What Size Generator Do I Need? Wattage Chart for Your Home
For a power cut, most households can ride it out with a 3,500 to 5,000 running-watt generator: enough for the refrigerator, a few lights, the furnace blower and a sump pump. Running the whole house, including central air, an electric water heater and a dryer, takes 15,000 watts or more. This guide shows how to work out the number for your home, and why the starting surge matters as much as the running load.
Running watts and starting watts
Every appliance is labelled with how much power it uses, in watts (or in amps and volts, where watts = amps × volts). Two numbers matter:
- Running (rated) watts is what the appliance draws once it is going.
- Starting (surge) watts is the much larger draw for a second or two when a motor starts. A refrigerator that runs at 700 W can need 2,200 W to start, and a central air conditioner can surge to three times its running load.
The generator has to supply the running watts of everything that is on, plus the surge of the biggest motor that starts. Because you can start motors one at a time, you only add the largest surge, not all of them.
The formula
- List everything you want to power, and add up the running watts.
- Find the appliance with the biggest surge above its own running watts. Add that extra to your total to get the peak watts.
- Choose a generator whose running rating is at least 20% above your total running watts, and whose peak rating covers your peak watts.
The generator size calculator does all three steps from a checklist of common appliances.
Typical wattages
These are typical values. The label on your own appliance always wins, and soft-start devices can cut the surge of air conditioners.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Freezer | 500 | 1,500 |
| Sump pump (1/3 hp) | 800 | 2,100 |
| Well pump (1/2 hp) | 1,000 | 2,100 |
| Furnace blower (1/2 hp) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (2 to 3 ton) | 3,500 | 10,500 |
| Electric water heater | 4,000 | 4,000 |
| Microwave | 1,000 | 1,000 |
| Coffee maker or kettle | 1,000 | 1,000 |
| Dishwasher | 1,500 | 1,500 |
| Washing machine | 1,150 | 2,300 |
| Electric dryer | 5,400 | 6,750 |
| Garage door opener | 550 | 1,100 |
| Space heater | 1,500 | 1,500 |
| Fans | 150 | 300 |
| LED lights (about 10 bulbs) | 100 | 100 |
| TV and cable box | 250 | 250 |
| Laptop, router and phone chargers | 150 | 150 |
Examples, from small to whole-house
Each row adds to the one above it. The generator size includes 20% headroom on the running load.
| What you power | Running | Peak | Generator to shop for |
|---|---|---|---|
| Essentials: fridge, lights, phone and laptop | 950 W | 2,450 W | 2,000 running watts |
| Essentials + sump pump | 1,750 W | 3,250 W | 3,000 running watts |
| Essentials + sump pump + furnace blower + microwave | 3,550 W | 5,100 W | 5,000 running watts |
| The above + well pump + window AC | 5,750 W | 8,150 W | 7,500 running watts |
| The above + electric water heater + dryer | 15,150 W | 17,550 W | 20,000 running watts |
| The above + central air conditioner | 18,650 W | 25,650 W | Whole-home standby generator |
The big jumps come from the electric heating loads (the water heater and dryer) and central air. Leaving those off the generator, or switching them to gas, is the cheapest way to shrink the generator you need.
Portable, inverter or standby?
- Conventional portable generators are the cheapest per watt and come in large sizes, but are loud and their power is less clean for sensitive electronics.
- Inverter generators produce clean power, are quieter, and use less fuel, but cost more per watt and generally top out at smaller sizes. Best for camping, tailgating and electronics.
- Standby generators are permanently installed on a pad, run on natural gas or propane, and start automatically through a transfer switch. They cost much more, but are the only practical way to run a whole house.
Safety, which matters more than the size
- Carbon monoxide. Never run a generator inside a home, garage, shed or anywhere near windows, doors or vents. Keep it at least 20 ft away from the house with the exhaust pointing away, and use battery-powered CO alarms indoors.
- Connect it properly. Use heavy-duty outdoor extension cords for individual appliances, or have an electrician install a transfer switch or interlock. Never plug a generator into a wall outlet to back-feed the house, which can electrocute utility workers and damage the generator.
- Fuel. Let the generator cool before refuelling, and store fuel in approved containers away from the house. Use a fuel stabilizer if fuel will sit for months.
- Maintenance. Run it under load every few months, change the oil on schedule, and keep the fuel fresh, so it starts when you need it.
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.