Economic vs capacity balance point: heat pump methodology
Economic balance point = temperature where the heat pump and backup fuel cost the same per unit of heat.
Thermal / Capacity balance point = outdoor temperature where the heat pump’s heating capacity equals the home’s heat loss. Below this temperature the heat pump alone cannot keep the house warm.
Calculating the economic switchover temperature
Heat pump cost per 10,000 BTU = electricity price × 10,000 / (COP at outdoor temperature × 3,412). Backup fuel cost per 10,000 BTU = fuel price × 10,000 / (BTU per fuel unit × effective efficiency).
Heating oil contains approximately 138,500 BTU/gallon, propane 91,500 BTU/gallon, and natural gas 100,000 BTU/therm. Effective efficiency applies AFUE and a cycling derate. The two costs are compared across outdoor temperatures to find the economic crossover.
Manufacturer COP and estimated performance
Manufacturer COP points are interpolated by outdoor temperature. Users can enter a COP table or select equipment. An HSPF2-derived curve is a simplified fallback, not a manufacturer measurement. The displayed confidence range is a modeling allowance, not a statistical guarantee.
Optional thermal capacity and home heat loss
Enter capacity in BTU/h at two or more outdoor temperatures plus a design heat-loss estimate or rough floor area and insulation. Manufacturer heating capacity and home heat loss are complementary inputs; neither alone determines capacity balance.
At an assumed indoor temperature of 70°F, heat loss(T) = UA × max(0, 70 − T). UA = design heat loss / (70 − design outdoor temperature). The rough home estimate uses UA = square feet × (0.45 − 0.03 × insulation rating) × ceiling height / 8, with insulation rated 1–10.
Capacity is linearly interpolated only between entered points. The warmest crossing where capacity equals heat loss is reported; capacity is never extrapolated. If the crossing is outside entered data, the result is a bound rather than an exact setting.
Recommended practical switchover
Usually choose the higher of the economic and capacity balance points, respecting the equipment minimum operating temperature. A heat pump that is economical but undersized cannot cover the home’s load. Out-of-range or unresolved thresholds require installer review.
Annual heating costs and limitations
Annual economic comparisons use climate normals and either rough home inputs or a 15,000 BTU/h reference load. Reference-load figures show relative cost, not the total home heating bill. Annual costs do not simulate capacity shortfall. Time-of-use and seasonal rates use heating-use-weighted averages, not hourly simulations; solar marginal cost includes the value of forgone exports.
These estimates are not Manual J, equipment-sizing advice, measured savings, or a guarantee. Review defrost, staging, thermostat controls, and local conditions with an HVAC professional.