This generator size calculator totals the running watts of the appliances you check, applies the largest motor’s starting surge, and recommends a generator size with working headroom.

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Running Watts Keep the Lights On; Starting Watts Decide the Size

Feeding the generator’s output through long cords costs voltage — see our Voltage Drop Calculator. Part of our Construction Calculators collection.

Worked Example (Verified)

  • Essentials set (defaults: fridge, furnace fan, sump pump, lights, TV/internet): running 2,800 W; largest surge = the fridge’s +1,500 W → peak 4,300 W; with 20% headroom → ≥ 5,400 W class — the popular 5,500–6,500 W portables and 7.5 kW standbys live here
  • Adding the 3-ton central AC changes the species of the answer: +3,500 running and a +7,000 surge (soft-start kits can halve that surge — the cheapest generator-size reduction in the catalog)
  • Resistive loads (water heater, space heater, stove) have no surge but eat running watts whole: one 4,000 W water heater outweighs the entire essentials list

Sizing Discipline

  • Whole-home standby units are quoted in kW with managed-load panels shedding big loads — a 14 kW managed system often outperforms a naive 20 kW sum.
  • Inverter generators sip fuel and protect electronics (clean sine, low THD); conventional sets win on price per watt.
  • Never backfeed through a dryer outlet — transfer switch or interlock, always; linemen’s lives are the footnote.
  • Fuel runtime is the other half of sizing: gallons on hand ÷ burn rate at half load is your outage endurance.

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Frequently Asked Questions

How do I size a generator for my house?

Add the running watts of everything you want powered, add the largest single motor’s extra starting watts, then add ~20% headroom. The essentials default here (fridge, furnace fan, sump, lights, electronics) needs 2,800 W running / 4,300 W peak → a 5,500 W class machine. This calculator does the sum from a checklist.

What are starting watts vs running watts?

Motors (fridges, pumps, compressors) draw 2–3× their running wattage for the second they start. Generators publish both ratings: continuous (running) and peak/surge. Size the continuous rating above your running load, and the surge rating above running + biggest motor surge.

What size generator runs a central AC?

A 3-ton unit typically needs ≈ 3,500 W running with surges toward 10,500 W without a soft starter — pushing most homes to 9–14 kW standby class, or a soft-start kit that tames the surge and lets a large portable cope. Read the AC nameplate’s LRA figure for the authoritative surge.

Can a generator be too big?

Economically yes: oversizing costs purchase price, fuel and noise, and chronic light loading isn’t kind to engines (wet stacking in diesels). The 20% headroom rule threads it — enough margin for surges and altitude, not a size class of waste.

How long will the fuel last?

A typical 5.5 kW portable burns ≈ 0.5–0.75 gal/hr at half load — a 5-gallon tank ≈ 7–10 hours. Runtime at your actual load, propane derating (~10% less power), and safe refueling practice belong in the outage plan beside the wattage.

Is this generator size calculator free?

Yes — free, instant, no sign-up.