Calculate the total wattage needed for your generator by adding up appliances with starting and running watts.
The Generator Wattage Calculator helps you determine the right generator size by adding up the power requirements of all the appliances and devices you need to run. Every appliance has two wattage ratings: starting (surge) watts and running (continuous) watts. Your generator must handle both to operate reliably.
Undersizing a generator causes circuit breaker trips, unstable power, and potential equipment damage. Oversizing wastes fuel and money. This calculator lists common household and commercial appliances with their starting and running wattage, lets you build a custom load list, and recommends the appropriate generator size with a safety margin.
Whether you're preparing for power outages, equipping a job site, planning an outdoor event, or going off-grid, accurate wattage planning ensures your generator can handle the full electrical load without straining.
Generator sizing is easiest to get wrong when startup surges are ignored. This calculator keeps the running load, surge load, and safety margin in one place so you can choose a generator that will actually start everything you care about.
Total Running Watts = Sum of all appliances' running watts. Maximum Starting Load = Highest single starting watt + Sum of other running watts. Recommended Generator = Max Starting Load × 1.20 safety margin.
Result: ~4,400 watts minimum, 5,000W generator recommended
Refrigerator (1200 start/200 run) + sump pump (2150 start/800 run) + 5 lights (300 run) + TV (200 run) gives a 3,650W starting case. With a 20% safety margin, the calculator recommends about 4,380W, which rounds to a 5,000W generator for practical sizing.
Generators have two ratings: peak (starting) watts and rated (running) watts. Peak watts handle brief surges when motors start. Rated watts is the continuous output. A "5,000W generator" typically means 5,000 running watts and 6,250 peak watts. Always check both numbers when purchasing.
Essential items during an outage: Refrigerator (200W run, 1200W start), Furnace Fan (700W run, 2350W start), Sump Pump (800W run, 2150W start), LED Lights (10W each), Phone Charger (25W), and WiFi Router (15W). These total about 2,000W running with a 3,550W peak — a 3,500W generator can handle essentials if you stagger startups.
Portable generators (1,000-10,000W) are affordable and mobile but require manual startup. Inverter generators (1,000-4,000W) produce cleaner power for electronics but cost more. Standby generators (7,000-50,000W) start automatically during outages and power entire homes. Choose based on your load calculation, budget, and whether you need automatic backup.
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Starting (surge) watts is the brief spike of power needed to start a motor or compressor. Running (continuous) watts is the steady-state power draw. Starting watts are typically 2-3× higher.
No — you add all running watts plus the highest single starting watt difference. Appliances don't all start simultaneously, so you account for the largest surge on top of the steady load.
A 5,000-7,500W generator covers essentials (fridge, furnace, lights, well pump). A 10,000-12,000W generator runs most home circuits. A 20,000W+ whole-house generator powers everything.
A 5,000 BTU window unit needs about 1,800 starting watts. A central AC needs 3,000-6,000 starting watts. You need a large portable or standby generator for central AC.
Most generators are rated in watts (real power). Some appliances have a low power factor, drawing more apparent power (VA). For safety, the 20% margin accounts for power factor differences.
A 20% margin handles typical variations. If you're in extreme heat (AC runs harder) or extreme cold (heating loads increase), consider 25-30%.