How to Size a Generator: Running Watts, Starting Watts, and Real-World Sizing

Running Watts vs Starting Watts

Every motorized appliance draws two different amounts of power: running watts (the steady-state power during normal operation) and starting watts (the surge of power needed to start the motor, typically lasting 1-3 seconds).

A refrigerator might run at 150 watts but needs 400 watts to start the compressor. A well pump might run at 1,000 watts but surges to 3,000 watts on startup. This surge is why generator sizing is more complicated than just adding up the wattage of everything you want to run.

How to Calculate Your Needed Size

Add up the running watts of everything you want to power simultaneously. Then find the single largest starting watt surge among those appliances. Your generator needs to handle all running watts plus that single largest surge happening at the same time.

Example: Refrigerator (150W running, 400W starting) + well pump (1,000W running, 3,000W starting) + lights (200W, no surge) + phone charger (15W, no surge). Total running: 1,365W. Largest surge above running: 2,000W (the well pump's extra 2,000W above its 1,000W running). Generator needs: 1,365 + 2,000 = 3,365W peak. Add 20% headroom: 4,038W. Buy a 4,000W or 5,000W generator.

The 20% Rule

Never run a generator at 100% capacity continuously. It burns more fuel, runs hotter, produces dirtier power (voltage fluctuations), and shortens the engine's life dramatically. The sweet spot is 50-75% of rated capacity. Sizing your generator with 20% headroom above your calculated need keeps you in this efficient range.

Fuel Types

Gasoline generators are the most common and cheapest. Drawback: gas goes stale in 3-6 months without stabilizer, and you can't store large quantities safely indoors.

Dual-fuel (gas + propane) generators give flexibility. Propane stores indefinitely in its tank, making it ideal for emergency backup that sits unused for months. Propane produces about 10% less power than gasoline, so a 5,000W gas generator runs about 4,500W on propane.

Diesel generators are the most fuel-efficient and last the longest, but they're louder and cost more upfront. Common in large standby systems (10,000W+).

Portable vs Standby

Portable generators ($400-$3,000) are wheeled units that you roll outside, start manually, and connect via extension cords or a transfer switch. They handle most home backup needs up to about 12,000W.

Standby generators ($3,000-$15,000 installed) are permanently mounted outside your home, connected directly to your electrical panel through an automatic transfer switch. When power goes out, they start automatically within seconds. No human intervention required. This is the true passive option: you set it up once and it handles every outage for the next 20 years.

Safety

Carbon monoxide from gas and diesel generators kills people every year during power outages. Never run a generator indoors, in a garage (even with the door open), or within 20 feet of any window, door, or vent. Always use a carbon monoxide detector in your home when running a generator. Never connect a generator to your home's wiring without a transfer switch since backfeed through the panel can electrocute utility workers repairing the lines.