The complete 2025 guide to generator sizing — for power outages, camping, job sites, and whole-home backup. Know the math, know your options, make the right buy.
Buying a generator that's too small means it trips the breaker the moment your refrigerator compressor kicks on. Too large, and you've spent $1,000 more than you needed to, and the engine runs inefficiently at low loads. Getting the size right matters — and it's easier than you think once you understand two numbers: running watts and starting watts.
💡 Skip the reading, get the answer: Use our free generator size calculator to check off your appliances and get an instant recommendation. Come back here if you want to understand the why behind the numbers.
Every generator has two power ratings printed on its label or in its spec sheet:
The continuous power the generator produces. This is the number you'll see most prominently advertised. A "7,500W generator" produces 7,500 watts continuously — theoretically. In practice, run it at 80% capacity (6,000W) for extended periods to avoid overloading and overheating.
The brief burst of extra power a generator can supply for 2–3 seconds when a motor starts. Most generators can produce 20–50% above their rated output for these short surges. This number is critical because motors demand much more power to start than to run.
The rule: Your generator's starting watts must be greater than your home's peak starting demand, and your generator's running watts must exceed your total continuous load.
Resistive loads — light bulbs, toasters, electric heaters — draw exactly the wattage listed on the box. They have no starting surge. But anything with a motor — refrigerators, AC units, well pumps, sump pumps, circular saws, air compressors — needs a surge of power to overcome inertia and get spinning. This surge can be 2× to 4× the running wattage, and it happens every single time the motor starts.
A 10,000 BTU central AC unit running at 1,500 watts might demand 4,500 watts to start. That 3,000W surge difference is the gap that kills most undersized generators.
There are three steps. The full walkthrough with a worked example lives on our how to calculate generator size page, but here's the condensed version:
Be realistic. You probably don't need the electric dryer running during a power outage, but the refrigerator, sump pump, some lights, and phone chargers are reasonable. Use our wattage chart to find the watts for each.
Sum the running watts for every appliance on your list. This is your continuous load — the minimum wattage your generator must sustain.
Find the appliance with the biggest difference between starting and running watts. Add that difference to your total running watts. The result is your peak demand — the wattage your generator must reach during starting surges. Your generator's starting/surge watts must equal or exceed this number.
Round up to the nearest standard size: Common generator sizes are 2,000W, 3,500W, 4,000W, 5,000W, 6,500W, 7,500W, 10,000W, and 12,000W. Always go one size up from your calculated need for a comfortable safety margin.
Run on gasoline (or dual-fuel gas/propane). Range from 1,000W to 17,000W. Must be started manually and kept outdoors or in a well-ventilated area. Connect appliances via extension cords or a transfer switch. Cost: $300–$3,000.
Permanently installed, connected to your natural gas or propane line. Start automatically within 10–30 seconds of a power outage — you don't even have to be home. Range from 7,000W to 20,000W+. Require professional installation. Cost: $3,000–$15,000 installed.
Most homeowners are well-served by a quality portable generator in the 5,000–10,000W range, connected to essential circuits via a manual transfer switch. This setup runs $800–$2,500 all-in and handles 90% of real-world outage needs.
To keep the refrigerator, some lights, phone chargers, the TV, and a window AC running during a summer outage, most homes need 3,500–5,000 watts. Add a well pump or sump pump and you're looking at 5,000–7,500 watts of starting capacity.
Storm prep means prioritizing: refrigerator, sump pump (critical if flooding is a risk), furnace fan for winter storms, a few lights, and device charging. The sump pump or furnace fan is usually the largest motor surge. Most storm-prep setups land at 4,000–6,500 watts.
Camping demands are light: a coffee maker (1,000W), a few lights (100W), phone chargers (30W), and maybe a portable AC (1,000–1,500W). A 2,000–3,000W inverter generator covers everything and is quiet enough not to anger your campsite neighbors. Honda EU2200i and WEN 56200i are the classic choices here.
Job site demands are tool-dependent. A circular saw (1,400W running / 2,300W starting), a drill (600W / 1,000W), and some LED work lights (200W) together need about 4,000–5,000 starting watts. Add a table saw and you may need 7,500–10,000 watts of starting capacity.
Running everything — central AC, electric water heater, range, washer/dryer — is a different story. The electric dryer alone draws 5,400 running watts and the electric water heater draws 4,000W. Running both simultaneously would need 12,000+ watts. In practice, whole-home backup with these heavy loads means a standby generator or accepting that some 240V loads simply stay off.
| Generator Size | Best For | Typical Appliances Supported |
|---|---|---|
| 2,000 W | Camping, RV, small backup | Fridge, lights, phone chargers, small TV, CPAP |
| 3,500 W | Moderate home backup | + Window AC (5,000 BTU), sump pump (1/3 HP) |
| 5,000 W | Most home outage needs | + Window AC (10,000 BTU), well pump, furnace fan |
| 7,500 W | Full home essentials + small HVAC | + Central AC (10,000 BTU), multiple motor loads |
| 10,000 W | Large home, central AC | + Central AC (2 ton), washer, multiple heavy loads |
| 12,000 W+ | Large home or whole-home | Nearly everything except electric dryer and full range |
| Standby (22 kW) | True whole-home backup | Everything, automatic, on natural gas or propane |
Yes, but you need a big generator. A 10,000 BTU central AC unit needs up to 4,500 starting watts, and a 2-ton (24,000 BTU) unit needs up to 7,000 starting watts. The generator's surge capacity must meet or exceed that number. A 7,500W generator with a 9,375W surge rating can handle a 2-ton AC, but barely — and that's before you add anything else. Add 1,000–2,000W of headroom for other loads.
Running watts (also called rated watts) is the continuous power output the generator can sustain. Starting watts (also called surge watts or peak watts) is the brief burst of extra power it can produce for 2–3 seconds when a motor starts. Most generators can surge 20–50% above their rated output. When the spec sheet says "7,500W running / 9,375W starting," that means 9,375W is only available for those brief startup moments.
It depends entirely on what "whole house" means to you. If you want to run central AC, a refrigerator, lights, electronics, and the microwave — but skip the electric dryer and range — a 10,000–12,000W generator gets you there. If you want every circuit powered including electric heating and cooking, you're looking at standby generators in the 20–22 kW range. The average American home's whole-house portable backup typically lands between 7,500W and 12,000W.
For essentials during an outage — refrigerator, lights, phone chargers, a small TV, and a window AC (5,000 BTU) — a 3,500W generator does the job for most homes. It won't power a well pump, sump pump, or central AC unit. If any of those are on your list, step up to at least 5,000W.
Choose an inverter generator if you're powering sensitive electronics (laptops, TVs, medical devices, CPAP machines). Inverter generators produce cleaner power (lower THD) that won't damage electronics, run quieter, and are more fuel-efficient at partial loads. Choose a conventional open-frame generator if you need maximum wattage per dollar for motor loads, tools, and appliances — and don't mind the noise.
NEVER operate a generator indoors, in a garage, or near windows or vents — even with the garage door open. Carbon monoxide is odorless and kills within minutes. The CPSC recommends at least 20 feet from any door, window, or vent, with the exhaust pointing away from the house. Use a quality heavy-gauge extension cord (14 AWG for short runs, 12 AWG or 10 AWG for longer runs) rated for outdoor use.
Yes — strongly recommended. A transfer switch (or interlock kit) disconnects your home from the utility grid before connecting the generator, preventing dangerous backfeed that could electrocute utility workers. It also lets you power hardwired appliances (furnace, well pump, etc.) without running extension cords everywhere. A licensed electrician can install a manual transfer switch for $300–$800, making your generator dramatically more useful and safe.
Most portable generators are designed to run 8–12 hours on a tank of gas before refueling, and they can run continuously for as long as they have fuel — provided the oil is changed on schedule (typically every 50–100 hours) and the unit isn't run above 80% load for extended periods. Standby generators connected to a natural gas line can run for days or weeks with periodic maintenance checks. Always follow the manufacturer's service intervals.
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Running and starting watts for 40+ appliances, organized by category.
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