How to work out what you need to run
Start by writing down what truly has to stay on in an outage. Usually that is the refrigerator, some lights and charging, and perhaps a sump pump or a furnace fan in winter. Leave out what can wait, and you may save a lot of money and fuel.
Then read the labels. Charts disagree widely: a refrigerator is 132 to 192 running watts by Honda, 400 by Champion and 700 by Lowe's and Generac. A plate on the back of the appliance, or the manual, beats any chart. Look for the running watts and the additional watts to start, and enter the additional figure, not the total, because that is what this method adds.
Where it runs matters more than its size. The CPSC says about 100 people die each year in the US from carbon monoxide from portable generators. Run one outside only, at least 20 feet from the house. Never run it in a home, garage, basement, crawlspace or shed, and not on a porch or in a carport. Opening doors or windows will not provide enough ventilation.
A worked example
This is printed on Champion's chart: a TV, a desktop computer, a microwave, a refrigerator, a blender and a stove. These lines come from the same formula the calculator above runs.
Champion's printed example
- Running: 80 + 100 + 800 + 400 + 500 + 2000 = 3,880 W
- Largest additional surge: the refrigerator, 2,200 W
- Starting: 3,880 + 2,200 = 6,080 W
Only the refrigerator's surge is added, not the microwave's, because you start one thing at a time. Lowe's worksheet prints the same method for its own example. Generac does it differently, tripling the largest motor's running watts, and you should not mix the two. To see what running these costs when the power is back, see the electricity cost calculator.
Generator size for common outages
These use Champion's chart, with no margin added. A generator has to meet both columns. If your label says something different, use the label.
| What runs together | Running | Starting |
|---|---|---|
| Refrigerator only | 400 | 2,600 |
| Refrigerator, TV and computer | 580 | 2,780 |
| Refrigerator and a sump pump, 1,050 W, 2,150 W to start | 1,450 | 3,650 |
| Refrigerator, microwave, TV and computer | 1,380 | 3,580 |
| Refrigerator, window AC, TV and computer | 1,780 | 3,980 |
| Refrigerator, window AC, washer, stove | 4,100 | 6,300 |
Mistakes that can be dangerous or expensive
- Running it close to the house. A window nearby or an attached garage lets carbon monoxide in, and you cannot smell it. Use battery carbon monoxide alarms.
- Backfeeding a panel. A generator plugged into a wall outlet can energize lines outside and kill a utility worker. Use a transfer switch fitted by an electrician.
- Mixing running and starting. Sizing to running watts alone leaves you stalled the moment the refrigerator kicks in.
- Starting everything at once. The Lowe's total assumes intermittent starting of devices. Bring them on one by one.
- Double counting. A total starting figure from one chart added to another chart's running watts counts the appliance twice.
Questions
What size generator do I need to run a refrigerator?
A generator of at least about 2,600 starting watts will run one refrigerator, on Champion's chart of 400 running and 2,200 more to start. Other charts put a refrigerator at 700 running, so read the label on yours.
Running watts or starting watts?
You need both. Running watts keep things going, and starting watts cover the extra surge a motor draws for a moment as it starts. A generator lists a figure for each, and yours has to meet both figures here.
How do I calculate wattage for a home generator?
Add up the running watts of everything you want on at once, then add the single largest additional starting watts. That is the method on both the Champion and Lowe's charts. Only one surge is added, because you start motors one at a time.
What size generator do I need to run my whole house?
That needs an electrician rather than this calculator, because a whole house involves central air, a well or electric heat, and a transfer switch. Most people size a portable generator for essentials, and add up only those.
How many watts to run a sump pump?
A half horsepower sump pump needs 1,050 running watts and 2,150 additional starting watts on the Lowe's chart. Enter your own from the pump's label in the further section, since sizes vary.
Should I oversize a generator?
We found no primary source that sets a margin, so we do not add one here. Generac warns against loading past capacity, and a generator run near its limit has no room for a surge you forgot. Choose a bigger one if your budget allows.
When do I need a professional?
You need a licensed electrician for any connection to your house wiring, including a transfer switch. Never backfeed a panel or plug a generator into a wall outlet, because it can energize lines and kill a utility worker.
This generator size is for planning, from typical wattage charts that differ widely, so read your own appliance labels. Connecting one to house wiring must follow the National Electrical Code as adopted locally and be done by a licensed electrician, because backfeeding a panel can kill a utility worker. Carbon monoxide kills too: never run one inside, and keep it 20 feet from the house.
Sources and review
- Choosing a generator, wattage chart flyer, Champion Power Equipment, 2025.
- Portable generator wattage worksheet, Lowe's, 2021.
- GP series portable generator owner's manual, Generac, 2020.
- CPSC safety tips: carbon monoxide and generators, U.S. Consumer Product Safety Commission, 2025.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Starting watts
Starting watts = All running watts + The largest additional starting watts
- All running watts
- Everything running together
- The largest additional starting watts
- The biggest motor's extra surge, one only
Worked through: 3880 + 2200 = 6080 W
The constants, and where they come from
WATTS
Published range, narrowedRunning watts and additional starting watts, by appliance
- refrigerator
- running 400, starting 2200
- windowAc
- running 1200, starting 2000
- washer
- running 500, starting 1200
- microwave
- running 800, starting 2000
- stove
- running 2000, starting 0
- tv
- running 80, starting 0
- computer
- running 100, starting 0
Applies to: Typical appliances. Your own labels are better
Source: Choosing a generator, wattage chart flyer, Champion Power Equipment, published 2025-02, read 2026-11.
Also from: Portable generator wattage worksheet, Lowe's, published 2021-09, read 2026-11.
Tables disagree widely. A refrigerator runs at 132 to 192 by Honda, 400 by Champion and 700 by Lowe's and Generac. We use one table and tell you to check the label. Generac and others use another method, three times running for the largest motor, and mixing the two counts the largest appliance twice.
WATTS_PER_KILOWATT
DefinitionalWatts in one kilowatt
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
The prefix kilo means a thousand, so 1,000 watts is one kilowatt.
What the labels mean
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the generator size calculator
- 1
Choose what has to run together, and leave out what can wait
- 2
Add anything else from its own label in the further section
- 3
Read the running watts and the starting watts
- 4
Compare both with a generator's rated figures
- 5
Start motors one at a time when you use it
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Motors are started one at a time, so only the largest starting surge is added
- Starting watts are the additional watts on top of running watts. Do not enter a total starting figure
- No margin is added, because we found no primary source for one. Choosing a larger generator than the minimum is up to you
- Wattage figures are typical values from one chart, and your appliances may differ
Pro tips
- Leave the biggest motor until the others are running, or start it first and let it settle
- Decide what truly must run before you buy
- Test the generator before the storm, outside and well clear of the house