October 24, 2026 · Climate & Energy · 6 min read

Heat Pump Sizing: COP, Heating Load and Running Costs

A heat pump is the most efficient way to heat a home — but only when it's sized and used correctly. An undersized unit runs flat out on the coldest days and never catches up; an oversized one short-cycles, wastes money and costs more to install. Both failures trace back to the same estimate: the heating load.

Step 1: Estimate the heating load

Heating load is the heat your home loses at the coldest design temperature, and floor area multiplied by an insulation-dependent factor is a solid starting point:

• Poor insulation (old single-glazed, leaky): 90–120 W/m²
• Average insulation (double-glazed, some loft insulation): 60–80 W/m²
• Good insulation (modern build, high standards): 35–50 W/m²

A well-insulated 150 m² home needs about 150 × 60 = 9 kW of heat; the same size poorly insulated could need 15 kW or more. Climate matters too — the colder your design temperature, the higher the factor within each band.

Step 2: Understand COP

The coefficient of performance (COP) is heat delivered divided by electricity consumed. A heat pump with COP 3.5 delivers 3.5 kWh of heat for 1 kWh of electricity — 350% efficiency, impossible for any burner. But COP isn't fixed: it falls as the outdoor temperature drops and as the water or air delivery temperature rises. When comparing quotes, look at COP at your climate's real conditions, not the laboratory rating point.

Step 3: Compare running costs

The cost per kWh of useful heat is:

At $0.15/kWh and COP 3.5, useful heat costs about $0.043/kWh. Compare that with gas heating at, say, $0.06/kWh of gas and 90% boiler efficiency → $0.067/kWh of useful heat. The heat pump wins by roughly a third. Run the numbers with your own tariffs — the margin shifts with prices, but the physics doesn't.

Sizing mistakes to avoid

• Installing a nominal 12 kW unit and expecting 12 kW at −10 °C — check the manufacturer's output curve at low temperature.
• Sizing for the coldest day but forgetting that average days matter more for annual cost; modulating (inverter) units cover the gap.
• Ignoring insulation upgrades: improving the envelope first lets a smaller, cheaper heat pump do the job.

If cooling is the bigger concern where you live, Cooling Load Basics: How Many BTUs Does Your Room Need? covers the summer side of the same equation.