Generator Size for Heat Pump Calculator
Find the running and starting watts — and the generator size needed to run your heat pump.
Running watts vs starting watts
Two numbers matter when running a heat pump on a generator. Running watts is the steady draw once it's going (roughly 900–1,400 W per ton). Starting watts is the brief surge when the compressor kicks on — for a standard single-stage compressor that can be 2.5–3× the running watts. Your generator must handle the surge, not just the running load.
What size generator do you need?
Size the generator to the starting surge plus headroom for other appliances (fridge, lights, well pump). A 3-ton standard heat pump can surge to around 10,000 W, so a 12 kW+ generator is a safe target. A smaller home or an inverter unit needs much less.
Soft starts change everything
A soft-start kit (or an inverter-driven heat pump) ramps the compressor up gradually, cutting the startup surge by 50–70%. With one, the same heat pump may start on a generator little bigger than its running watts — often letting a portable or smaller standby unit do the job.
Frequently asked questions
What size generator do I need to run a heat pump?
A 3-ton standard unit needs roughly 10,000 W to start (about a 12 kW generator); an inverter or soft-start model needs far less. Add headroom for other appliances.
Why is the starting wattage so high?
A standard compressor draws a large surge (locked-rotor amps) at startup — often 2.5–3× its running watts. Soft-start kits cut this dramatically.
Will a 12,000-watt generator run a heat pump?
For a 1.5–3 ton standard unit, usually yes — its surge fits within about 12 kW. Larger units, or running other appliances at the same time, may need more or a soft-start kit.
Do I need a generator that runs my whole house?
No — size it to the heat pump surge plus the essentials you want during an outage (fridge, lights, well pump). A whole-house standby unit is only needed if you want everything on at once.
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⚠️ Rule-of-thumb estimate, not a substitute for a professional Manual J load calculation. Based on US DOE / ENERGY STAR guidance.