Home Generator Size Calculator
Determine the right generator size (in kilowatts) to keep your home's essential appliances running during a power outage. Use it when planning an emergency generator purchase or sizing a transfer switch installation.
Last updated: September 2026
Formula below · 2 sources (nfpa.org, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
This calculator estimates the generator capacity needed for critical home loads. First, the running watts of lighting, refrigeration, the furnace blower and the well pump are added. Motors draw 2–3× their running wattage for a moment when they start, but they normally start one at a time, so only the largest motor’s extra starting draw is added: largest motor × (starting factor − 1). Lighting is resistive and has no surge. The peak is then multiplied by 1.25 for 25% headroom, converted to kilowatts and rounded up. Formula: Generator Size (kW) = ⌈(lights + refrigeration + blower + pump + max(refrigeration, blower, pump) × (startingFactor − 1)) × 1.25 / 1000⌉. If motors could start together (for example when a transfer switch reconnects everything at once), add each motor’s extra starting watts or choose a generator with load management.
How to use
Suppose your home needs: Essential Lighting = 500 W, Refrigeration = 800 W, Furnace Blower = 600 W, Well Pump = 1,000 W, and a Motor Starting Factor of 2.0. Step 1 — sum running loads: 500 + 800 + 600 + 1,000 = 2,900 W. Step 2 — add the largest motor’s extra starting draw: the well pump, 1,000 × (2.0 − 1) = 1,000 W, for a peak of 3,900 W. Step 3 — add 25% headroom: 3,900 × 1.25 = 4,875 W. Step 4 — convert and round up: ⌈4.875⌉ = 5 kW. The defaults (800, 700, 600, 1,200 W at 2.5×) give 3,300 + 1,200 × 1.5 = 5,100 W × 1.25 = 6,375 W, so 7 kW.
Frequently asked questions
What is a motor starting factor and why does it matter for generator sizing?
Electric motors draw significantly more current the moment they start than they do while running steadily—this is called inrush or starting current. A motor starting factor of 2.0–3.0 accounts for this surge. If you ignore it, your generator may stall or trip under the sudden load spike when the refrigerator compressor or well pump kicks on. Using the correct starting factor ensures the generator can handle both steady-state running and startup peaks without damage.
How many watts do I need to run essential home appliances on a generator?
Typical essential loads total 3,000–8,000 running watts. A refrigerator runs at roughly 150–400 W (but starts at up to 1,200 W), a furnace blower at 300–800 W, a well pump at 750–1,500 W, and basic lighting at 200–600 W. Add the largest motor’s extra starting draw and 25% headroom, which usually gives 5–10 kW for most homes. Always list every device you want to power and check its nameplate wattage.
Why should I add a 25% safety margin when sizing a home generator?
Running a generator at or near 100% of its rated capacity accelerates wear, increases fuel consumption, and risks overloading if an unexpected load starts. The 25% headroom (multiplying by 1.25) keeps the generator operating in its optimal efficiency range and leaves room for devices you may have overlooked. Most generator manufacturers and electricians recommend sizing to no more than 80% of rated capacity for continuous operation, which is exactly what the 1.25 multiplier enforces.