Heat-pump energy and resistance-heating cost
Electricity is split between heat-pump input and resistance backup. The selected comparison uses the same delivered heat. A negative cost reduction means the system costs more. The matching electricity tariff uses only modeled running costs; the combined heat/input ratio is shown as zero when no heat is required.
If you already know measured electrical watts and operating hours, use the Appliance Electricity Cost Calculator instead of estimating input from thermal demand.
The formula
COP is the ratio of delivered heat to electrical input using the same energy units. A COP of three means three units of heat per unit of electricity at the modeled conditions. Direct resistance backup is represented with a COP of one.
Worked example
For 1,200 kWh of delivered heat, 10% resistance backup supplies 120 kWh. The heat pump supplies the remaining 1,080 kWh using 360 kWh at COP 3. Total electricity is 480 kWh, costing 96 at 0.20 per kWh, compared with 240 for all-resistance heating. The same thermal demand can be entered as 5 kW average output for 240 hours. Against fuel at 90% efficiency and 0.08 per input kWh, the system saves about 10.67, and its matching electricity price is about 0.222222 per kWh.
How to use this calculator
- Choose known heat energy or average delivered heat output and hours; these describe thermal output, not purchased electricity.
- Enter period-average COP, the delivered-heat backup share and your electricity tariff. Changing an energy or price unit preserves its value.
- Choose electric resistance or fuel heating for comparison. For fuel, enter its input-energy price and seasonal efficiency in the same currency.
Thermal demand is not the electricity bill
A building needs a quantity of heat, while the heating system purchases electricity to deliver it. Those two quantities differ for a heat pump. Do not enter existing heat-pump electrical consumption as heat demand unless you first convert it using an appropriate performance estimate. This tool does not calculate building heat loss from floor area.
COP changes with operating conditions
Outdoor temperature, delivery temperature, defrosting, controls, and auxiliaries affect system performance. A point rating may not represent a season. Use a period-average figure that includes the relevant electrical loads; the scenario table illustrates sensitivity instead of predicting the weather. HSPF or HSPF2 values use different units and should not be typed directly into a dimensionless COP field.
Compare fuel input with delivered heat
Fuel price must be per kWh of purchased fuel energy, not per kWh of useful heat. A 90% seasonal efficiency needs 1,200/0.9 = 1,333.333 kWh of fuel to deliver 1,200 kWh of heat. At an illustrative 0.08 per input kWh, its energy cost is 106.6667, versus 96 for the worked heat-pump case. Boiler electricity, standing charges and capital costs are excluded.
Assumptions & limitations
What this calculation assumes
- Resistance backup has COP 1 and shares the same electricity tariff.
What to keep in mind
- No building heat-loss sizing, weather simulation, changing tariff schedule, fuel-system auxiliary electricity, standing charges or capital-cost payback.
Common questions
What does the backup percentage describe?
It is the share of delivered heat supplied by resistance backup, not the share of operating hours or purchased electricity.
Does COP 3 mean 300% of electricity is created?
No. A heat pump moves heat from another source as well as using electrical work; COP relates delivered heat to the electricity input.
Does the currency selector convert prices?
No. It labels costs. Enter both energy prices in that currency; cents or pence are divided by 100.
Sources & further reading
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