What Size Generator Do I Need?
Generator size depends on simultaneous running watts, the largest realistic startup surge, voltage requirements and operating margin.
The advertised peak wattage is not the main sizing number; continuous running watts are.
This guide provides practical ranges, but appliance nameplates and measured startup loads are more reliable than generic estimates.
Add Running Watts, Largest Surge and 15–25% Margin
Many homes need 3,000–5,000 running watts for basic essentials, 7,000–10,000W for pumps and broader circuit backup, and more for central HVAC or electric heating.
Sizing Verdict
Size from the exact load list and choose enough 120/240V capability for the circuits involved. Oversizing slightly is safer than operating continuously at the limit.
Generator Sizing Formula
Typical Appliance Wattage
| Load | Running Watts | Starting Watts | Notes |
|---|---|---|---|
| Refrigerator | 150–800W | 1,000–2,200W | Compressor surge |
| Freezer | 150–700W | 1,000–2,000W | May cycle with refrigerator |
| Furnace blower | 400–900W | 800–1,800W | Check motor label |
| Sump pump | 800–1,500W | 2,000–4,000W | High surge |
| Well pump | 1,000–2,000W | 3,000–6,000W | Often 240V |
| Window AC | 500–1,500W | 1,200–3,000W | Varies by BTU |
| Microwave | 900–1,500W | Near running draw | Intermittent |
| Lights/router/TV | 100–500W total | Minimal | Easy load |
Common Home Generator Sizes
2,000–3,000W
Electronics, lights, refrigerator and small essentials.
3,000–5,000W
Basic household essentials and selected furnace loads.
5,000–7,500W
Several appliances, sump pump and broader backup.
7,500–10,000W
Well pump, furnace and multiple circuits.
10,000–15,000W
Large home-essential systems with load management.
15,000W+
Heavy circuits, but not automatically unrestricted whole-home power.
RV Sizing
A 2,000W inverter generator handles charging and electronics. A 3,000–4,500W model is more suitable for selected RV air conditioners, depending on soft-start equipment and other simultaneous loads.
Pumps and HVAC
Well pumps, sump pumps and compressors can dominate generator size because of startup surge and 240V requirements.
Common Sizing Mistakes
Using Peak Watts
Compare running output.
Ignoring Voltage
A high-watt 120V unit cannot run 240V loads.
Adding Every Surge
Use realistic startup overlap.
No Margin
Avoid continuous maximum operation.
Ignoring Fuel Rating
Propane and natural gas reduce output.
Skipping Transfer Planning
Connection capacity must match.
Frequently Asked Questions
What size runs a refrigerator and freezer?
Often 2,000–3,000 running watts with adequate surge.
What size for basic house backup?
Commonly 3,000–5,000 running watts.
What size for a well pump?
Often 5,000–10,000W with 240V, depending on pump size.
Can 3,500W run an RV AC?
It may run selected units, especially with a soft start.
Should I oversize?
Add practical margin, but excessive oversizing wastes fuel.
Do propane watts differ?
Yes, output is usually lower.