What Size Generator Do I Need? Wattage Chart and Calculator (2026)
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Researched by Moses Washika · Updated November 2026
- Add up the running watts of everything you want on at once, then add the largest starting surge from a motor like a fridge or well pump.
- Most homes that want the fridge, sump pump, furnace fan, lights and internet need about 4,000 to 5,000 starting watts. Add a well pump and you are near 6,000 to 7,500.
- Running central air or an electric water heater usually means a large portable or a home standby unit.
- Use the calculator below and buy a size 20% above your peak.
Buying a generator that is too small means tripped breakers and a warm fridge. Buying one that is too big means paying for fuel and weight you never use.
This guide shows you how to size a generator in three steps, gives typical wattages for common appliances, and matches your number to the right type of generator.
Generator size calculator
Generator size calculator
Tick what you want to run at the same time. We add running watts, then the biggest starting surge, and suggest a size with a safety margin.
How to size a generator in 3 steps
- List what must run at the same time. In an outage that is usually the fridge, freezer, sump or well pump, furnace fan, some lights and the internet.
- Add the running watts. This is the power each item uses once it is on. Look for it on the label or in the manual.
- Add the biggest starting surge. Motors pull two to three times their running watts for a second or two when they start. You only add the largest one, because motors rarely start at the exact same moment.
Then add about 20% so the generator is not working at its limit all night. That margin also covers a cold start or an item you forgot.
A worked example
Say you want a fridge (700 W), sump pump (800 W), furnace fan (800 W), lights (100 W) and TV with router (250 W). That is 2,650 running watts.
The biggest surge is the furnace fan, which can jump from 800 to 2,350 W, adding 1,550 W. Your peak is 4,200 W, so a generator of about 5,000 W gives you a safe margin.
Appliance wattage chart
These are typical numbers used in manufacturer sizing guides. Your own appliances may differ, so check their labels for the most accurate figure.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Chest freezer | 500 | 1,500 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Well pump (1/2 HP) | 1,000 | 2,100 |
| Gas furnace fan (1/2 HP) | 800 | 2,350 |
| LED lights (10 bulbs) | 100 | Same |
| TV and Wi-Fi router | 250 | Same |
| Laptop and phone chargers | 150 | Same |
| Microwave | 1,000 | Same |
| Coffee maker | 1,000 | Same |
| Space heater | 1,500 | Same |
| Window AC (10,000 BTU) | 1,200 | 1,800 |
| Washing machine | 1,150 | 2,250 |
| CPAP machine | 60 | Same |
| Electric water heater | 4,000 | Same |
| Circular saw | 1,400 | 2,300 |
| Central AC (3 ton) | 3,800 | 11,000 |
| Electric range (one burner) | 1,500 | Same |
Typical values from manufacturer wattage guides, including Generac and Champion.
What each generator size can run
| Generator size | What it can usually run | Good for |
|---|---|---|
| 2,000 to 2,500 W | Fridge, lights, phones, TV, CPAP | Camping, tailgates, light home backup |
| 3,500 to 4,500 W | Fridge, freezer, sump pump, lights, internet | Most short outages in a small home |
| 5,500 to 7,500 W | Above plus furnace fan or well pump, microwave | Most homes with a transfer switch |
| 9,000 to 12,500 W | Most of the house, one window AC, well pump | Larger homes, long outages |
| 14 kW and up (standby) | Whole house, including central AC | Areas with frequent long outages |
Inverter, portable or standby?
Inverter generators
Inverter generators make clean, stable power that is safe for laptops, TVs and medical devices. They are quieter and more fuel efficient, but most top out around 4,000 watts.
Pick one if you need under 4,000 W or care most about noise.
See inverter generators on Amazon →
Conventional portable generators
Open frame portables give you the most watts for the money, often 5,000 to 12,000 W. They are louder and heavier, and many are dual fuel so you can run them on propane.
This is the usual pick for backing up a home through a transfer switch. Our guide to the best portable generators for home backup compares popular models.
See portable generators on Amazon →
Home standby generators
Standby units are installed outside by a pro and start on their own when the power goes out. They run on natural gas or propane and can power central air.
They cost far more once you add installation, so they make sense if outages are frequent or long where you live.
Connect your generator safely
Never run a generator indoors, in a garage or near open windows. The CDC warns that carbon monoxide from generators can kill within minutes.
- Keep the generator at least 20 feet from the house with the exhaust pointed away.
- Use a carbon monoxide alarm inside, with fresh batteries.
- Never plug a generator into a wall outlet (“backfeeding”). It can kill utility workers.
- Use a transfer switch or interlock installed by an electrician.
- Use heavy gauge cords, like the ones in our guide to generator extension cords.
- Check your hookup with a voltage tester before you touch any wiring.
Safety source: CDC carbon monoxide poisoning guidance.
Frequently asked questions
What size generator do I need to run a house?
Most homes need 5,000 to 7,500 watts to run the essentials: fridge, freezer, sump or well pump, furnace fan, lights and internet. To run the whole house with central air, you usually need a 14 kW or larger standby generator.
Will a 2,000 watt generator run a refrigerator?
Usually yes, on its own. A typical fridge runs at about 700 watts but can surge to 2,000 watts or more when the compressor starts, so a 2,000 to 2,500 watt inverter is the smallest safe choice.
What size generator do I need for a well pump?
A 1/2 HP well pump needs about 1,000 running watts and 2,100 starting watts. With a fridge and lights on too, plan for at least 5,000 watts, and check if your pump runs on 240 volts.
How many watts does a furnace need?
A gas furnace blower with a 1/2 HP motor uses about 800 watts running and up to 2,350 watts to start. The furnace usually needs a clean connection through a transfer switch or interlock.
Is it better to oversize a generator?
A margin of about 20% is good. Going much bigger costs more, burns more fuel and makes the generator heavier to move.
