Generator Size Calculator

How to use the generator size calculator

Enter the total running watts of everything you intend to power at once and the additional starting surge of the single largest motor-driven appliance, and the tool returns the peak demand and a recommended generator size with 20 percent headroom already built in. Read the second field carefully, because it is the one people fill in wrong: it wants the extra watts the biggest motor needs to get spinning, on top of the running figure you have already counted, not that appliance total starting wattage. If a wattage chart lists a refrigerator at 700 running and 1,200 starting, the 700 belongs in the running total and the 500 difference is what belongs in the surge field. Getting that distinction right is the difference between a sensible recommendation and one that is 700 watts too big.

Running watts against starting watts is the single most common generator-sizing mistake, and it exists because of how induction motors behave at standstill. A stationary rotor produces no back-EMF to oppose the applied voltage, so the winding impedance is just copper resistance and leakage reactance, and the current that flows for the first fraction of a second is far above the running figure. General-purpose induction motors on a direct-on-line start draw roughly five to eight times full-load amps at that instant, which is why manufacturer wattage charts show refrigerators, pumps, compressors and washing machines with starting figures well above their running ones. Size a generator to running watts alone and it will hold your lights beautifully and then stall the moment the well pump kicks in.

You add the surge of only one appliance, and that is a deliberate simplification rather than an oversight. Motors on independent thermostats and pressure switches almost never start on the same instant, so the realistic worst case is everything running plus one motor starting. Adding every surge together would oversize the generator badly, costing money at purchase and fuel every hour thereafter, since a lightly loaded engine is running well below its efficient band. If you genuinely have two large motors that can start together because they share a control — some heat pump and well pump arrangements do — then enter both surges combined, but treat that as the exception you can justify rather than the default.

The 20 percent headroom is there because a generator should not live at 100 percent of its rating. Continuous flat-out operation runs the engine hot, burns more fuel per watt delivered, and leaves nothing at all for the next thing somebody plugs in. Sizing to 120 percent of peak demand keeps the unit in a band it can hold for hours. Then round up, because generators are sold in fixed steps rather than to the watt. One safety point that matters more than any sizing question: the US Consumer Product Safety Commission says never to operate a portable generator inside a home, garage, basement, crawlspace or shed, or on a porch, and to run it outdoors at least 20 feet from the house with the exhaust directed away. Opening a window does not make an enclosed space safe.

The formula

The generator has to survive its worst instant, not its average hour. That worst instant is everything you have running drawing its steady current while one motor is trying to start, so peak demand is the running total plus a single starting surge. The recommendation then adds a fifth on top, because a generator held at its exact rating has no margin left for error, for the next appliance, or for a hot afternoon.

The headroom multiplier of 1.20 is a sizing convention rather than a physical constant. It exists for the same reason you do not choose a car engine that can only just reach the speed limit: continuous operation at the limit is hot, thirsty per watt delivered, and unforgiving of anything unexpected. Manufacturer sizing guidance commonly recommends adding around 20 percent above calculated demand for exactly this reason.

peak demand = running watts + largest starting surge
recommended size = peak demand × 1.20
3,000 W + 2,200 W = 5,200 W peak
5,200 W × 1.20 = 6,240 W recommended

Worked example with the defaults: 3,000 running watts plus a 2,200 W additional starting surge gives a peak demand of 5,200 W, and 5,200 × 1.20 = 6,240 W recommended. No generator is sold at 6,240 W, so you round up to the next standard size, which in practice means a 7,000 or 7,500 W unit. Remove the motor loads entirely and set the surge to zero and the same 3,000 running watts needs only 3,000 × 1.20 = 3,600 W, which a 3,500 W unit very nearly covers and a 4,000 W unit covers comfortably.

Notice how much of that recommendation the surge alone is responsible for. The 2,200 W surge adds 2,640 W to the recommendation once the headroom multiplier is applied, which is 42 percent of the final figure produced by an event lasting a fraction of a second. That is why the choice of which appliance surge to enter deserves a moment of thought, and why swapping a motor-driven appliance for one with a soft-start or inverter-driven compressor can drop the generator you need by a whole size class.

What this calculation does not do is design an electrical installation. It sizes a portable generator against a list of loads you choose to plug into it directly. A generator that will feed house circuits needs a transfer switch and a proper load calculation, both of which are a licensed electrician job, and back-feeding a panel through an outlet is dangerous to you and to line workers. The safety rule that matters most is separate from sizing altogether: the US Consumer Product Safety Commission directs that portable generators be operated outdoors only, at least 20 feet from the house, with the exhaust pointed away from any building anyone could enter.

Frequently asked questions