The single most common mistake homeowners make when buying a standby generator isn’t picking the wrong brand — it’s picking the wrong size. Oversizing means paying thousands more than necessary; undersizing means a generator that can’t actually keep your home running when you need it most. Here’s how to get it right.
Start With Two Numbers: Running Watts and Starting Watts
Every appliance and system in your home draws two different amounts of power:
- Running watts: the power it uses continuously once it’s on
- Starting watts (or surge watts): a short burst of extra power needed to start up — often 2-3x the running wattage, especially for anything with a motor (AC units, well pumps, refrigerators)
Sizing based only on running watts is the most common cause of an undersized system, because the generator needs enough headroom to handle the highest starting-watt spike, not just the sum of everything running steadily.
Typical Wattage by Appliance
| Appliance/System | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 600-800 | 1,200-2,200 |
| Central AC (3-ton) | 3,500-4,000 | 8,000-10,000 |
| Well pump (1/2 HP) | 1,000 | 2,000-3,000 |
| Electric water heater | 3,000-4,500 | — |
| Furnace fan/blower | 600-1,000 | 1,500-2,500 |
| Sump pump | 800-1,050 | 1,300-2,200 |
| Window AC unit | 900-1,500 | 1,800-3,000 |
| Microwave | 1,000-1,500 | — |
| Lighting circuit (whole home) | 600-1,200 | — |
These are general ranges — check the nameplate on your actual appliances for exact figures where possible.
Step-by-Step: Calculating Your Generator Size
- List every circuit or appliance you want to run during an outage. Be realistic — this is about what you need, not everything in the house.
- Add up the running watts of everything that could reasonably run at the same time.
- Identify the single largest starting-watt appliance (usually central AC or a well pump) and add its starting watts — not running watts — on top of your running total. You don’t need to add starting watts for every appliance, just the largest one, since appliances rarely all start at the exact same instant.
- Add a buffer of 20% on top of your total to give the generator headroom rather than running at its absolute maximum capacity constantly.
Example calculation for a mid-size home:
- Refrigerator (700W) + lighting (1,000W) + furnace fan (800W) + sump pump (900W) = 3,400W running
- Largest starting load: central AC starting watts (9,000W) minus its own running watts already counted separately
- Total with buffer: roughly in the 14-16kW range
This is exactly why most mid-size homes land in the 14-20kW range covered in our full cost breakdown by size.
When to Just Get a Professional Load Calculation
DIY wattage math gets you a solid ballpark, but it’s worth having your installer run a formal load calculation before finalizing your purchase — especially if:
- You have a well pump (starting watts vary a lot by pump size and depth)
- You have multiple HVAC zones
- You’re deciding between two adjacent sizes, like 22kW vs 24kW, and want to be sure which side of the line you actually fall on
Common Sizing Mistakes
- Sizing off square footage alone instead of actual appliance load — two homes the same size can have very different electrical needs
- Forgetting well pump starting watts, which is one of the most common causes of a generator that «should» work on paper but stalls in practice
- Not accounting for future additions — if you’re planning to add central air or a hot tub soon, size with that in mind rather than resizing later
Frequently Asked Questions
Is it safer to just oversize «to be safe»? Not really — oversizing mainly costs you more money and slightly more fuel, without meaningfully improving reliability. A correct load calculation is a better use of money than blanket oversizing.
Do I need to run literally everything during an outage? No — most homeowners choose to run only essential and comfort circuits, not every outlet in the house. This is a personal decision, not a technical requirement.
Can I use an online wattage calculator instead of doing this by hand? Yes, most generator brands offer one, and they’re a reasonable starting point — but they still rely on you entering accurate appliance data, so the manual understanding above helps you use them correctly.
Bottom Line
Correctly sizing a generator comes down to running watts, the single largest starting-watt spike, and a reasonable buffer — not square footage or «bigger is safer» thinking. Getting this calculation right before you buy is the best way to avoid both overpaying and ending up with a system that can’t handle your home’s real demand.