Quick Answer
To size a generator, add the running watts of everything you run at once, then add the single largest starting (surge) wattage among those items. Motor-driven appliances (refrigerator, well pump, AC, sump pump) draw a brief surge several times their running watts when they start, so the generator must cover that spike.
Running & Starting Watts by Appliance
| Appliance | Running Watts | Additional Surge | Total Starting Watts |
|---|---|---|---|
| Refrigerator / Freezer | 700 | 2,200 | 2,900 |
| Sump pump (1/3 HP) | 800 | 1,300 | 2,100 |
| Furnace fan blower (1/3 HP) | 700 | 1,400 | 2,100 |
| Window AC (10,000 BTU) | 1,200 | 3,600 | 4,800 |
| Well pump (1/2 HP) | 1,000 | 2,100 | 3,100 |
| Space heater | 1,800 | 0 | 1,800 |
| Microwave | 1,000 | 0 | 1,000 |
| Electric stove (one element) | 2,100 | 0 | 2,100 |
| TV (49" LED) | 85 | 0 | 85 |
| Light bulb (each) | 75 | 0 | 75 |
| Laptop charging | 50 | 0 | 50 |
| Phone charger | 5–20 | 0 | 5–20 |
| CPAP machine | 30–100* | 0 | 30–100* |
Figures are typical values that vary by model. "Additional surge" is the extra spike at start-up; total starting watts = running + additional surge. *CPAP: roughly 30–60 W without a heated humidifier, up to ~100 W with one, per equipment manufacturer ranges.
How to Size Your Generator
Pick the appliances you need to run at the same time, total their running watts, then add the largest single surge:
| Item Running at Once | Running Watts | Surge (if largest) |
|---|---|---|
| Refrigerator | 700 | +2,200 (largest surge) |
| Furnace fan | 700 | — |
| 5 LED lights | 375 | — |
| Phone + laptop charging | 70 | — |
| Minimum generator size | 1,845 running + 2,200 surge ≈ 4,000+ watts | |
Add a 10–20% safety margin, and never run a generator at 100% load continuously. For a precise build, use the calculator below.
Recommended Backup Power
Safety Warning
Never run a gas generator indoors, in a garage, or near windows; carbon monoxide is deadly. Keep it at least 20 feet from the house. Wattages here are typical estimates that vary by model; check your appliance nameplate for exact figures. Have a licensed electrician install any transfer switch for connecting to home wiring.
Frequently Asked Questions
How do I size a generator for my home?
Add the running watts of everything you run at the same time, then add the single largest starting (surge) wattage among those items. That total is the minimum generator size, before adding a safety margin.
What is the difference between running watts and starting watts?
Running (rated) watts are the continuous power used while operating. Starting (surge) watts are the brief higher spike when a motor-driven appliance like a refrigerator, well pump, or AC first turns on. A generator must handle both.
How many watts does a refrigerator need on a generator?
A typical refrigerator or freezer uses about 700 running watts but can spike to roughly 2,900 total watts when the compressor starts. Plan for that surge so the generator does not overload when the fridge cycles on.
Sources
- Generatorist — Generator Wattage Chart (manufacturer-aggregated running and surge data)
- Generator sizing methodology: running watts + largest single surge (industry standard)
Related Calculators & Guides
- Generator Size Calculator — exact sizing for your appliance list
- Generator Runtime Calculator
- Best Portable Generators 2026
- Emergency Food Calories Reference