How to size
a generator.
Sizing is just arithmetic once you know the two numbers that matter. This guide walks through both, gives you the wattage of everything in a typical home, and shows the math on a real example — so you buy enough power, and not a watt more than you need.
Every appliance has two wattage numbers.
A generator is rated in watts. To size one, you need to know how many watts your home draws — both while everything hums along, and in the split second a motor first kicks on.
What it draws while it runs.
The steady power an appliance uses once it's up and going. A refrigerator holding temperature, a furnace blower pushing air, lights staying lit. Add these up for everything you want on at the same time — that's your continuous load.
The jolt when a motor wakes up.
Anything with a motor — fridges, pumps, air conditioners — pulls a brief surge to overcome inertia, often two to three times its running draw. It lasts a fraction of a second, but your generator has to cover it or it stalls.
You only size for one surge at a time. Motors rarely start in unison, so the peak your generator must cover is your total running watts plus the single largest startup surge on the list — not the sum of every surge. That distinction is the whole game.
Decide how much of the house stays on.
Most people land in one of four places. Find the one that sounds like your outage, and you're within a model or two of the right size.
A few critical things
2,000 – 3,000 W- Lights and phone charging
- Refrigerator in rotation
- Internet and a laptop
- A space heater or fan
The essentials, comfortably
5,000 – 8,000 W- Fridge and stand-alone freezer
- Furnace blower and sump pump
- Well pump and water
- Lights, internet, a window AC
Most of the house
10,000 – 16,000 W- Everything in Level 02
- Central air conditioning
- Electric range and oven
- Starts automatically · standby
The whole house, automatically
18,000 – 24,000 W- Nothing turns off
- Central AC, electric heat, dryer
- Range and water heater at once
- Large or multi-zone homes
What everything draws.
Typical figures for common household loads. Check the nameplate on the appliance for your exact number, then size up to the nearest figure here.
| Appliance | Running W | Starting W | Notes |
|---|---|---|---|
| Kitchen | |||
| Refrigerator | 700 | 2,200 | Compressor surge on startup |
| Stand-alone freezer | 700 | 2,200 | Same as a fridge |
| Microwave | 1,200 | 1,200 | No surge · rated by cook power |
| Electric range (per element) | 3,000 | 3,000 | Resistive · no startup spike |
| Dishwasher | 1,500 | 1,800 | Heated dry draws most |
| Coffee maker | 1,000 | 1,000 | Brief, while heating |
| Climate | |||
| Furnace blower (gas heat) | 800 | 2,350 | Fan motor surge |
| Central air conditioner | 3,500 | 8,750 | Largest surge in most homes |
| Window AC unit | 1,200 | 3,600 | Per 12,000 BTU unit |
| Space heater | 1,500 | 1,500 | Resistive · runs flat out |
| Water & well | |||
| Well pump (½ hp) | 1,500 | 4,500 | Three-times surge · plan for it |
| Sump pump | 1,000 | 3,000 | Critical in a flood |
| Electric water heater | 4,500 | 4,500 | Heavy · cycle it, don't run constant |
| Laundry | |||
| Electric dryer | 5,400 | 6,750 | Drum motor adds a small surge |
| Washing machine | 1,200 | 2,300 | Spin cycle is the peak |
| Living & essentials | |||
| Lights (whole home, LED) | 400 | 400 | No surge |
| Internet + computers | 300 | 300 | Router, modem, a laptop or two |
| Television | 200 | 200 | Per large flat-panel |
| Garage door opener | 550 | 1,100 | Surge only while lifting |
| Medical & safety | |||
| Home medical equipment | 600 | 800 | Confirm your device's nameplate |
| CPAP machine | 60 | 60 | Add 200 W if heated humidifier |
Let's size a real home.
A 2,500 sq ft house on a well, in a storm-prone county. The owners want the essentials to ride through a multi-day outage in comfort.
First, list everything that should run at the same time and add up the running watts. For this home that's the refrigerator, furnace blower, well pump, sump pump, whole-home lighting, internet, and one window AC — 5,900 watts of continuous load.
Next, account for surge. You don't add every appliance's starting watts — only the single largest one, because motors don't all kick on at the same instant. Here the well pump is the worst case: it runs at 1,500 W but surges to 4,500 W. The extra it adds on top of its running draw is 3,000 watts.
Add that one surge to the running total and you get the peak the generator must cover: 8,900 watts. A 9,500 W unit carries it with room to spare — which is exactly the headroom you want, not waste.
Same watts, different tank.
Two generators can be identical on paper and feel completely different in an outage. Fuel decides how long it runs, how much it stores, and whether you ever have to think about it.
The most available fuel and the lowest sticker price. You refill by hand, so plan on storing cans for a long outage.
Runs on gasoline or propane and lets you change between them without stopping. The most flexible portable option when supply is uncertain.
Plumbed straight to your gas line, so a standby unit runs as long as the outage lasts. No refilling, no storage, no thinking about it.
Stores indefinitely and burns clean, fed from a tank you own. A strong standby choice where there's no natural gas line.
We always leave a little room.
Size to about a quarter over your peak.
A generator run flat-out all day wears faster, runs hotter, and leaves nothing for the toaster you forgot about. We size every recommendation to roughly 25% above the calculated peak — enough headroom for real life, without paying for capacity you'll never touch. It's the difference between a generator that copes and one that's always straining.
Let the sizing tool pick it.
Answer three quick questions and we'll name the right category — no math required.
Or let us size it for you.
On a well, running medical equipment, or just want a second opinion? Leave us a message and we'll reply the same day.