Generator Sizing Calculator

Every generator is advertised with two numbers, and the gap between them exists because of one specific physical fact about electric motors. A motor at rest has almost no back-EMF opposing the supply, so at the instant it is switched on it draws several times the current it will settle at once it is spinning. This is called locked-rotor current or inrush, and for a typical household induction motor it lasts a fraction of a second and reaches two to three times the running draw.

Running watts — sometimes called rated or continuous watts — is what the generator can produce indefinitely without overheating. Starting watts, also called surge or peak watts, is what it can deliver for a few seconds by exploiting the engine’s flywheel inertia and the alternator’s thermal mass.

A generator has to satisfy both constraints, and they fail differently. Undersize the continuous rating and the machine runs at its limit for hours, gets hot, drinks fuel and shortens its life. Undersize the surge and the engine bogs down the first time the well pump kicks in — the voltage sags, the motor draws even more current trying to start, and either the generator’s breaker trips or the motor sits there humming and cooking itself.

A great many sizing guides tell you to add up the running watts of everything you want to power, then add up all the starting watts, and buy a generator that meets both totals. The first half is right. The second half is wrong, and it is wrong in an expensive direction.

Adding every appliance’s starting wattage assumes that the refrigerator, the freezer, the furnace blower and the sump pump all happen to switch on within the same fraction of a second. They do not. Each is controlled by its own thermostat or float switch, running on its own schedule, and the probability of a simultaneous start across four independent devices is negligible — and in the rare event it happened, the surge would last well under a second and most generators would ride through it.

The realistic worst case, and the one to size for, is that everything is already running at its continuous draw when the largest motor in the house starts. That is the running total plus one surge — the biggest one. In the worked example above, the difference between the two methods is 5,250 W against 7,250 W, which in practice is the difference between a mid-size portable and a machine costing well over twice as much.

The three categories solve genuinely different problems, and price is not the only axis they differ on.

A generator’s nameplate output is measured at sea level in moderate conditions. An internal combustion engine makes power by burning fuel with oxygen, and thinner air holds less oxygen per cubic foot, so a naturally aspirated engine loses roughly 3% of its output for every 1,000 feet of elevation. At Denver’s 5,280 feet that is about a 16% loss; in a Colorado mountain town at 9,000 feet it approaches 27%.

This is not a subtlety buried in a footnote — it is stated in the manual of essentially every generator sold — but it is missing from almost every online sizing chart, and it is exactly the sort of margin that turns a correctly sized machine into one that stalls. Fuel-injected and turbocharged engines lose less, and some manufacturers publish specific derate tables.

High ambient temperature has a smaller effect in the same direction. Propane, meanwhile, carries about 10% less energy per unit than gasoline, so a dual-fuel generator’s propane rating is usually lower than its gasoline rating — check which number you are reading before sizing against it.

The genuinely hazardous mistake with home generators is not getting the wattage wrong. It is backfeeding: connecting a generator to the house wiring through a dryer outlet or a "suicide cord" with two male plugs. This energises the house circuits and, through the service drop, the utility lines outside — where a lineman restoring power has every reason to believe the conductors are dead. The step-up through the pole transformer turns 240 volts into thousands.

The correct connection is a transfer switch or an interlock kit, both of which make it physically impossible for the generator and the utility supply to be connected at the same time. Both require an electrician and a permit in most jurisdictions, and both are inexpensive next to the generator itself.

The other reliable killer is carbon monoxide. Portable generators produce it in quantities that have repeatedly proved fatal in attached garages and under porches, including with the garage door open. The rule is outdoors only, well clear of any window, door or vent — and newer units sold with a CO shutoff sensor are worth the small premium.