What size generator do you need to power your home during an outage? This free generator size calculator adds up the running watts and startup surge of every appliance you pick, then recommends the right generator size and type, from a small inverter generator to a whole-house standby unit. Pick a preset or choose your own appliances below.
Generator Size Calculator
Add your appliances to find the right generator size for power outages.
Wattages are typical values. Check the label on your own appliances for exact numbers. Always use a transfer switch or interlock, never back-feed through an outlet, and run generators outdoors at least 20 ft from the house.
How to Use the Generator Size Calculator
- Click Essentials for a typical outage setup or Whole house for full backup, then fine-tune.
- Use + and − to set how many of each appliance you want to run at the same time.
- Read the recommended size. It covers your total running watts plus the single biggest motor surge, with at least 20% headroom.
Running Watts vs Starting Watts
Running watts are what an appliance uses while it works. Starting watts (surge watts) are the short burst a motor needs to start, often 2 to 3 times higher. Refrigerators, sump pumps, well pumps, furnace fans and air conditioners all have big startup surges.
Generator watts needed = total running watts + the largest single starting surge. You only add one surge because motors rarely start at the same second.
Example: a fridge (700 W), sump pump (1,050 W), furnace fan (800 W), lights and router (220 W) total 2,770 running watts. The furnace fan has the biggest extra surge (1,550 W), so peak demand is 4,320 W. A 4,500 W generator covers it.
Generator Wattage Chart for Common Appliances
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Sump pump (1/2 HP) | 1,050 | 2,200 |
| Well pump (1/2 HP) | 1,000 | 2,100 |
| Furnace fan (1/2 HP) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3 ton) | 3,800 | 9,000 |
| Microwave | 1,000 | – |
| Electric water heater | 4,000 | – |
| Washing machine | 1,150 | 2,250 |
| Electric dryer | 5,400 | 6,750 |
Which Type of Generator Should You Buy?
| Size | Best type | What it can run |
|---|---|---|
| 2,000 – 3,500 W | Inverter generator | Fridge, lights, Wi-Fi, phones, CPAP. Quiet and safe for electronics. |
| 4,500 – 9,500 W | Portable + transfer switch | Essentials plus sump or well pump, furnace, microwave, window AC. |
| 12,000 – 26,000 W | Home standby | Most or all of the home, including central AC, starting automatically. |
Generator Safety Tips
- Never run a generator indoors, in a garage or near windows. Keep it at least 20 ft from the house, exhaust pointing away.
- Install battery-powered carbon monoxide alarms on every floor.
- Connect through a transfer switch or interlock installed by a licensed electrician.
- Store fuel safely and add stabilizer if it will sit for more than a month.
- Keep your Wi-Fi and security cameras on a small UPS so they stay online while the generator starts.
Frequently Asked Questions
What size generator do I need to run a house?
Most homes need 5,000 to 7,500 watts to run essentials like the fridge, freezer, sump pump, furnace fan, lights and Wi-Fi. Powering the whole house with central AC usually takes a 14,000 to 22,000 watt standby generator.
Will a 3,500 watt generator run a refrigerator?
Yes. A typical refrigerator needs about 700 running watts and 2,200 starting watts, so a 3,500 W generator can run it along with lights, a router and phone chargers.
What size generator do I need for central air?
A 3-ton central AC uses about 3,800 running watts and up to 9,000 starting watts. Together with other essentials, plan for at least a 12,000 to 14,000 W generator, or add a soft-start kit to use a smaller one.
Can I run a generator without a transfer switch?
Only by plugging appliances directly into the generator with heavy-duty extension cords. Never plug a generator into a wall outlet; back-feeding can electrocute utility workers and damage your home’s wiring.
Is an inverter generator better?
Inverter generators produce clean, stable power that is safe for laptops and TVs, and they are much quieter and more fuel efficient. They cost more per watt and are usually limited to about 7,000 W.
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