SolarToolsHow This Tool Works
The Generator Sizing Calculator tells you exactly what size generator you need to back up your essential loads. The trick to generator sizing isn't the running watts — it's the starting surge. Induction motors in refrigerators, well pumps, AC compressors, and furnaces draw 3–6× their running wattage for the first 1–3 seconds as they spin up. If your generator can't deliver that surge, the motor stalls or the generator trips off.
This calculator lets you check off the appliances you want to back up, automatically sums their running watts, applies the appropriate surge multiplier to motor loads, and recommends a generator size with 20% headroom. The result includes a fuel type recommendation (gasoline for small, propane/gasoline for medium, natural gas/propane for whole-home standby) and an estimated cost.
For most US homes backing up essentials (fridge, lights, furnace fan, internet), a 7–10 kW generator is sufficient. For whole-home including AC, you'll need 18–26 kW. South African homes with geysers and electric cooking may need 15–22 kW. The calculator shows exactly where your specific load list lands.
- Check the appliances you want to back up. Defaults cover the essentials: fridge, lights, furnace fan, TV/internet, chargers. Add or remove based on your priorities.
- Set the motor starting multiplier. 3× for modern soft-start motors and inverter generators, 4× for standard motors, 6× for older motors or large well pumps and compressors.
- Read the recommended size. The calculator adds 20% headroom to the peak surge and shows the recommended continuous kW.
- Compare to generator specs. Generators have both a continuous (running) rating and a surge (starting) rating. Make sure both exceed your calculated needs.
If the recommended size exceeds 25 kW, consider load shedding — backing up only critical loads and accepting that AC or the dryer won't run during outages. This can halve the generator cost.
When to Use This Calculator
Why motor starting surge matters
Induction motors (fridge compressors, well pumps, AC compressors, furnace fans) draw 3–6× their running wattage for 1–3 seconds at startup. A 750W well pump can pull 4,500W at startup. If the generator can't deliver that surge, the motor stalls — and stalled motors draw even more current, often tripping the generator's breaker or burning out the motor.
Inverter generators vs conventional
Inverter generators (Honda EU series, Predator inverter, Generac PowerPact) produce clean power with surge capacity 1.5–2× rated output. Conventional generators have surge capacity 1.0–1.3× rated. For motor loads, inverter generators handle surges better and protect sensitive electronics.
Natural gas vs propane vs gasoline
Natural gas: unlimited supply (no refueling), but lower energy density means 10–15% less power than propane. Best for permanent standby installations.
Propane: stores indefinitely, slightly more power than NG, requires large tank (250–500 gallons for whole-home). Common for standby generators.
Gasoline: highest power per gallon, but goes bad in 3–6 months, requires manual refueling, and is dangerous to store in quantity. Portable generators only.
The 20% headroom rule
Generators should run at 80% of rated load for continuous operation. Running at 100% stresses the engine, increases fuel consumption, and shortens lifespan. The calculator's 20% headroom accounts for this and provides buffer for future loads.
When to choose battery instead
For short outages (under 12 hours) and quiet operation, batteries (Tesla Powerwall, Enphase) are superior. For multi-day outages or whole-home including AC, generators win. The Generator vs Battery comparison breaks down the trade-offs.
Frequently Asked Questions
For essentials only (fridge, lights, furnace fan, internet): 7–10 kW. For whole-home including AC: 18–26 kW. For large homes with electric heat and well pump: 26–35 kW. Use this calculator with your specific load list for an exact number.
AC compressors have large induction motors that surge 4–6× running watts at startup. If your generator can't deliver that surge, it trips. Solutions: install a hard start kit on the AC (cuts surge by 50%), upgrade to a larger generator, or use an inverter generator with higher surge capacity.
Portable ($500–$2,000) for occasional short outages, manual setup, gasoline. Standby ($5,000–$15,000 installed) for frequent or long outages, automatic transfer, natural gas/propane, weatherproof enclosure. Standby adds home value; portable doesn't.
Yes, but inefficiently. A 7 kW generator running 4 hours adds ~28 kWh to an EV — about 100 miles of range. Costs about $15 in fuel. Compare to $4 for the same charge from home electricity. Reserve generator fuel for essential loads.
Portable gasoline: 8–12 hours at 50% load. Standby with 250-gallon propane tank: 5–10 days at 50% load. Standby with natural gas: unlimited (as long as gas service is on). Size your fuel supply for the longest expected outage in your area.
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Energy & Solar Glossary
Plain-English definitions of every term used in this calculator and across the site.