Understanding your result
The result separates compressor-system electricity from resistance backup. The comparison uses direct electric resistance heating for the same delivered heat and tariff. A negative reduction means the modeled system costs more than that reference.
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.
How to use this calculator
- Enter the required thermal energy for a defined period.
- Use an average COP appropriate to that period and operating conditions.
- Set the share of heat supplied by resistance backup and your electricity rate.
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.
Assumptions & limitations
What this calculation assumes
- Resistance backup has COP 1 and shares the same electricity tariff.
What to keep in mind
- No building sizing, gas-furnace comparison, weather simulation, 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.