Portable Generator Sizing Calculator
Match a portable or inverter generator's running and surge watts to camping, job-site, and storm-backup appliances.
Common conversions
| Input | Result |
|---|---|
| 2,000 W inverter | Phone/laptop charging, small cooler, solo camping |
| 3,500 W inverter | Weekend RV/tailgating: fridge + lights + TV |
| 4,000-5,000 W | Storm backup: fridge, sump pump, furnace blower, lights |
| 7,500 W | Job site: circular saw, compressor, shop lights, plus a backup fridge |
| 10,000 W | Extended outage: multiple circuits, small window AC, furnace, fridge |
| Twin 2,200 W inverters, paralleled | ≈4,000-4,400 W combined |
| Runtime at 50% load (5,000 W class) | ≈8-10 hrs per 5 gal gasoline |
| Runtime at 25% load (3,000 W inverter) | ≈8-10 hrs per 1.1 gal gasoline |
| Typical motor surge multiplier | 2-3× running watts |
The math behind it
- Running total = 700 + 300 + 1,000 + 500 = 2,500 W
- Sump pump surge adds roughly 2.5× its running watts, about 1,500 W extra for under a second
- Peak demand ≈ 2,500 + 1,500 = 4,000 W
Everything you need to know
Portable and inverter generators are the right tool for camping, tailgating, job sites, and short-term storm backup, and they're rated in running watts and surge (starting) watts rather than kW. Sizing one means adding up what you'll run at once, checking the surge from the single largest motor, and then thinking about fuel run-time for however long you expect to need power.
Running watts vs. surge watts
Running watts is the steady draw a device pulls once it's already on; surge watts is the brief spike a motor needs to start turning. A 700-watt refrigerator can spike to 1,800-2,200 watts for less than a second at startup, and a 1,000-watt sump pump can briefly draw 2,000-3,000 watts. Add up running watts for everything you'll use at once, then add the surge of just the largest motor on top, since it's very unlikely two big motors start in the exact same instant. Most portable generator spec sheets list both numbers separately; buy to the surge number if you plan to run any motor-driven appliance.
Choosing a size for your use case
Solo camping and phone charging rarely need more than 2,000 watts, which is also the sweet spot for the quietest and lightest inverter units. A weekend at an RV site with a small fridge, lights, and a TV fits comfortably on 3,000-3,500 watts. Storm backup covering a fridge, a sump pump, a furnace blower, and some lighting usually needs 4,000-5,000 watts of running capacity with surge headroom. Job sites running a circular saw, an air compressor, and shop lights typically need 6,500-7,500 watts, and two smaller inverter generators can often be run in parallel with a cable kit to reach that same output more quietly than one large open-frame unit.
Fuel type and run-time
Gasoline units are the cheapest to buy but the fuel goes stale within a month or two unless treated with stabilizer, which matters for a generator that sits unused between outages. Propane and dual-fuel units store fuel indefinitely and burn cleaner, at the cost of roughly 10-15% less output than the same engine on gasoline. A typical 5,000-7,500 watt gasoline unit running at half load burns through a 5-gallon tank in about 8-10 hours, so multi-day outages mean planning fuel storage and safe refueling, not just generator capacity.
Common applications
List essential loads (fridge, freezer, sump, furnace blower, a few lights). Total their running watts and add the largest motor's surge to size a 4,000-5,000 W unit.
Tools, lights, and a battery charger usually total under 5 kW running. A 6,500-7,500 W unit, or two paralleled inverters, covers most light contractor work.
An inverter generator under 2,500 W handles a small fridge, lights, and phone charging quietly enough for shared-site use.
Parallel-ready 2,000-2,200 W inverters connect with a cable kit to reach 4,000+ W, useful when you need more power occasionally but want the lighter units the rest of the time.
Common mistakes
Refrigerators and pumps need 2-3× running watts to start. Without surge headroom the generator's breaker trips repeatedly or the engine bogs down.
Open-frame portables emit carbon monoxide. Operate at least 20 ft from any window, door, or vent regardless of how 'temporary' the use is.
A light-duty household cord run at 20+ amps causes voltage drop and heat buildup. Use a 12-gauge or heavier outdoor-rated cord matched to the load and kept under 100 feet.
Plugging a generator into a household outlet without an interlock kit or transfer switch can send power back through the meter and electrocute utility workers on the line.
Frequently asked questions
What size portable generator do I need for a power outage?+
Most homes cover the essentials, fridge, sump pump, furnace blower, and some lights, with 4,000-5,000 running watts. Add up your specific appliances with the calculator above since kitchens with electric well pumps or window AC units need more.
Is an inverter generator worth the extra cost?+
Yes, if you'll power electronics, a laptop, a CPAP machine, or a TV. Inverter generators produce cleaner, more stable power than conventional models and also run quieter and lighter, though usually at a smaller maximum wattage.
Can I run two portable generators together for more power?+
Yes, but only if both units are explicitly parallel-capable and you use the matched cable kit from the same manufacturer. Two 2,200 W parallel-ready inverters can combine for roughly 4,000-4,400 W of usable output.
Do I need a transfer switch for a portable generator?+
Yes, for powering any home circuit. Backfeeding a generator into a wall outlet without an interlock kit or transfer switch can energize utility lines and electrocute a lineworker, and it's illegal in most jurisdictions.
How many watts does a window air conditioner need from a portable generator?+
A 10,000 BTU window unit needs about 1,200 W running and up to 3,600 W surge at startup, so it needs at least a 3,500-4,000 W generator on its own before adding anything else.
Is it safe to run a portable generator in a garage with the door open?+
No, never. Carbon monoxide from a running generator can build up in an attached garage even with the door open. Run it outdoors, at least 20 feet from any window, door, or vent.
How long does gasoline last in a stored portable generator?+
About 1-2 months untreated before it starts to degrade and gum up the carburetor. Adding fuel stabilizer extends stored gasoline to roughly 12 months, and propane or dual-fuel units avoid the issue entirely.
What gauge extension cord should I use with a portable generator?+
Use a 12-gauge outdoor-rated cord for loads up to 20 A and 10-gauge for up to 30 A, and keep the run under 100 feet. Thinner household cords cause voltage drop that can damage sensitive electronics.
Can a 3,500 W generator run a refrigerator and a microwave at the same time?+
Usually yes. A fridge draws about 700 W running (up to 2,200 W surge) and a microwave draws 1,000-1,200 W with no meaningful surge, so the combined running load of roughly 1,700-1,900 W fits comfortably under 3,500 W.
How much bigger should a portable generator be than my calculated running watts?+
Add at least 15-20% headroom above your calculated running total, and always buy to the surge number of your largest single motor, not just the running total.