Generator Size Calculator

How it works

Minimum running output = total simultaneous running watts × (1 + headroom ÷ 100). Minimum starting output = (total running watts + the largest additional starting watts) × (1 + headroom ÷ 100). This follows the common sizing method of adding one largest additional motor-starting load to the simultaneous running total; it assumes only one large motor starts at a time.

Worked example: 5,000 running watts plus a 2,000-watt start

Suppose all loads that may run together total 5,000 W. The largest motor needs 2,000 additional watts for a brief start, and you choose 20% planning headroom.

Running requirement = 5,000 × 1.20 = 6,000 W.

Starting requirement = (5,000 + 2,000) × 1.20 = 8,400 W.

Compare 6,000 W with the generator’s continuous or rated output and 8,400 W with its starting or surge output. A model advertised as “8,500 watts” may still be too small if that number is only its peak rating and its continuous rating is below 6,000 W.

Build the running-load total before choosing a generator

List only the equipment that must operate at the same time during an outage or off-grid session. Use the rated or running watts from each nameplate or manual, then add them. Do not use a utility bill’s average watts for motors, pumps, refrigeration, air conditioning, medical equipment, or other loads whose momentary demand matters.

Check voltage, frequency, individual outlet current, and whether any load needs 240 V as well as total watts. A generator can have enough total wattage while one receptacle, winding, phase, or connection is still unsuitable.

“Additional starting watts” means the extra burst—not the full total

Running watts are the continuous demand after equipment is operating. Starting watts are the brief higher demand when some motors or compressors begin. In this calculator, enter the largest additional amount above running. For example, if a pump uses 800 W while running and 2,150 W while starting, its additional starting load is 1,350 W—not 2,150 W.

The Generac worksheet and Champion operating guidance both use total simultaneous running watts plus the single highest additional starting load. If multiple large motors can start together, sequence the loads or use qualified manufacturer guidance instead of assuming one surge.

Ratings can change with fuel, altitude, weather, and load type

Check the exact model manual rather than relying on a marketplace title. A dual-fuel generator can have different output on gasoline and propane. High altitude and high temperature may require derating, and sensitive electronics, pumps, compressors, battery chargers, welders, and other nonlinear loads may need model-specific guidance.

This calculator does not choose a fuel, waveform, voltage, receptacle, cord, transfer equipment, grounding arrangement, or installation. It also does not size conductors, breakers, inlets, or transfer switches.

Portable-generator safety is non-negotiable

CPSC says portable generators must be used outdoors only, at least 20 feet from the home, with exhaust directed away from windows, doors, and vents, and with working carbon-monoxide alarms. Never operate one in a house, garage, basement, crawlspace, or shed—even with doors open.

OSHA warns never to connect a generator directly to a building electrical system unless a qualified electrician has properly installed a transfer switch. Never backfeed through a wall outlet. Use the manufacturer’s instructions and properly rated, intact outdoor cords, keep the generator dry, shut it down before refueling, and follow local electrical and fuel rules.

Sources used for assumptions: Generac — running watts, additional starting watts, and sizing worksheet; Honda Power Equipment — generator wattage estimation guide; Champion Power Equipment — running-load and largest-starting-load method; U.S. Consumer Product Safety Commission — portable generator safety; OSHA — using portable generators safely.

Frequently asked questions