3. Your result
Running load0 W
Peak starting0 W
Recommended inverter generator
Select appliances
Look for at least – running / – starting watts.
Load on recommended unit: –
Min. rated (running) watts–
Min. starting (surge) watts–
Estimated fuel use–
Runtime per tank–
Fuel for your hours / day–
Largest motor start–
How to size a portable inverter generator
Buying a generator that is too small is the most common mistake people make: the unit overloads the first time the refrigerator compressor or the RV air conditioner kicks on. Buying one that is far too large wastes money, adds weight and burns more fuel at idle. The calculator above follows the same three-step method that dealers and electricians use.
Step 1 – Add up the running watts
Running (or rated) watts are the continuous power an appliance draws once it is operating. Look on the device's data plate or power adapter. If you only see amps, multiply amps by 120 volts (or 230 V outside North America). Add the running watts of everything that could be switched on at the same time. This is the number that must stay below the generator's rated output, because rated output is what the engine can deliver for hours without overheating.
Step 2 – Find the biggest starting surge
Motors, compressors and pumps need two to three times their running power for a fraction of a second when they start. That is why a 700 W refrigerator can demand around 2,200 W at start-up. Generators publish a second number – starting, peak or surge watts – for exactly this reason. You rarely start every motor at the same instant, so the correct peak is your total running load plus the extra surge of the single largest motor. The calculator does this automatically and shows which device sets the peak.
Step 3 – Add a safety margin
Inverter generators are quietest and most fuel-efficient between 25 % and 75 % load. Planning on 20 % headroom keeps the engine out of the red, handles devices whose label understates their draw and leaves room for one more phone charger or a coffee maker. If you live at high altitude, add more: output drops roughly 3.5 % for every 1,000 ft (300 m) above sea level.
Typical inverter generator classes
| Class (starting W) | Typical running W | Weight | Good for |
|---|
| 1,000 – 1,200 W | 800 – 1,000 W | 28 – 35 lb | Phones, laptops, lights, CPAP, small fan |
| 2,000 – 2,500 W | 1,600 – 1,900 W | 40 – 50 lb | Camping, mini fridge, microwave, coffee maker (one at a time) |
| 3,000 – 3,500 W | 2,800 – 3,000 W | 60 – 95 lb | RV with 13.5k BTU AC, tailgating, light home backup |
| 4,000 – 4,500 W | 3,400 – 3,700 W | 95 – 105 lb | RV 30 A, refrigerator + freezer + lights |
| 6,000 – 7,000 W | 5,000 – 5,500 W | 120 – 140 lb | Home essentials, well pump, larger RVs |
| 8,500 – 9,500 W | 7,000 – 7,600 W | 170 – 210 lb | Large home backup, 50 A RVs |
Inverter vs. conventional generators
A conventional open-frame generator spins its alternator at a fixed 3,600 rpm to produce 60 Hz power directly. An inverter generator produces high-frequency AC, converts it to DC and then electronically rebuilds a clean sine wave. The result is power with very low total harmonic distortion (usually under 3 %), which is safe for laptops, TVs, medical devices and modern furnace controls. Because the engine speed can follow the load ("eco mode"), inverter units are typically 50 – 60 dB at quarter load – about as loud as a normal conversation – and use up to 40 % less fuel at light loads.
The trade-off is price per watt. Above roughly 7,000 running watts a conventional or home standby generator is usually cheaper. If you need lots of power for a house, compare the result here with a dedicated standby generator sizing calculator.
Fuel consumption and runtime
Fuel use depends far more on load than on generator size. The estimates in the calculator are based on published runtime tables of popular models: a 2,000 W class unit burns roughly 0.1 gal/h at 25 % load and 0.3 gal/h at full load. Propane delivers about 10 % less power and is consumed by weight – a standard 20 lb barbecue cylinder usually lasts 8 – 12 hours at light load on a 2,000 – 3,000 W dual-fuel unit. Gasoline stored for emergencies should be treated with stabilizer and rotated every 6 – 12 months.
Running two generators in parallel
Most inverter generators can be linked with a parallel kit. Two 2,200 W units combined give about 3,600 running watts while each half weighs under 50 lb – easier to carry than one 95 lb unit. Parallel kits often include a 30 A RV outlet. Only parallel identical or manufacturer-approved models.
Buyer's checklist
- Running watts at least the "Min. rated watts" shown above.
- Starting watts at least the "Min. starting watts" shown above.
- Outlets: TT-30R for RVs, L5-30 or L14-30 for a transfer switch, USB for small devices.
- CO safety shutdown sensor (now standard on many models) – it stops the engine if carbon monoxide builds up.
- Noise rating measured at 23 ft (7 m) and the load at which it was measured.
- Electric start, fuel gauge and wheels for anything above 60 lb.
- Emissions certification (EPA, and CARB if you live in California).
Safety: never run a generator indoors, in a garage, under an awning or near windows. Keep it at least 20 ft (6 m) from the house with the exhaust pointing away. Never back-feed a home through a dryer outlet – use a transfer switch or interlock installed by an electrician.
Frequently asked questions
What size inverter generator do I need for a 30 A RV?
A 30 A, 120 V RV service can deliver up to 3,600 W. With a 13,500 BTU air conditioner (about 1,300 running / 2,800 starting watts) plus a converter and a few small loads, most owners need 3,000 – 3,500 running watts. With a soft-start kit on the AC, a 2,500 – 3,000 W class unit often works.
Can a 2,000 W inverter generator run a refrigerator?
Usually yes. A modern refrigerator needs about 150 – 700 W running and 1,200 – 2,200 W for a moment at start-up. Avoid starting other large loads at the same time.
Is an inverter generator safe for computers and TVs?
Yes. Quality inverter generators produce a clean sine wave with low harmonic distortion (typically below 3 %), comparable to or better than utility power.
How long will an inverter generator run on one tank?
Typically 8 – 10 hours at 25 % load and 3 – 4 hours at full load, depending on tank size. The calculator estimates runtime for your actual load.
Why does my generator overload even though running watts are below its rating?
The starting surge of a motor is probably exceeding the generator's peak rating, or two motors are starting at once. Stagger start-ups or add a soft starter to the compressor.
Does altitude affect generator output?
Yes. Thinner air reduces engine power by roughly 3.5 % per 1,000 ft of elevation. Many manufacturers sell high-altitude jet kits; otherwise choose a larger safety margin.
About GenPilot
GenPilot is an independent, free calculator that helps campers, RV owners, tailgaters and homeowners pick a portable inverter generator with the right running watts, starting watts and fuel capacity. We are not affiliated with any generator manufacturer or retailer, and we do not sell generators.
Every formula on this page runs entirely in your web browser. The numbers you type are never uploaded, stored or shared. Our goal is simple: give you an honest starting point so you can compare product specifications with confidence and avoid paying for capacity you do not need – or buying something too small to do the job.
The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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The calculator and articles on this website are provided free of charge for general information and educational purposes only. Results are estimates based on typical values and simplifying assumptions; real-world performance depends on the specific product, installation, climate, wiring and how you use your equipment.
Nothing on this site is professional engineering, electrical or financial advice. Always follow the manufacturer's instructions and local electrical codes, and have permanent installations, transfer switches and grid-connected systems designed and installed by a licensed electrician or qualified installer. Never operate fuel-burning generators indoors or in enclosed spaces – carbon monoxide can kill within minutes.
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