To find the recommended heat strip size, subtract your heat pump’s output at design temperature from the total heat needed at that temperature — the shortfall is the kW your heat strip needs to cover.
Heat Strip Size Calculator
Electric heat strip (auxiliary/emergency heat) kW size needed to supplement a heat pump air handler, from BTU shortfall.
Sizing an Electric Heat Strip for Backup Heat
Electric heat strips (auxiliary or emergency heat) supplement a heat pump’s air handler when the heat pump’s own output falls short of the home’s total heating need at design temperature — the coldest temperature your system is designed to handle. This calculator finds that shortfall and expresses it as the kW the heat strip needs to cover.
This covers related searches such as heat strip size calculator, how many kW heat strip do I need, and auxiliary heat sizing for a heat pump air handler.
How the Heat Strip Size Calculator Works
Shortfall to cover = max(0, Total heat needed at design temp − Heat pump output at design temp)
Recommended heat strip size (kW) = Shortfall converted from BTU/h
Worked Example
60,000 BTU/h needed at design temperature, heat pump delivering 36,000 BTU/h at that temperature: Shortfall = max(0, 60,000 − 36,000) = 24,000 BTU/h, converted to kW for the heat strip size.
Frequently Asked Questions
Why does a heat pump’s output drop at design temperature?
Heat pump capacity generally decreases as outdoor temperature falls, so a unit rated at a certain BTU/h under mild conditions delivers less at your coldest design temperature — heat strips exist to cover that gap.
How do I find my heat pump’s output at design temperature?
Manufacturer performance data (often called an extended performance or capacity table) lists a heat pump’s output at various outdoor temperatures — check your specific model’s documentation or ask your installer.
What is design temperature?
Design temperature is the coldest outdoor temperature your heating system is engineered to handle for your specific region, typically based on historical climate data rather than the coldest temperature ever recorded.
Is it bad to oversize a heat strip?
An oversized heat strip mainly means higher potential electrical demand and cost if it runs at full capacity, but it generally doesn’t harm the system — undersizing is the bigger practical concern since it may leave a shortfall uncovered on the coldest days.
Does a heat strip run constantly during cold weather?
No — heat strips are typically designed to activate only when needed, supplementing the heat pump during the coldest periods or during defrost cycles, rather than running continuously.
Does electrical panel capacity affect heat strip sizing?
Yes — larger heat strips draw more electrical current, so confirm your home’s electrical panel and wiring can support the heat strip size you’re considering; an electrician can verify this.
Can I skip a heat strip if I have a backup furnace instead?
Yes — some systems use a dual-fuel setup with a gas furnace as backup instead of an electric heat strip; in that configuration, the furnace is sized to cover the same shortfall this calculator estimates.
Does a heat strip affect my thermostat settings?
Most heat pump thermostats are configured to call for auxiliary heat automatically when the heat pump alone can’t maintain the setpoint, so proper thermostat configuration matters alongside correct heat strip sizing for efficient operation.
How is heat strip size expressed on equipment nameplates?
Heat strips are typically sold and labeled in fixed kW increments (like 5 kW, 10 kW, 15 kW), so round your calculated shortfall up to the nearest available size when selecting an actual unit.
