Generator Size Calculator

About This Tool

When a power outage hits, having the right size generator means the difference between comfortable waiting and a freezing house with spoiling food. This calculator helps you determine exactly how many watts you need by adding up your essential appliances. It accounts for both running (continuous) wattage and starting (surge) wattage, which is critically important for motor-driven appliances that can draw 2-3 times their running watts during startup.

Running Watts vs. Starting Watts

Every motorized appliance has two wattage ratings. Running watts (also called rated watts) is the continuous power draw during normal operation. Starting watts (also called surge or peak watts) is the extra burst of power needed to start the motor. A refrigerator might run at 700W continuously but needs 2,200W to kick on the compressor. This surge only lasts a fraction of a second, but your generator must be able to handle it or the breaker will trip. Always size your generator based on starting watts, not running watts.

Why 20% Headroom Matters

Running a generator at 100% capacity constantly causes excessive wear, reduces fuel efficiency, and shortens its lifespan. The 20% headroom buffer protects against multiple appliances surging simultaneously (like the fridge and sump pump kicking on at the same time), accounts for wattage variations in older or less efficient appliances, provides a safety margin for anything you might have forgotten to include, and keeps the generator running at a more efficient and quieter load level. Most manufacturers recommend operating at 50-75% of maximum capacity for optimal performance and longevity.

Portable vs. Standby Generators

Portable generators (3,000-14,000W) are less expensive ($500-$3,000), run on gasoline, and connect via extension cords or a manual transfer switch. They require manual setup during outages and need refueling every 8-12 hours. Standby generators (7,000-30,000W+) are permanently installed, run on natural gas or propane, turn on automatically within seconds of an outage, and run as long as fuel is available. They cost $5,000-$15,000 installed but provide seamless whole-house coverage. Inverter generators are a premium portable option that produce cleaner power safe for sensitive electronics and run more quietly than conventional portables.

Fuel Type Considerations

Gasoline is the most common fuel for portable generators but has a limited shelf life (3-6 months) and can be hard to find during widespread outages. Propane stores indefinitely and is readily available from home tanks but produces slightly less power per gallon. Natural gas is piped in continuously (no refueling) but requires a permanent installation. Dual-fuel generators that run on either gasoline or propane offer the most flexibility. For emergency preparedness, keep fuel stabilizer with your stored gasoline and rotate it every 6 months.

Safety Warnings

Generator safety is non-negotiable. Never run a generator indoors, in a garage, or in any enclosed space — carbon monoxide poisoning kills dozens of Americans each year from improper generator use. Place the generator at least 20 feet from the house with the exhaust pointing away from windows and doors. Never plug a generator directly into a wall outlet (backfeeding), as this can electrocute utility workers and start fires. Always use a transfer switch or heavy-duty extension cords rated for the wattage. Let the generator cool before refueling — gasoline on a hot engine causes fires.