What Size Generator Do I Need? The Complete Wattage Guide
Figuring out what size generator you need comes down to one thing: adding up the wattage of everything you want to run at the same time. Get that number wrong in either direction and you're either lugging around a machine that's overkill, or watching your generator shut down the moment you flip on the microwave.
The #1 Mistake: Confusing Running Watts and Starting Watts
Running Watts
The continuous power an appliance draws during normal operation. This is the number most people look at — and it's only half the story.
Starting Watts (Surge)
The spike of power needed when an appliance first starts. Electric motors (fridges, AC, sump pumps, power tools) require 1.5–3× their running watts to start.
Appliance Wattage Reference Table
Common Household Appliances
| Device | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator (full-size) | 150–400W | 800–1,200W |
| Chest freezer | 100–200W | 500–900W |
| Window AC (5,000 BTU) | 500W | 1,500W |
| Window AC (10,000 BTU) | 1,000W | 2,200W |
| Central AC (2.5 ton) | 3,500W | 7,500–8,000W |
| Microwave (1,000W) | 1,000–1,500W | No surge |
| Coffee maker | 600–1,200W | No surge |
| LED lights (10 bulbs) | 60–100W | No surge |
| Box fan | 100–200W | No surge |
| Ceiling fan | 65–175W | No surge |
| 55" LED TV | 80–150W | No surge |
| Laptop | 50–100W | No surge |
| Phone/tablet charger | 5–20W | No surge |
Power Tools
| Device | Running Watts | Starting Watts |
|---|---|---|
| Circular saw (7-1/4") | 1,400–1,800W | 2,300–4,500W |
| Miter saw (10") | 1,800–2,400W | 3,600–4,800W |
| Table saw (10") | 1,800W | 4,500W |
| Air compressor (1 HP) | 1,000–1,500W | 3,000–4,500W |
| Air compressor (2 HP) | 1,500–2,000W | 4,500–6,000W |
| Shop vacuum | 600–1,400W | No surge |
| Angle grinder | 800–1,000W | No surge |
Medical Devices
| Device | Running Watts | Starting Watts |
|---|---|---|
| CPAP (no humidifier) | 30–60W | Minimal |
| CPAP (with heated humidifier) | 60–250W | Minimal |
| Home oxygen concentrator (5L) | 150–300W | No surge |
| Nebulizer | 75–175W | No surge |
Sump Pumps & Utilities
| Device | Running Watts | Starting Watts |
|---|---|---|
| Sump pump (1/3 HP) | 300–500W | 800–1,300W |
| Sump pump (1/2 HP) | 500–900W | 1,400–2,150W |
| Well pump (1/2 HP) | 750–1,000W | 1,700–2,000W |
| Well pump (1 HP) | 1,000–1,500W | 3,000–4,000W |
| Garage door opener | 350–600W | 900–1,200W |
Always Add 20–25% Headroom
Efficiency Loss
Generators shouldn't run at 100% rated output continuously. 80–85% is a reliable long-term operating point.
Startup Stacking
Even staggered, a motor starting while others run creates a surge. You need margin above your peak calculation.
Future Flexibility
Needs change. You'll add another appliance or tool. Being at the edge of capacity with no margin is a problem waiting to happen.
Generator Size Classes Explained
Camping, tailgating, phones/fans/lights. Not for refrigerators or AC.
Small RV AC, fridge, lights, CPAP. Home emergency essentials.
Most common homeowner class. Fridge, sump pump, lights, small tools.
Adds window AC alongside home essentials. Job site with one large tool.
Central AC territory. Heavy contractor use. Consider a standby generator.
5 Use Cases with Specific Recommendations
1. Camping / Tailgating
Recommended: 1,000–2,000W inverterPhone chargers, LED lights, a fan, and a small cooler total around 350W running. A 2,000W inverter gives you headroom for a small coffee maker or induction burner. Go inverter — quieter, fuel-efficient, and safe for electronics.
2. RV Travel
Recommended: 3,000–3,500WA 13,500 BTU rooftop AC (1,800W running / 3,200W starting) is the anchor load. A 3,500W generator handles it — but just. Don't run the microwave simultaneously with the AC on a 3,000W unit. For a 15,000 BTU AC, step up to 4,000W.
3. Home Backup — Essentials Only
Recommended: 3,000–3,500WRefrigerator + sump pump + lights + fans + CPAP = ~1,550W running. But the sump pump starting while the fridge compressor runs demands ~2,500W peak. A 3,500W unit handles this comfortably; a 2,000W unit is risky.
4. Home Backup — Comfortable (with AC)
Recommended: 5,000–6,500WAdding a 10,000 BTU window AC pushes running load to ~2,700W and worst-case peak to ~4,000W. A 5,500W generator with ~6,875W peak handles this cleanly and lets you run the microwave briefly.
5. Job Site / Contractor
Recommended: 7,500W+ (10,000W+ peak)A 2HP air compressor starts at ~6,000W surge. That alone demands a generator with serious peak capacity. Running it alongside lights, chargers, and occasional power tool use requires 7,500W+ rated output and a 10,000W+ surge rating.
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