Heat Pump Size Calculator: What Size Heat Pump Do I Need?

Heat pump sizing is more complex than AC sizing because the unit must handle both heating and cooling. In cold climates, the heating load dominates and the heat pump's capacity drops as outdoor temperature decreases. Here is how we calculate it:

1. Calculate both heating and cooling loads using simplified ACCA Manual J methodology (envelope, infiltration, solar, internal gains, latent).
2. Size for the dominant load: In cold climates (Zone 5+), size by heating load. In warm climates, size by cooling load.
3. Apply cold-climate derating: Based on NEEP (Northeast Energy Efficiency Partnerships) data, heat pump capacity drops 12-50% at design temperatures. Zone 5: -25%, Zone 6: -33%, Zone 7: -42%.
4. Calculate balance point: The outdoor temperature where heat pump capacity equals building heat loss. Below this, supplemental heat is needed.
5. Calculate supplemental heat: The gap between heating load and derated heat pump capacity, converted to kW of electric strip heat.
6. Estimate savings: Compare annual heating cost with current fuel vs. heat pump COP (varies by zone, 2.0-4.0).

This is our key differentiator: most online calculators ignore cold-climate derating entirely, leading to undersized heat pumps that cannot keep homes warm on the coldest days.