Generator Sizing Calculator

Running watts is the steady draw once a device is up to speed. Starting watts, sometimes called surge or locked-rotor watts, is the brief spike a motor pulls in the first fraction of a second before it spins up. A 1,000 W well pump can pull 3,000 to 5,000 W for well under a second. Generators are rated both ways: a set advertised as 7,000/5,500 W means 7,000 surge and 5,500 continuous. Always match your continuous number to the smaller figure and your surge number to the larger one.

Because the odds of two motors starting in the same half-second are low, and sizing for it would nearly double the generator you buy for no practical gain. The standard approach is running watts for everything plus the surge of the single largest motor. If you have two hard-starting loads you cannot control — say a well pump and a central AC — either add a soft-start kit to the AC compressor, which typically cuts inrush by 60 to 70%, or stagger them manually so only one kicks on at a time.

Yes, if you want to power house circuits. Back-feeding a panel through a dryer outlet or a double-male cord is illegal in every US jurisdiction and it kills line workers, because your generator energizes the utility side of the transformer at several thousand volts. The legal options are a listed manual transfer switch, a service-rated automatic transfer switch, or a panel-specific mechanical interlock kit with a generator inlet. All require a permit and inspection, and a licensed electrician should do the install.

Outdoors only, never in a garage, carport, crawlspace, basement, or shed, even with the door open. The CPSC and every manufacturer specify at least 20 feet from the house with the exhaust pointed away from doors, windows, and vents. Carbon monoxide from a portable generator can reach lethal concentrations in an attached garage in minutes, and CO is what kills the majority of generator victims each year. Install battery-backed CO alarms on every level of the house before you ever need the generator.

Divide watts by voltage. A 7,000 W generator at 240 V delivers about 29 A, which fits a 30 A inlet and a typical 10-circuit transfer switch. A 200 A service panel does not mean you need 200 A of generator — that would be 48,000 W. You are backing up a selected set of circuits, not the whole service. Whole-house standby units in the 18 to 26 kW range cover most homes at 75 to 108 A, usually with load management shedding the AC or range when demand peaks.

A gasoline portable in the 5,000 to 7,500 W class burns roughly 0.6 to 0.9 gallons per hour at half load, so 8 hours a day is about 5 to 7 gallons. That is 35 to 50 gallons a week, which is more than most people store safely. Gasoline also degrades in 3 to 6 months without stabilizer. Propane burns cleaner and stores indefinitely but yields about 10% less power; natural gas standby units sidestep storage entirely if you have a gas service.

Treat it as a planning estimate. It uses the generator-industry rule of thumb for starting watts rather than the measured locked-rotor amps on your specific equipment, assumes only one motor starts at a time, and does not perform an NEC Article 220 load calculation, apply power factor, or account for altitude and temperature derating of the engine — most sets lose roughly 3% of output per 1,000 feet of elevation. It also says nothing about the transfer equipment, the generator circuit conductors and overcurrent protection, grounding and bonding, or whether the neutral must be switched. The locally adopted code and a licensed electrician govern the final design, and a permanent generator, inlet, or transfer switch needs a permit and inspection in essentially every jurisdiction.