Most folks who call me about backup power have already made up their minds on a number. “I’m thinking 10,000 watts,” they’ll say, before I’ve asked a single question about their house. Sometimes they’re right. More often they’re about to spend a thousand dollars they didn’t need to, or worse, buy a unit that stalls the first time the well pump and the fridge kick on together.
The math is not hard. You don’t need to be an electrician to do it, and you don’t need to pay one to run the numbers before you shop. What you need is a list of what you actually want running during an outage and a little patience with two kinds of wattage. I’ll walk you through exactly how I do it on a service call.
By the end you’ll have a real number, not a guess.
Key takeaways
- A generator sizing calculator works by adding up running watts, then adding the single biggest motor startup surge on top.
- Motors (fridges, pumps, AC) draw two to three times their running watts for a second or two when they start.
- Most homes that want essentials only land between 5,000 and 7,500 watts; whole-house comfort runs higher.
- Size for what you’ll actually run at once, not everything you own, then leave about 20 percent headroom.
What a generator sizing calculator is really doing
A generator sizing calculator is just a structured way to add up two numbers: your total running watts, and your largest starting surge. Every online tool, including the fancy ones from manufacturers, is doing that same arithmetic behind the scenes. Understanding it means you can sanity-check whatever a website spits out.
Running watts are what a device pulls once it’s humming along. A LED-lit living room might be 200 watts. A microwave, 1,000. Starting watts are the extra jolt a motor needs to get spinning, and they only last a moment.
Here’s the part people miss: you only add one startup surge, not all of them. Your fridge and your well pump almost never start at the same instant. So the honest formula is total running watts plus the single largest starting surge in your lineup.
Step one: list what you actually want to run
Grab a notepad. Walk the house and write down only the things you’d genuinely keep on during a multi-hour outage. This is where discipline saves you money.
A mistake I see all the time is folks listing every outlet in the house. You are not running the guest-room TV and the basement dehumidifier at 2 a.m. during an ice storm. Be realistic about what’s on at once.
For a typical “keep the essentials alive” list, most of my customers land on something like this:
- Refrigerator and/or chest freezer
- A few lighting circuits
- Furnace blower or boiler (gas heat still needs electric to run)
- Well pump, if you’re not on city water
- Sump pump, if your basement needs it
- Phone and laptop charging, a router, maybe one TV
- Microwave or coffee maker (used briefly, not continuously)
If you want central air or electric heat in the mix, that’s a whole different conversation, and I’ll point you to it below.
Step two: find the watts for each item
Now put a number next to everything on your list. Three places to look: the nameplate sticker on the appliance, the owner’s manual, or a plug-in watt meter that costs about $25 and pays for itself in peace of mind.
If a label only gives you amps, multiply amps by volts to get watts. A pump rated at 8 amps on a 240-volt circuit pulls roughly 1,920 running watts (8 × 240). Standard household outlets are 120 volts, so a 6-amp appliance there is about 720 watts.
To save you the digging, here are the ballpark figures I use when a nameplate is missing or unreadable. Motors are the ones with a big gap between the two columns.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator/freezer | 700 | 2,200 |
| Well pump (1/2 HP) | 1,000 | 2,300 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Furnace blower (1/2 HP) | 800 | 2,350 |
| Microwave | 1,000 | 1,000 |
| Lights (per room, LED) | 60 | 60 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
These are averages. Your actual fridge might be 500 watts or 900. When you can, use your own labels over any chart, including mine.
Why motors get their own column
Anything with a compressor or a motor, your fridge, freezer, pumps, air conditioner, spikes hard at the moment of startup. That inrush lasts maybe a second or two, but if your generator can’t supply it, the unit either trips or browns out and stalls the motor. That’s the number-one reason a generator that “should” be big enough isn’t.
Step three: do the actual generator sizing math
Add up every running watt on your list. Then look down your list, find the item with the highest starting surge, and add only that one surge figure on top.
Let me run a real example from a job last winter. The homeowner wanted fridge, furnace blower, well pump, a sump pump, six rooms of LED lights, a router, and the occasional microwave. Running watts came to:
Fridge 700 + blower 800 + well pump 1,000 + sump 800 + lights 360 + router 20 + microwave 1,000 = 4,680 running watts.
The biggest single startup surge in that group was the furnace blower at 2,350 (the fridge is close). Running watts minus the blower’s own running figure, plus its full startup: 4,680 − 800 + 2,350 = 6,230 starting watts.
So this house needs a generator that delivers at least 4,680 running watts and can handle a 6,230-watt surge. A 7,500-watt unit fits with room to spare. A 5,500 would be too tight once that blower kicks.
Add about 20 percent headroom to your final number. It keeps the generator off its redline, runs quieter and cooler, and leaves room for the day you plug in something you forgot to count. Sizing to the exact watt is how people end up frustrated.
Where the big loads change everything
Two appliances blow up these numbers fast, and they deserve their own homework. The first is central air conditioning. A 3-ton central AC can surge past 12,000 watts on startup all by itself, which is why sizing a generator for central air conditioning often pushes people toward a large portable or a standby unit.
The second is a deep well pump. Those 240-volt motors surge hard, and getting it wrong means no water. If you’re on a well, read through sizing a generator for a well pump before you settle on a number, because a marginal generator will short-cycle the pump and burn it out.
The fridge is the appliance people ask about most, so I wrote a whole piece on what size generator to run a refrigerator, surge figures and all.
Never plug a generator into a wall outlet to power your house. That “backfeeding” can electrocute a lineman working to restore power and can burn your home down. Use a proper interlock or transfer switch. The CPSC also warns to run any generator at least 20 feet from the house because of carbon monoxide.
From your number to the right generator
Once you’ve got a running-watt figure and a surge figure with headroom added, you’re ready to shop with confidence instead of guessing. If your total lands under about 7,500 watts, a good portable will cover it. If you’re pushing whole-house comfort with central air and electric appliances, look hard at a whole-house standby generator that starts automatically.
Either way, plan for how it connects. A generator without a safe hookup is just a noisy paperweight, so understand your options for transfer switches before you buy. And if you want to double-check my method against another walkthrough, our fuller guide on how to size a generator for your home covers the same steps with a few more examples.
Go make your list tonight. Twenty minutes with a notepad and the labels on your appliances gets you a number you can trust, and it beats every gut-feel guess I’ve ever had to talk a customer back from. For manufacturer wattage references, energy.gov’s appliance energy estimates are a solid cross-check, and the CPSC portable generator safety page is worth five minutes before your first run.
