Generator sizing is simple arithmetic once you know two things about each appliance: how much power it draws while running, and how much extra it needs for a moment when it starts. This guide gives you the method, a table of typical figures and a worked example. Our generator size calculator does the same sums automatically.
Quick answer
Add up the running watts of everything you want to power at the same time. Add the extra starting watts of the single largest motor. Then look for a generator whose running (rated) watts are at least 20 percent above your running total, and whose starting (peak) watts cover the total with the motor surge. For most homes that means 5,000 to 7,500 running watts for essentials.
In this guide
Running watts and starting watts
Every generator carries two numbers. Running watts (also called rated or continuous watts) is the power it can supply all day. Starting watts (peak, surge or maximum watts) is a short burst, usually only a few seconds, available to start motors. Anything with a motor or compressor, such as a refrigerator, pump, furnace blower or air conditioner, needs two to three times its running watts for that moment. Heaters, lights and electronics have no surge.
Manufacturers list both figures. For example, Honda publishes its EU2200i inverter as 2,200 watts maximum and 1,800 watts rated. The rated figure is the one to size on.
The four-step method
- List what will run at the same time. Not everything you own, just what has to be on together during an outage.
- Add up the running watts. Use the nameplate on each appliance or the table below.
- Add the largest extra starting load. Find the appliance with the biggest gap between starting and running watts, and add that gap once. Motors rarely all start at the same instant, and you can stagger them deliberately.
- Add headroom. Multiply the running total by about 1.2. A generator working at 100 percent runs hot, wastes fuel and has nothing in reserve.
Wattage chart for common loads
These are typical planning values, in line with commonly published wattage charts. Your own appliances will differ, so check the nameplate or manual where you can. If a label gives amps, multiply by volts: 8 amps at 120 volts is 960 watts.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator or freezer | 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 blower, 1/2 HP | 800 | 2,350 |
| Window air conditioner, 10,000 BTU | 1,200 | 3,600 |
| Central air conditioner, 24,000 BTU | 3,800 | 11,400 |
| Electric water heater | 4,500 | 4,500 |
| Electric space heater | 1,500 | 1,500 |
| Microwave oven | 1,500 | 1,500 |
| Coffee maker | 1,000 | 1,000 |
| Electric range, one element | 2,100 | 2,100 |
| Washing machine | 1,150 | 2,250 |
| Electric clothes dryer | 5,400 | 6,750 |
| Lights, several rooms | 300 | 300 |
| Television, router, chargers | 300 | 300 |
| Garage door opener, 1/2 HP | 875 | 2,350 |
| Air compressor, 1 HP | 1,600 | 4,500 |
| Circular saw | 1,400 | 2,300 |
A worked example
Suppose a family wants to keep the following running through a winter outage: refrigerator (700 W), gas furnace blower (800 W), sump pump (800 W), lights (300 W) and the television, router and phone chargers (300 W).
- Running total: 700 + 800 + 800 + 300 + 300 = 2,900 watts.
- Largest extra starting load: the furnace blower needs 2,350 W to start, which is 1,550 W above its running figure. Add that once: 2,900 + 1,550 = 4,450 watts of starting demand.
- Headroom: 2,900 × 1.2 = about 3,500 running watts.
A generator rated around 3,500 running watts and 4,500 or more starting watts covers this comfortably. Adding a microwave used occasionally pushes the running figure to 4,400 W, so a 5,000 W class generator would give a margin for the odd extra. Adding a central air conditioner would push the starting demand past 15,000 W, which is why air conditioning so often decides the size.
Mistakes to avoid
- Sizing on starting watts. The big number on the box is the peak. Compare rated watts.
- Forgetting 240 V loads. Well pumps, central air and electric dryers need a generator with a 240 V outlet, not just more watts.
- Ignoring altitude and heat. Engines lose roughly 3 percent of their power for every 1,000 feet above sea level, and more on very hot days. At 5,000 feet a 7,000 W generator behaves more like a 6,000 W one.
- Oversizing by a wide margin. A generator that is far larger than the load burns more fuel, costs more and can be worse for the engine when run lightly loaded for long periods.
- Running everything at once. Start the biggest motor first, let it settle, then add loads one at a time.
Generator size classes
| Running watts | Typical use | Typical type |
|---|---|---|
| 1,600 to 2,500 | Camping, a refrigerator, lights and electronics | Small inverter |
| 3,000 to 4,500 | RV with air conditioning, home essentials without a well pump | Mid-size inverter or small portable |
| 5,000 to 7,500 | Essentials in most homes, including a well pump or furnace | Open-frame portable |
| 8,000 to 12,000 | Larger homes, 50 A transfer switch, a window air conditioner or two | Large portable |
| 14,000 and up | Whole-house backup with central air | Standby generator |
Frequently asked questions
What size generator do I need to run a refrigerator?
A typical household refrigerator runs on about 700 watts but needs around 2,000 to 2,200 watts to start the compressor. A 2,000 to 2,200 W inverter generator will usually run one refrigerator plus a few lights and chargers.
Will a 7,500 watt generator run my whole house?
It will run the essentials in most houses: refrigeration, a gas furnace, a well or sump pump, lights, electronics and a few kitchen appliances. It will not run central air conditioning, electric heating or an electric water heater alongside those loads.
How do I find the wattage of an appliance?
Look at the nameplate, usually on the back or inside the door, for watts or amps. If only amps are listed, multiply by the voltage (120 or 240). Motor loads list horsepower; as a rough guide, 1 HP needs about 750 to 1,000 running watts and two to three times that to start.
What size generator do I need for a well pump?
Most residential well pumps are 1/2 to 1 HP and need 240 V. A 1/2 HP pump needs about 1,000 running watts and 2,000 or more starting watts; a 1 HP pump needs roughly double that. Plan for a generator of at least 5,000 running watts with a 240 V outlet, and have the wiring done through a transfer switch.
