Heat Pump Backup Heat Calculator (Auxiliary Electric Heat Sizing)

Cooling and heating loads are estimated from your square footage × a BTU/sqft figure for your IECC climate zone, adjusted for ceiling height, insulation quality, sun/window exposure (cooling only), and occupancy beyond 2 people (cooling only) — the same shared formula this site's AC & Furnace Size Calculator uses. The cooling-based baseline sizes the heat pump to the COOLING load only (rounded UP to the nearest standard nominal tonnage), then approximates its rated heating capacity at 47°F as roughly equal to that tonnage in BTU/hr — a widely used rule of thumb, though real products vary ±10-20% by manufacturer and model. This is a baseline, not the only valid way to size a heat pump — ANSI/ACCA 3 Manual S-2023 (3rd edition) also permits HEATING-led sizing of variable-capacity equipment in heating-dominant climates, evaluated against a roughly 0.90-1.30 ratio of your CALCULATED cooling load, plus latent capacity, actual heating capacity, and minimum-compressor checks that need manufacturer-specific data this calculator doesn't have. The Heating-Led Planning Alternative shown above checks only the size-ratio part of that allowance (capped against your calculated cooling load, not the already-rounded baseline, since capping against the rounded number would let the result balloon past ACCA's actual ratio) — treat it as a bounded planning check, not a completed Manual S selection. Rated capacity is derated for a representative winter design temperature — a zone-level estimate from this calculator's 6-bucket climate selector, not your exact location's own 99% design temperature — using a typical capacity-retention curve (100% at 47°F, and a Standard- or Cold-Climate-specific retention at 17°F and 5°F, per AHRI's own rating points — extrapolated and floored below 5°F, since no standard test data exists there; the Cold-Climate profile's 5°F retention matches ENERGY STAR's own Cold Climate Heat Pump minimum, though that certification also requires a minimum COP this calculator does not model). The balance point is the outdoor temperature where derated capacity first drops below the heating load, found by scanning from 47°F down to that design temperature; if capacity never drops below load in that range, no backup heat is needed at all. Backup heatis the electric resistance capacity (in kW) needed to cover any shortfall at the design temperature, rounded UP to a standard heat-strip size (5, 7.5, 10, 15, or 20 kW) — note the snapped kW figure can stay the same between the baseline and the heating-led alternative even when the underlying BTU shortfall shrinks, since both may round to the same standard size. This is a square-footage planning estimate, not an ACCA Manual J load calculation, and the retention curve is a typical rule of thumb, not a specific manufacturer's AHRI-rated extended performance data — get a Manual J load calculation, a full Manual S evaluation, and a specific unit's manufacturer performance data from a licensed HVAC contractor before purchasing. Cost estimation, if enabled, covers equipment only (priced per ton / per kW of backup heat, based on the cooling-based baseline) — installation labor, ductwork, and electrical work are not included.