The short answer: add up the running watts of everything you want on at the same time, then add the extra starting watts of the single biggest motor (a fridge, freezer, sump pump, well pump or air conditioner). That total is the smallest generator output that will carry your list. Running watts are what an appliance draws while it works. Starting watts are the short surge a motor needs for a few seconds to get going. The chart below gives rough typical numbers for common household items, but the label on your own appliance always wins. We explain how to do the math, not which model to buy.
Running watts vs starting watts
Running watts (also called rated or continuous watts) are the steady power an appliance uses once it is on. A coffee maker, toaster or space heater uses about the same power from the first second to the last, so it has no meaningful surge.
Starting watts (also called surge or peak watts) apply to anything with a motor or compressor: refrigerators, freezers, pumps, furnace fans, washers and air conditioners. Virginia Cooperative Extension notes that when you measure a motor, the meter will show about three times more current in starting the motor than when it is running smoothly. That surge lasts only a few seconds, but if your generator cannot supply it, the motor stalls or the generator's breaker trips.
Generators are usually rated the same two ways: a running (rated) output it can hold, and a higher starting (surge) output it can give for a moment. Compare your running total with the first and your surge total with the second.
Generator wattage chart: common household appliances
These are rough typical ranges, so check your own label. The first table lists nameplate wattage ranges published by Virginia Cooperative Extension. Nameplate watts are the maximum an appliance is rated to draw; many use less most of the time.
| Appliance | Typical nameplate watts |
|---|---|
| Refrigerator (frost-free, 16 cu. ft.) | 725 |
| Microwave oven | 750–1,100 |
| Coffee maker | 900–1,200 |
| Toaster | 800–1,400 |
| Portable heater | 750–1,500 |
| Hair dryer | 1,200–1,875 |
| Clothes washer | 350–500 |
| Clothes dryer (electric) | 1,800–5,000 |
| Dishwasher | 1,200–2,400 |
| Dehumidifier | 785 |
| Furnace fan | 750 |
| Water pump (deep well) | 250–1,100 |
| Water heater (40 gallon, electric) | 4,500–5,500 |
| Ceiling fan | 65–175 |
| Flat-screen TV | 120 |
| Laptop | 50 |
The second table shows running and starting estimates for motor loads from one electric cooperative's generator sizing chart. The starting column is the total watts needed in the first few seconds.
| Motor load | Running watts | Starting watts |
|---|---|---|
| Refrigerator/freezer | 500 | 750 |
| Freezer (frost-free, 24 cu. ft.) | 720 | 1,890 |
| Furnace fan | 1,100 | 2,000 |
| Sump pump (1/3 hp) | 800 | 1,250 |
| Clothes washer | 1,150 | 2,200 |
| Garage door opener (1/4 hp) | 550 | 1,000 |
| Central air conditioner (3 ton) | 5,400 | 7,200 |
Notice that the two sources do not agree exactly (the furnace fan, for example). That is normal: wattage varies a lot by size, age and model, which is why the steps below start with your own labels.
Step 1: List what you must keep running
Walk through your home and write down only what matters during an outage. For most families that is the refrigerator and freezer, a few lights, phone chargers and the internet router, plus anything critical for your home: a sump pump, a well pump, a furnace fan in winter, or a medical device. Mark the items you would run at the same time. You do not need to power the dryer and the microwave together if you can take turns. Our power outage preparation checklist can help you build the list.
Step 2: Find the real watts on each label
Look for a nameplate or sticker on the back, bottom or inside the door of each appliance. If it lists watts, use that number. If it lists only amps, Virginia Cooperative Extension explains that you can multiply the current (amps) by the voltage: most U.S. appliances use 120 volts, while larger ones such as clothes dryers and electric cooktops use 240 volts. For example, a label reading 5 amps at 120 volts means about 600 watts.
Starting watts are rarely printed on the label. If your appliance has a motor or compressor and you cannot find a starting figure in its manual, use the chart above as a rough guide or ask the manufacturer.
Step 3: Add up your running watts
Add the running watts of every item you marked to run at the same time. This is the output your generator must hold steadily for hours. Write the total down.
Step 4: Add the biggest starting surge
Motors rarely all start at the same moment, so you only need to cover the largest single surge on top of everything else. Find the motor with the biggest gap between its starting and running watts and add that extra amount to your running total.
The cooperative's chart gives a worked example: a 100 watt light bulb, a 200 watt slow cooker, a 1,200 watt refrigerator with a start up wattage of 2,900 watts and a 750 watt TV would require 3,950 watts. In other words, the light, slow cooker and TV (1,050 watts) plus the refrigerator at its starting peak (2,900 watts). Once the fridge settles down, the same list runs on about 2,250 watts.
Step 5: Leave headroom and stagger your starts
Do not plan to run a generator at its very limit for hours. Leave some margin above your running total so the generator is not straining and you have room to add a load later. When the power goes out, the same cooperative recommends that you always start the largest electric appliance first, then plug in other items, one at a time. Starting motors one at a time keeps surges from stacking up.
Step 6: Plan the connection safely
A generator must always run outside. Ready.gov says generators and fuel should always be used outdoors and at least 20 feet away from windows, doors and attached garages, and every home should have battery-backed carbon monoxide detectors. Never plug a generator into a wall outlet to feed your house. To power hardwired loads such as a furnace, well pump or central air, a licensed electrician should install a transfer switch or interlock kit; see our guide to transfer switch vs interlock kit.
What about 240-volt appliances?
Well pumps, central air, electric water heaters and electric dryers often run on 240 volts. They need a generator with 240-volt output and a proper connection through your panel, not an extension cord. These loads are also the biggest numbers on any wattage chart, so decide early whether you truly need them during an outage. Dropping the water heater or dryer from your list can cut your required size dramatically.
Where a portable power station fits
Battery power stations are rated in watts too (how much they can power at once), plus watt-hours (how long they can do it). The same running and starting math applies to the watt rating. For runtime, the U.S. Department of Energy's formula, (wattage × hours used per day) ÷ 1,000 = daily kilowatt-hours, helps you estimate how much stored energy a load uses. See how long a power station will run a refrigerator for a step-by-step example.
When you know your numbers, use our guide to what size generator you need, compare standby vs portable vs solar generators, or weigh home battery backup vs a generator. If you will run laptops and other sensitive electronics, read inverter vs regular generators, and for fuel choices see dual fuel generators explained. Then browse our inverter and dual-fuel generators, portable power stations, solar generators and home battery backup systems.
Frequently asked questions
How many watts does a refrigerator need on a generator?
It varies by size and age. Virginia Cooperative Extension lists about 725 nameplate watts for a 16 cubic foot frost-free model, and one cooperative's chart estimates 500 running and 750 starting watts for a refrigerator/freezer. Check your fridge's label and plan for its starting surge.
What is the difference between rated watts and surge watts on a generator?
Rated (running) watts are what the generator can supply continuously. Surge (starting) watts are a higher amount it can supply for a few seconds while motors start. You need both numbers to be high enough for your list.
Can I add up the starting watts of every appliance?
You can, but it usually oversizes the generator. Motors rarely start at the exact same moment, so the common method is your total running watts plus the single largest extra starting surge. Starting the biggest motor first makes this work in practice.
Why does my generator trip when the fridge or pump kicks on?
The motor's starting surge is pushing the total above what the generator can deliver. Turn off a few other loads, start the motor first, then add the rest one at a time.
Do I need to power my whole house?
Most people do not. Covering the refrigerator, freezer, lights, internet, phone charging and one critical pump or fan keeps a home livable through most outages, and it takes far less power than running everything.
Need help with your numbers?
Send us your list of appliances and their label ratings, and we will help you work out the running and starting watts you need. Contact us with your questions.
This guide is general information. Wattages vary by model; always check your appliance labels and manuals, and have a licensed electrician handle any connection to your home's wiring.
Sources
- Virginia Cooperative Extension (Virginia Tech), Estimating Appliance and Home Electronic Energy Use (Feb. 26, 2020)
- Howell-Oregon Electric Cooperative, Choosing a Generator: Which Size Is Right?
- Ready.gov, Power Outages
- U.S. Department of Energy, Energy Saver, Estimating Appliance and Home Electronic Energy Use