an estimate).
Below it the bill climbs. The 4 kW unit cannot cover the cold days alone, so the electric backup heater supplies 1,296 kWh, one kWh of electricity for each kWh of heat. That is where the 320 EUR a year more comes from: 1,413 EUR against 1,095 EUR.
Run your own house
Your house will not give the same numbers. Your heat loss, your climate and your tariff decide where the right size sits, and each installer's quote is one more run. Enter the house, pick the size from each quote, run it and compare, the way I did here.
The method behind the numbers
The Library article shows where these numbers come from, why the water in your heating circuit matters as much as the size on the label, and what the model leaves out.