Choosing the Right Generator Size for Your Home
In This Guide
What size generator do I need is the question that decides whether your backup power actually works when it matters, and getting it wrong is expensive in both directions. Too small and it trips out or damages appliances, too large and you have overspent on the unit and burn extra fuel. The good news is that sizing a generator is simple arithmetic once you understand two numbers, and this guide walks through both.
First: Essentials or Whole House?
The single biggest question is what you actually want to run. There are two very different goals, and they need very different generators.
| Goal | Typical size |
|---|---|
| Essentials only (fridge, lights, outlets, small AC) | 5,000 to 7,500 watts |
| Essentials plus central air | 10,000 watts or more |
| Whole house, most appliances | 15,000 to 20,000 watts |
| Large home, dual HVAC, well pump | 22,000 watts and up |
Most people who just want to ride out an outage comfortably land in the 5,000 to 7,500 watt range. Whole house living-as-normal is a different and larger machine.
The Two Numbers: Running Watts and Starting Watts
Every appliance has two power figures, and both matter.
- Running watts are the continuous power an appliance draws while operating.
- Starting watts are the brief surge it needs at the moment it switches on. Anything with a motor, a fridge, a well pump, an air conditioner, can surge to two or three times its running watts for a second or two.
Typical Appliance Wattages
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 150 to 400 | 800 to 1,200 |
| Sump pump | 800 to 1,050 | 1,300 to 2,900 |
| Well pump | 1,000 to 2,000 | 2,000 to 4,000 |
| Window AC | 1,000 to 1,500 | 1,500 to 3,000 |
| Microwave | 1,000 to 1,500 | Same |
| Lights (LED, whole house) | 100 to 300 | Same |
| Furnace fan | 600 to 900 | 1,500 to 2,000 |
These are typical figures. Your appliances' own labels and manuals give the exact numbers, which is what you should size from.
How to Do the Calculation
1. List what you want to run at once
Be realistic. You will not run everything simultaneously, so group the essentials you genuinely need together.
2. Add up the running watts
Total the continuous draw of everything on your list.
3. Add the single largest starting surge
Find the appliance with the highest starting watts and add that one surge on top. Only one motor typically starts at any instant.
4. Add 20 to 25 percent headroom
Multiply the total by about 1.25. This buffer keeps the generator off its limit, which is easier on the engine and safer.
Do Not Oversize Either
Bigger is not automatically better. An oversized generator costs more to buy, burns more fuel, and running one under very light load for long periods can actually cause problems like wet stacking, where unburned fuel builds up in the exhaust. Size it to your real needs plus a sensible buffer, not to the largest unit you can afford.
Portable or Standby?
Sizing interacts with type. Portable generators typically top out lower and suit essentials backup. Whole house sizes usually mean a standby unit wired in through a transfer switch. If your calculation lands above roughly 10,000 watts, you are generally into standby territory. Our repair guide covers the differences.
Rather Have a Real Technician Handle It?
A Wildwood technician comes to your home, diagnoses the generator on site, and gets it running, usually in about an hour. More than 10,000 repairs completed since 2019, with flat rates quoted before we arrive so there are no surprises. When you need power back, you need it back now.