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What Size Generator Do I Need?
Pick what you want to keep running during an outage. You'll get the generator size, how many hours a battery power station would last, and the models that can actually handle your list.
- Generators and power stations
- Counts motor surge + 20% margin
- Runs in your browser, nothing sent
| Appliance | Qty | Run W | Start W | Hrs/day | Remove |
|---|
More options: altitude
Matched to your list
Models that can run your list
The smallest models whose manufacturer specs cover your running watts and your biggest motor start. Smaller usually means cheaper, lighter and quieter.
Power station
Silent, safe indoors, no fuel. Best for fridges, CPAP and electronics.
Inverter generator
Quiet, clean power for electronics. Runs for days with fuel. Outdoors only.
Portable generator
The most watts per dollar, often dual fuel and 240 V. Loud. Outdoors only.
We may earn a commission if you buy through these links, at no extra cost to you. It never changes which models appear: they're picked only by the math above. How we pick.
How to size a generator in 4 steps
This is the same method generator manufacturers use, and the one the planner above follows:
- List what must run at the same time. Not everything in the house, only what you'd turn on together during an outage.
- Add up the running watts. You'll find them on each appliance's label (or multiply volts × amps). Our generator wattage chart has typical values for 50+ appliances.
- Add the biggest starting surge, only once. Motors (fridges, pumps, AC units) need a spike for a second or two to start. They rarely start at the exact same moment, so you add only the largest extra surge.
- Add a 20% safety margin. It covers older appliances, heat, altitude and that one thing you'll want to plug in later. Running a generator near 100% all the time also shortens its life.
Generator running watts = (total running watts) × 1.2 · Starting watts ≥ total running watts + biggest extra surge
Example: a modern fridge (150 W running, 1,200 W starting), a 1/2 HP sump pump (1,050 W, 2,150 W starting), LED lights (100 W) and Wi-Fi (20 W) add up to 1,320 running watts. The biggest extra surge is the sump pump: 2,150 − 1,050 = 1,100 W, so the peak is 2,420 W. You need a generator with at least 1,600 running watts (1,320 × 1.2, rounded up) and 2,500 starting watts.
Common setups and the size they need
Calculated with typical appliance values. Your own appliances may use more or less, so use the planner with your labels for a real answer.
| Setup | Generator (running / starting) | Energy per day |
|---|---|---|
| Just the essentials Refrigerator (modern, Energy Star), LED lights (10 bulbs), Wi-Fi router + modem, Phone chargers (2), TV (50" LED) | 500 W / 1,500 W | 3,100 Wh |
| Fridge + CPAP Refrigerator (modern, Energy Star), CPAP with heated humidifier, Wi-Fi router + modem, Phone chargers (2), LED lights (10 bulbs) | 500 W / 1,500 W | 3,320 Wh |
| Storm prep (sump pump) Refrigerator (modern, Energy Star), Chest freezer, Sump pump (1/2 HP), LED lights (10 bulbs), Wi-Fi router + modem, Phone chargers (2) | 1,800 W / 2,600 W | 6,080 Wh |
| Most of the house Refrigerator (modern, Energy Star), Chest freezer, Furnace blower fan (gas furnace, 1/2 HP), Well pump (1/2 HP), Microwave (1,000 W cooking power), 2× LED lights (10 bulbs), TV (50" LED), Wi-Fi router + modem, Washing machine | 6,200 W / 6,600 W · 240 V | 19,820 Wh |
| Whole house + central AC Refrigerator (modern, Energy Star), Chest freezer, Central AC (3 ton), Well pump (1/2 HP), Microwave (1,000 W cooking power), 3× LED lights (10 bulbs), 2× TV (50" LED), Wi-Fi router + modem, Washing machine, Gas dryer (motor only) | 10,400 W / 15,000 W · 240 V | 32,120 Wh |
| RV (30 amp) RV air conditioner (13,500 BTU), RV fridge (electric mode), Microwave (1,000 W cooking power), RV converter / battery charger, TV (50" LED) | 4,200 W / 5,000 W | 11,990 Wh |
| RV (50 amp, 2 ACs) 2× RV air conditioner (15,000 BTU), RV fridge (electric mode), Microwave (1,000 W cooking power), RV converter / battery charger, TV (50" LED), Coffee maker | 7,500 W / 8,200 W | 20,450 Wh |
Generator or power station?
The planner shows both, because the right answer depends on your outages:
- Power station (battery): silent, no fuel, no fumes, safe indoors. Recharges from the wall, solar panels or a car. Best for short outages (hours to a day or two) and for a fridge, CPAP, Wi-Fi and electronics. The energy per day in your result tells you how big a battery you need: a 1,000 Wh unit delivers roughly 850 Wh to your appliances after inverter losses.
- Inverter generator: quieter and with clean power that's safe for electronics. Great up to about 4,000 watts. Runs as long as you keep adding fuel.
- Portable (open-frame) generator: the most watts per dollar, often dual fuel (gasoline or propane) and with 240 V for well pumps and central AC. Loud, and must be connected to your house through a transfer switch or interlock installed by an electrician.
- Never run a generator indoors, in a garage (even with the door open), a basement or near windows and vents. Keep it at least 20 feet from the house with the exhaust pointing away.
- Install battery-powered carbon monoxide alarms. Prefer generators with an automatic CO shutoff.
- Never plug a generator into a wall outlet ("backfeeding"). It can kill utility workers and neighbors. Use a transfer switch or interlock installed by a licensed electrician.
- Let the generator cool before refueling, and store fuel away from the house.
Frequently asked questions
What size generator do I need to run a house?
For the essentials (refrigerator, freezer, lights, Wi-Fi, a sump pump and phone charging) most homes need about 3,000 to 5,000 running watts. Adding a well pump, a furnace blower or a window AC usually pushes it to 6,000 to 8,000 watts. Running central air, an electric water heater or an electric dryer at the same time needs 10,000 watts or more, which is usually a job for a home standby generator.
How do I calculate what size generator I need?
Add the running watts of everything you want to run at the same time. Then find the appliance with the biggest starting surge (usually a motor: fridge, pump or AC) and add only its extra starting watts. Finally, add a 20% safety margin. The generator's running rating should cover the total with margin, and its starting rating should cover the surge.
What is the difference between running watts and starting watts?
Running (rated) watts are what an appliance uses while it works. Starting (surge) watts are the short spike a motor needs for a second or two to start, often two to three times the running watts. Generators list both numbers, and you need to cover both.
Will a 2,000-watt generator run a refrigerator?
Usually yes. A modern refrigerator runs on about 150 to 200 watts and needs around 1,200 watts for a moment to start, so a 2,000-watt inverter generator can run it with room for lights and phone charging. An older refrigerator can need 2,000 watts or more to start.
Is a portable power station better than a generator?
It depends on how long your outages last. A power station is silent, has no fumes and can be used indoors, which is ideal for a fridge, CPAP or electronics during outages of a day or less. A gas generator can run for days as long as you have fuel and gives more watts per dollar, but it must run outdoors, far from windows.
Do I need a 240-volt generator?
Only if you want to run 240-volt appliances: well pumps, central air, electric dryers, electric water heaters or an electric range. Most portable generators above about 7,000 watts have a 240-volt outlet. To power these appliances through your home's wiring you need a transfer switch or interlock installed by a licensed electrician.
Sources
- U.S. Consumer Product Safety Commission: Portable generator safety
- Ready.gov: Power outages
- Appliance wattage: typical values from generator manufacturers' wattage guides (Champion, Generac, Honda) and ENERGY STAR product data. See how we pick products and data.
Estimates only. Appliances vary: always check the label on yours and your generator's manual. Product specs are from manufacturers and were last checked October 21, 2026.