Electrical & Lighting · Calculator
Generator Wattage Calculator
This generator wattage calculator adds up the running and starting watts of everything you want to power at once, then tells you what size generator to buy. Check off your appliances, set quantities, and add your own items from their nameplates.
What do you want to run at once?
Check each item and set how many. Figures are typical values from generator maker wattage charts. Use the nameplate on your own appliance when you have it.
How the generator wattage calculator sizes your generator
Running watts are what your appliances use once they are going. Add them up for everything you want on at the same time. That total has to fit under the generator's running (rated) watts, with some room to spare.
Starting watts are the short surge a motor needs to spin up. Motors on a generator rarely start at the exact same moment, so you add only the single largest surge to your running total. That is the method generator makers use on their wattage worksheets.
Then add headroom. A generator run flat out is loud, thirsty and stressed. The calculator adds 20 percent to running watts and 10 percent to starting watts, and it assumes a generator's peak rating is about 1.25 times its running rating. That is why a big surge, like a 1 hp air compressor, can push you into a bigger class than its running watts suggest.
Generator wattage chart for common appliances
These are the same figures the calculator uses. They are typical values compiled from generator manufacturer wattage charts. Your own appliance can differ, so read its label when you can: watts = volts x amps.
| Appliance | Group | Running watts | Starting watts | Notes |
|---|---|---|---|---|
| Refrigerator | Essentials | 400 | 1,600 | Energy Star era fridge; older units run up to 700 W |
| Chest freezer | Essentials | 500 | 2,000 | |
| Sump pump, 1/3 hp | Essentials | 800 | 2,100 | |
| Sump pump, 1/2 hp | Essentials | 1,050 | 3,250 | |
| Well pump, 1/2 hp | Essentials | 1,000 | 3,100 | Many well pumps are 240 V: check the motor label |
| Furnace fan, 1/2 hp | Essentials | 800 | 3,150 | Gas or oil furnace blower |
| LED lights (per 10 bulbs) | Essentials | 100 | Same | |
| Wi-Fi router + modem | Essentials | 30 | Same | |
| TV (LED, 50 in class) | Essentials | 150 | Same | |
| Laptop | Essentials | 75 | Same | |
| Phone chargers (2) | Essentials | 25 | Same | |
| CPAP machine | Essentials | 60 | Same | Up to about 120 W with a heated humidifier |
| Window AC, 10,000 BTU | Comfort and kitchen | 1,200 | 3,600 | |
| Space heater, 1,500 W | Comfort and kitchen | 1,500 | Same | |
| Microwave (1,000 W cooking) | Comfort and kitchen | 1,500 | Same | A 1,000 W microwave draws about 1,500 W from the outlet |
| Coffee maker | Comfort and kitchen | 1,000 | Same | |
| Electric water heater | Comfort and kitchen | 4,500 | Same | 240 V. |
| Central AC, 3 ton | Comfort and kitchen | 3,500 | 9,500 | 240 V. A soft-start kit can cut the starting surge by half or more |
| Circular saw, 7-1/4 in | Tools, RV and jobsite | 1,400 | 3,700 | |
| Miter saw, 10 in | Tools, RV and jobsite | 1,800 | 3,600 | |
| Air compressor, 1 hp | Tools, RV and jobsite | 1,600 | 6,100 | |
| RV air conditioner, 13,500 BTU | Tools, RV and jobsite | 1,600 | 3,500 | A soft-start kit lets many 3,000 W inverters start one |
Generator sizes: what each class can run
Portable inverters (1,000 to about 3,500 W) are quiet, light and make clean power that is safe for laptops, TVs and CPAP machines. Two small inverters can often be paralleled. Our best inverter generator picks cover the sizes that make sense for home backup and RVs.
Open-frame portables (about 3,500 to 9,500 W) give you the most watts per dollar. The larger ones have a 240 V outlet that feeds a transfer switch, so you can run a well pump or furnace through the house wiring.
Home standby units are permanently installed, run on natural gas or propane, and start themselves when the power drops. If your list includes central air, see our generator sizing guide before you shop.
| Class | Size | Typical loads |
|---|---|---|
| Portable inverter | Up to about 3,500 running watts | Fridge, lights, Wi-Fi, TV, CPAP, a sump pump, or an RV air conditioner. Quiet and safe for electronics. |
| Portable (open frame) | About 3,500 to 9,500 running watts | Most essentials at once: fridge, freezer, sump or well pump, furnace fan and a window AC. Larger models have a 240 V outlet for a transfer switch. |
| Home standby | Above about 9,500 running watts | Central air, electric water heaters, ranges and most of the house. Permanently installed with an automatic transfer switch. |
Transfer switches, cords and safety
Carbon monoxide kills. Never run a generator indoors.
- Use a transfer switch or interlock to power hardwired loads like a furnace, well pump or anything 240 V. Never backfeed through a wall outlet.
- Run heavy cords. A 50 ft run to a sump pump or fridge should be at least 14 AWG, and 12 AWG for bigger loads. Check yours with the extension cord gauge calculator.
- Stagger your starts. Turn on the biggest motor first, then add loads one at a time. The surge math only works if motors do not all start together.
- Protect the sump pump first. In a storm it is the load that saves your basement. Know its horsepower (our sump pump guide explains how to tell) before you size.
- Sizing a window AC? Find the right BTU for the room with the BTU calculator, then add its watts here.
How much fuel will it use?
Fuel use depends on load, not on the size printed on the side. Check the spec sheet for run time at 25 and 50 percent load. As a rough guide, a 7,500 W open-frame portable burns about 0.7 gallon of gasoline per hour at half load, so a 24 hour outage can take 15 gallons or more.
Inverters throttle the engine down at light loads, which is why a small one can run a fridge and lights all night on a gallon or two. Store fuel with stabilizer and rotate it, and run your generator for a few minutes every month so it starts when you need it. Plug your own list into the generator wattage calculator above before an outage, not during one.
Frequently asked questions
What size generator do I need to run a house?
Most homes need 5,000 to 7,500 running watts to cover the essentials: refrigerator, freezer, sump pump, furnace fan, lights and electronics. Add central air or an electric water heater and you are into 10,000 watts and up, which is home standby territory.
How many watts does a refrigerator need on a generator?
A modern refrigerator runs on about 150 to 400 watts but needs roughly 1,200 to 1,600 watts for a few seconds when the compressor starts. Older fridges can run closer to 700 watts, so check the label.
Can a 3,500 watt generator run a sump pump and a fridge?
Usually yes, as long as they do not start at the same moment. A 1/2 hp sump pump and a fridge run on about 1,450 watts together, and the pump surge of about 2,200 watts brings the peak to roughly 3,650 watts, inside the surge rating of most 3,500 watt generators.
What is the difference between running watts and starting watts?
Running watts are what an appliance uses continuously. Starting watts are the brief surge a motor needs to get moving, often two to four times its running watts. Only motors have a big surge: heaters, lights and electronics start at their running wattage.
Can I plug a generator into a wall outlet?
No. Backfeeding through an outlet with a double male cord can energize the utility lines and electrocute line workers, and it bypasses your breakers. Use extension cords straight from the generator, or have an electrician install a transfer switch or interlock.
Will an inverter generator run central air?
Not a typical 3 ton central AC. It needs about 3,500 running watts, a starting surge several times higher and a 240 volt connection. A large portable with a soft-start kit on the condenser can do it, but most people use a home standby unit.
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