Generator Size Calculator
Enter how many of each appliance you want to run at the same time during an outage. You'll get the running watts and the starting (surge) watts your generator needs.
- Wattages from a utility's sizing chart
- Running and starting watts
- Add your own appliance
2 What size generator
Updates as you type| Refrigerator/freezer | 700 W running, +2,200 W to start |
|---|---|
| Sump pump (1/2 hp) | 1,050 W running, +2,200 W to start |
| Furnace fan blower (1/3 hp) | 700 W running, +1,400 W to start |
| Microwave (1,000 W) | 1,000 W running |
| TV | 400 W running |
| 2 × Phone charger | 40 W running |
| 6 × LED light bulb (15 W) | 90 W running |
| Total running | 3,980 W |
| Highest additional starting watts | 2,200 W (Refrigerator/freezer or Sump pump (1/2 hp)) |
| Total starting watts | 6,180 W |
Look for a generator whose running (rated) watts are at least the first number and whose starting (surge or peak) watts are at least the second. This assumes motors start one at a time, so switch on the refrigerator, pumps and furnace fan one after another rather than all at once.
Run a portable generator outdoors only, at least 20 feet from doors, windows and vents, and never plug it into a wall outlet to power the house. A transfer switch installed by an electrician is the safe way to connect it to home wiring.
Quick answer
Add the running watts of everything you want to run at once, then add the largest extra starting watts of any one motor. Using the Eugene Water & Electric Board's wattage chart, a refrigerator/freezer, a 1/2 hp sump pump, a 1/3 hp furnace fan, a 1,000 W microwave, a TV, 2 phone chargers and 6 LED bulbs come to 3,980 running watts and 6,180 starting watts.
Generator size for common outage plans
| What runs at once | Running watts | Starting watts |
|---|---|---|
| Refrigerator, lights, phone chargers | 830 | 3,030 |
| Plus furnace fan (1/3 hp) | 1,530 | 3,730 |
| Plus 1/2 hp sump pump | 2,580 | 4,780 |
| Plus 1/3 hp well pump | 3,580 | 5,780 |
| Refrigerator, lights, 12,000 BTU window AC | 4,080 | 8,030 |
| Refrigerator, lights, electric water heater | 5,330 | 7,530 |
Running and starting watts from the Eugene Water & Electric Board chart. Lights are six 15 W LED bulbs; every row also has two phone chargers. Use the calculator above for your exact size.
How to size a generator
A generator lists two wattages: running (or rated) watts, which it can supply continuously, and starting (surge or peak) watts, a short burst for starting motors. The Eugene Water & Electric Board, the city-owned utility in Eugene, Oregon, sets out the same method that Champion and Lowe's use on their wattage charts:
- Add up the running watts of everything you want to power at the same time.
- Find the one item with the highest additional starting watts and add that to the total. The result is the total starting watts.
For the example above: refrigerator/freezer 700 W + 1/2 hp sump pump 1,050 W + 1/3 hp furnace fan 700 W + 1,000 W microwave 1,000 W + TV 400 W + 2 phone chargers 40 W + 6 LED bulbs 90 W = 3,980 running watts. The largest extra starting load is 2,200 W, for the refrigerator/freezer or the 1/2 hp sump pump, so the generator should also handle 6,180 starting watts.
The method assumes motors start one at a time. Lowe's wattage worksheet notes its totals assume "intermittent starting of devices", and Honda advises starting appliances with high starting wattage at different times.
Typical appliance wattages
| Appliance | Running watts | Additional starting watts |
|---|---|---|
| Refrigerator/freezer | 700 | 2,200 |
| Sump pump (1/3 hp) | 800 | 1,300 |
| Sump pump (1/2 hp) | 1,050 | 2,200 |
| Well pump (1/3 hp) | 1,000 | 2,000 |
| Furnace fan blower (1/3 hp) | 700 | 1,400 |
| Window AC (12,000 BTU) | 3,250 | 3,950 |
| Space heater | 1,500 | |
| Electric water heater | 4,500 | |
| Microwave (1,000 W) | 1,000 | |
| Coffee maker | 1,000 | |
| Toaster | 850 | |
| TV | 400 | |
| Computer with monitor | 800 | |
| Laptop | 60 | |
| Phone charger | 20 | |
| LED light bulb (15 W) | 15 |
From the Eugene Water & Electric Board's Generator Sizing Considerations. Your appliances may differ: the label or manual gives the real figure, and if only amps are listed, volts × amps = watts.
Use a generator safely
- Outdoors only. The Consumer Product Safety Commission says never to run a generator inside a home, garage, crawlspace or shed, even with fans or open doors and windows, and to keep it at least 20 feet from doors, windows and vents. It also advises battery-operated carbon monoxide alarms, or plug-in alarms with battery backup.
- No backfeeding. Plugging a generator into a wall outlet to power the house wiring is "extremely dangerous", the CPSC says, and puts utility workers and neighbors at risk of electrocution. FEMA says to have a qualified electrician install a transfer switch, or ask your utility.
- Cords. Use heavy-duty outdoor extension cords rated for more than the total wattage of what is plugged into them (CPSC).
Frequently asked questions
What size generator do I need to run a refrigerator?
About 700 running watts and 2,900 starting watts for the refrigerator alone, using the Eugene Water & Electric Board's figures (700 running, 2,200 more to start). Check your refrigerator's label, since models differ.
What is the difference between running and starting watts?
Running watts are what an appliance draws while it runs. Motors such as refrigerator compressors, sump pumps and furnace fans need a short burst of extra power to start, the starting or surge watts. A generator has to cover your total running watts plus the biggest single starting surge.
Can a generator run a sump pump and a refrigerator?
Yes, if it is big enough. A refrigerator (700 W) and a 1/2 hp sump pump (1,050 W) run on 1,750 watts, and need 3,950 starting watts when one of them starts, by the Eugene Water & Electric Board's chart. Let one start before switching on the other.
How far from the house should a generator be?
At least 20 feet from doors, windows and vents, according to the Consumer Product Safety Commission, and never in a garage or other enclosed space.
Last reviewed 2026-11-12. See how we calculate for our sources and assumptions.