Generator Wattage Calculator: What Size Generator Do I Need?

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Quick answer: To size a generator, add up the running watts of everything that stays on, then add the biggest extra demand you expect, such as one motor starting or a microwave switching on. That total is the starting (surge) watts to look for. For running watts, divide what you run at the same time by 0.8, so the generator is not working above 80% of its rating for hours. A typical home backup of a fridge, sump pump, furnace fan, lights, TV and microwave comes to about 5,700 W. Our generator wattage calculator below does the math for you.

Disclaimer: This calculator gives estimates for planning only. It is not electrical or safety advice. Use the wattage labels on your own appliances, follow the generator maker’s instructions, and have a licensed electrician install any transfer switch or interlock kit. Never connect a generator to house wiring any other way.

Generator Wattage Calculator

Tick the appliances you want to power, or start with a preset for home backup, an off-grid cabin, an RV or a workshop. Change the quantity and watts to match your labels, and tick “Occasional” for things you only use for a few minutes. The result updates as you go.

Running Watts vs Starting Watts

Every generator lists two numbers. Running watts (also called rated watts) is what it can supply hour after hour. Starting watts (surge or peak watts) is the short burst it can give for a second or two while a motor gets going.

Anything with a motor or compressor needs that burst. A standard fridge runs on about 700 W but needs around 2,200 W to start, according to Generac’s guide to starting watts. Lights, TVs, chargers, coffee makers and heaters need about the same watts to start as to run.

How to Calculate the Generator Size You Need

The calculator follows the sizing method that generator makers recommend, including Generac and Champion Power Equipment: add the running watts of what you run together, then add the biggest starting demand. You can do it by hand in four steps:

1. List what stays on. Add the running watts of everything that runs for long stretches: fridge, freezer, sump or well pump, furnace fan, lights, TV, router and chargers. This is your continuous need.
2. Find the biggest extra demand. This is either the full starting watts of your largest motor, or the biggest item you only use now and then, such as a microwave or hair dryer. Use whichever is larger. This is your occasional need.
3. Add them together. Continuous need plus occasional need is the starting watts your generator should have.
4. Leave headroom on running watts. Add the running watts of your largest occasional item to the continuous need, then divide by 0.8. This is the 20% cushion Champion recommends so the generator does not run flat out for hours. The calculator lets you change this load limit.

Why not just add every starting watt together? Motors do not all start at the same moment, so that total overstates what you need. The calculator still shows the “everything at once” figure, so you can see the worst case.

Worked Examples

Home backup essentials. A fridge (700 W), 1/2 HP sump pump (1,050 W), furnace fan (700 W), six 60 W bulbs, a TV and two phone chargers come to 3,050 W running. The biggest extra demand is the fridge starting at 2,200 W, so you need at least 5,250 starting watts. Add a 1,000 W microwave (1,500 W) for 4,550 W running at once; divided by 0.8, that gives about 5,700 W. Look for a generator of about 5,700 running watts.

Off-grid cabin. An Energy Star fridge, a 1/2 HP well pump, four lights, a laptop, chargers, a fan and a coffee maker need about 1,800 W running. The well pump adds 2,000 W to start, so the cabin needs about 3,800 starting watts and 3,000 running watts. A 3,500 to 4,000 W generator covers it, and a smaller one can work if you start the pump while little else is on.

Generator Wattage Chart

Typical running and starting watts for common appliances. The figures are typical values from generator makers’ published wattage charts. The last column links to a typical model of each appliance on Amazon, so you can compare wattages. Your appliance’s label or manual is always more accurate, so use it when you have it.

How to Use a Smaller Generator

• Start the biggest motor first, on its own, then add other loads one at a time.
• Switch off occasional items, like the microwave or coffee maker, while a pump or fridge is starting.
• Pick efficient appliances. An Energy Star fridge can start on about half the watts of an older model.
• Ask an HVAC technician about a soft starter for an air conditioner. It can cut the starting surge a lot.
• For small loads, an inverter and battery may be enough. See our guide to using your car and a power inverter for backup.

Generator Safety Basics

Carbon monoxide from generators kills people every year. The U.S. Consumer Product Safety Commission and the American Red Cross give the same core advice:

• Run a portable generator outside only, at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents.
• Never run it in a home, garage, basement, crawlspace, shed, carport or on a porch, even with the doors open.
• Install battery-powered carbon monoxide alarms on every level of your home. If one sounds, get outside and call 911.
• Never plug a generator into a wall outlet. This is called backfeeding and can kill utility workers and neighbors. Use a transfer switch or interlock kit installed by a licensed electrician.
• Use heavy-duty, outdoor-rated cords that are rated for the load, and keep the generator dry.
• Turn the generator off and let it cool before refueling. OSHA’s generator fact sheet covers more hazards.

Planning a full backup setup with batteries and solar as well? Read our guide to setting up a reliable off-grid backup system.

Frequently Asked Questions About Generator Wattage Calculator