Enter what the generator has to run and it sizes the unit, then estimates the fuel it burns per day and what a tank of gas or a 20 lb propane bottle really gives you.
Running watts is everything on at once (a fridge is ~150W running, freezer ~100W, well pump ~1000W, lights and wifi ~100W). The surge is the single hardest-starting motor, usually the well pump or a compressor.
This calculator needs JavaScript turned on. With it off, the guide below walks through the math by hand.
Peak watts needed = the larger of (running watts x 1.25) or (running watts + largest motor surge); generator class = the next standard size up (2,000 / 3,500 / 4,500 / 6,500 / 9,000 / 12,000 / 15,000W). Fuel per day = (running watts x load factor / 1000) x hours per day / delivered kWh per gallon (6.0 gasoline, 4.6 propane, 9.5 diesel). One tank's hours = tank gallons x kWh per gallon / average kW.
2,000W running with a 1,500W motor surge needs max(2,500, 3,500) = 3,500W peak, a 3,500W-class unit. At 50% load for 4 hours a day on gasoline: 1.0 kW x 4 h / 6.0 = 0.7 gal/day, about 4.7 gal/week; a 5-gallon can lasts about 30 hours at that load.
| Case | Inputs | Expected output |
|---|---|---|
| GEN-001 | Running 2,000 W, largest surge 1,500 W, load factor 50%, 4 h/day, gasoline | Peak needed 3,500 W -> 3,500 W class; fuel ~0.67 gal/day (2.0 kW x 0.5 x 4 h / 6.0 kWh/gal), plus or minus ~30% |
| GEN-002 | Running 4,000 W, no surge entry, diesel, 6 h/day, 60% load | Peak 5,000 W -> 6,500 W class; fuel ~1.52 gal/day (4.0 x 0.6 x 6 / 9.5) |
Formula version 1.0, in effect since 2026-08-25. Changes to formulas or assumptions bump this version and are listed in the corrections log.
These are the exact numbers the calculator on this page runs, stated so you can check them. Planning estimates, not engineering; sizing that matters gets confirmed by a professional against local conditions and code. Spotted a problem with the method? Tell me.
Want the reasoning, not just the number? Lesson 6: Wire it so it does not burn in the free course walks through it, and the system planner carries the answer into a whole design.
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.
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.
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 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.
Add up everything that must run simultaneously, then add the starting surge of the hardest motor, usually the well pump. A typical cabin with a fridge, freezer, lights, and a 1,000 watt well pump needs a 3,500 to 4,500 watt generator. Buy one size up from "barely enough"; a generator run flat-out lives a short life.
A conventional gasoline generator delivers roughly 5 to 6 kWh per gallon in real use. Running 4 hours a day at a 1,000 watt average load burns about two-thirds of a gallon daily, or roughly 5 gallons a week. Propane holds a bit less energy per gallon but stores indefinitely without going stale.
Propane wins for backup duty: it never goes stale, so the generator starts after months idle, and it can plumb straight to the house tank. Gasoline is cheap and available everywhere but degrades in storage. Diesel delivers the most work per gallon and the engines last longest, best where the generator runs regularly.
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Products appear here only when they match this page's subject. Selection is based on documented specs and owner reports, never on compensation; pages with no relevant gear carry none.
Runs on gas or propane, quiet enough to live near, and inverter-clean for electronics. The size that covers a fridge, well pump, and lights on a cabin.
Why it's here: Listed as the size most cabins actually need. A bigger generator burns more fuel doing the same small job.
Researched, not personally tested
When the load is the whole homestead: 7,500 running watts on gas, electric start, and propane so the fuel never goes stale between outages.
Why it's here: Listed for whole-homestead loads. It is a lot of generator, and a lot of fuel, for a cabin that only needs lights.
Researched, not personally tested
The small inverter generator with the longest track record in owner reports. Quiet, sips fuel, and exactly right for topping up a battery bank on a cloudy week; see the spares page for the plug, carb, and oil to keep on the shelf.
Why it's here: Listed on its owner-report record rather than its spec sheet, which is the only long-run evidence any of us have.
Researched, not personally tested
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