Generator sizing & fuel calculator

This calculator does the two pieces of generator math people most often get wrong: sizing the machine for the loads it actually has to carry, and being honest about how much fuel it will drink. The first mistake leaves you with a generator that trips every time the well pump kicks on. The second leaves you hauling gas cans twice a week wondering where your budget went.

Running watts, starting watts, and why the difference matters

Every electrical load has a running draw, what it pulls once it is going, and motor loads have a starting surge on top of that, often two to four times the running number for the first second or so. A fridge that runs at 150 watts may want 600 to start. A 1,000 watt well pump can demand 2,500 or more at the instant the pressure switch closes.

The generator has to cover your total running load and, at the same moment, the surge of whichever motor happens to start next. That is why the calculator asks for both numbers and sizes with headroom. A generator run constantly at its limit lives a short, loud life; one loafing at 40 to 60 percent load lasts for years and burns cleaner.

Do not size for everything you own. Size for what genuinely must run at once, and plan to stagger the rest. During an outage, nobody needs the water heater, the toaster, and the shop compressor at the same time.

What fuel really costs you

A conventional generator delivers roughly 5 to 6 kWh per gallon of gasoline in real use, not the thermal number on paper. Propane holds a bit less energy per gallon but stores forever without going stale, which is why so many off-grid setups keep a propane generator plumbed to the house tank: no carburetor varnish, no rotating gas cans, it just starts. Diesel delivers the most work per gallon and the engines outlast everything else, but the machines cost more up front and dislike light loads.

The fuel-per-day number in the results is the figure readers tend to flinch at. Running a generator as primary power is the most expensive electricity you can make, often ten times solar's cost per kWh over time. The cheapest generator strategy off-grid is the one where it barely runs: solar and batteries carry the days, and the generator is the deep-winter and bad-week backstop. The solar sizing calculator and battery runtime calculator will size that side of the system.

Inverter generators, conventional generators, and standby units

Inverter generators throttle to match the load, so at light loads they can cut fuel use by a third or more, and their power is clean enough for sensitive electronics. They cost more per watt and top out smaller. Conventional open-frame units are cheap workhorses that run at full speed no matter the load. Standby units (the permanently installed kind with a transfer switch) shine where outages are long and frequent, and most run on propane, which ties neatly into the tank sizing in the propane calculator.

Whatever you buy, the boring rules pay the best: run it under load monthly, keep fresh fuel or propane, keep a spare spark plug and filter, and put the money you almost spent on an oversized unit into batteries instead. For deeper planning on the power side of an off-grid setup, the Solar & Power archive covers system design, backup strategy, and the mistakes to skip.