Most folks buy a generator the same way they buy a Christmas tree: they walk in, grab the biggest one they can afford, and hope it fits. I’ve pulled plenty of shiny 12,000-watt units out of garages that never once ran more than a fridge and a couple of lamps. That’s a few thousand dollars of overkill burning extra fuel every hour.
The opposite mistake stings worse. A homeowner grabs a 3,500-watt special, fires it up in an ice storm, plugs in the furnace, and the thing bogs down and dies. Now the family’s cold and the generator’s blinking an overload light.
Figuring out how to size a generator isn’t complicated once you stop guessing and start adding. That’s the whole trick, and I’ll walk you through the exact math I use on service calls.
Key takeaways
- Add up the running watts of only what you actually need during an outage, then pad it for startup surge.
- Motors (fridges, well pumps, AC) draw 2 to 3 times their running watts for a split second when they kick on.
- For most homes, a 7,500 to 12,000-watt unit covers the essentials without wild overspending.
- Match the generator to how you’ll connect it, whether that’s a few cords or a transfer switch.
Start with what you can’t live without
Before touching a wattage chart, make a list. Not a wish list, an outage list. What genuinely matters when the power’s out for three days?
For most people that’s the refrigerator, the furnace blower, a few lights, phone chargers, and maybe a well pump or sump pump. Everything else, the dryer, the oven, the hot tub, is a luxury you can skip while the utility crews work.
A mistake I see all the time: people size for their whole house as if every breaker will be on at once. Your house never runs everything simultaneously, even on grid power. Size for the loads you’ll actually run together.
How to size a generator by adding running watts
Every appliance has two numbers that matter: running watts (what it draws steadily) and starting watts (the surge when a motor spins up). You’ll usually find running watts on a sticker near the plug or in the manual, sometimes listed as amps instead. If it’s in amps, multiply amps by volts (120 for standard outlets, 240 for big appliances) to get watts.
Here’s a rough tally for a typical winter essentials setup:
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator/freezer | 700 | 2,200 |
| Furnace blower (1/2 hp) | 800 | 2,300 |
| Sump pump (1/3 hp) | 800 | 2,000 |
| Well pump (1/2 hp) | 1,000 | 3,000 |
| Lights (LED, whole floor) | 150 | 150 |
| TV + internet router | 250 | 250 |
| Microwave | 1,000 | 1,000 |
Add up the running watts of everything on your list. In this example that’s about 4,700 watts running. But you can’t stop there, and this is where the cheap generators get people.
The surge is what sizes the unit
Motors don’t sip power when they start, they gulp it. That fridge pulling 700 running watts might spike to 2,200 for a half-second when the compressor kicks on. If your well pump and fridge both decide to start at the same moment, that surge stacks up fast.
The safe way to handle it: take your total running watts, then add the single largest starting surge on top. In our example, that’s 4,700 running plus the well pump’s extra 2,000-watt surge, landing around 6,700 to 7,000 watts of headroom you’ll want available. A 7,500-watt generator handles this comfortably. If you want to run central AC too, you’re in different territory, and I’d point you toward the numbers in a dedicated look at sizing a generator for central air conditioning before you buy.
Never run a portable generator in a garage, basement, or near an open window. Carbon monoxide from one unit can kill in minutes. Keep it at least 20 feet from the house with the exhaust pointed away, and use a battery CO alarm indoors. The CPSC logs dozens of generator-related deaths every year, and nearly all of them are preventable.
Running watts vs. starting watts, in plain terms
Think of it like your own legs. Walking across the room all day is your running load, easy, steady. Jumping to catch something falling off a shelf is your surge, a huge burst for an instant.
A generator has both a running rating and a peak (or “starting”) rating. Buy based on the running number, not the peak on the box. Manufacturers love to plaster the surge figure in giant font because it’s bigger. A “9,000-watt” generator often runs at only 7,200 watts continuous.
If you want a slower walkthrough with a fill-in-the-blank worksheet, I put together a simple generator sizing calculator walkthrough that does this same math step by step.
Match the size to how you’ll actually connect it
How you get power into the house changes what size makes sense. There are basically three routes.
- Extension cords through a window: fine for a 3,500 to 5,000-watt unit running a fridge and a few cords. Cheap, but you’re babysitting cords and the door won’t shut all the way.
- An interlock kit or transfer switch: lets you power hardwired circuits like the furnace and well pump safely. This is where 7,500 to 12,000-watt portables shine.
- A permanent standby generator: wired in, fueled by natural gas or propane, starts itself. Usually 14,000 to 26,000 watts and up.
If you’re running anything hardwired, a transfer switch isn’t optional, it’s the law and it’s what keeps a lineman from getting shocked by your backfeed. Read up on how they work in this rundown on transfer switches explained before you settle on a portable versus a standby.
On a job last winter, a couple wanted a whole-house standby but really only cared about heat, fridge, and hot water. We sized a 14kW unit instead of the 22kW the salesman quoted, and it managed their loads by shedding the water heater when the furnace kicked on. Saved them roughly $2,500 and it still runs the whole house.
When a portable stops making sense
Somewhere around the 12,000-watt mark, portables get heavy, loud, and thirsty. If you want the fridge to just keep humming without you dragging a machine out at 2 a.m., a permanent unit earns its cost. I break down that trade-off fully in the guide to whole-house standby generators.
A quick reality check before you buy
The single most common question I get is about the fridge, so start there if you’re overwhelmed. Nail down that one appliance using the specifics in what size generator to run a refrigerator, then build the rest of your list on top of it.
Once you’ve got your running total and your biggest surge, add them and shop for a generator whose running rating clears that number with a little room to spare. Don’t chase the peak figure on the box, and don’t buy triple what you need because it was on sale. For the official safe-operation checklist, the Ready.gov generator safety guidance is worth five minutes before your first outage.
Write your list, do the addition, and you’ll walk into the store knowing exactly what you’re looking for instead of trusting the sticker with the biggest font.
