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.
About this calculator
This calculator estimates the minimum generator capacity needed to power critical home loads. The core loads—essential lighting, refrigeration, furnace blower, and well pump—are summed in watts. Because electric motors (refrigerators, pumps, blowers) draw 2–3× their running wattage at startup, a motor starting factor is applied to account for inrush current. The result is then multiplied by 1.25 to add a 25% safety headroom, ensuring the generator is never run at 100% capacity. Finally, dividing by 1,000 converts watts to kilowatts, and the result is rounded up to the nearest whole kW. Formula: Generator Size (kW) = ⌈((lights + refrigeration + blower + pump) × startingFactor × 1.25) / 1000⌉
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 loads: 500 + 800 + 600 + 1,000 = 2,900 W. Step 2 — Apply starting factor: 2,900 × 2.0 = 5,800 W. Step 3 — Add 25% safety margin: 5,800 × 1.25 = 7,250 W. Step 4 — Convert to kW and round up: ⌈7,250 / 1,000⌉ = ⌈7.25⌉ = 8 kW. You would need at least an 8 kW generator.
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 watts before applying a starting factor. 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. Adding a 25% safety buffer and accounting for motor start surges usually pushes the required generator size to 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.