What Size Generator Do I Need? Running vs Starting Watts, and the 3,500 W Unit That Couldn't Start My Fridge
My neighbour bought a 3,500-watt portable generator the week before an ice storm. He added up what he wanted to run — fridge, furnace fan, sump pump, a few lights — and got about 2,600 watts. Comfortable margin, he figured. The power went out at 2am, he wheeled the generator out, plugged in the fridge, and the generator coughed, bogged down, and tripped its breaker. Twice. He spent the first hour of the outage in the dark with a perfectly good machine that was, on paper, big enough.
The number he missed was starting watts. This guide is for anyone who is about to buy (or already owns) a backup generator and wants to size it properly before the next storm. By the end you'll know how to add up running watts, find the surge that actually decides the size, and use a Generator Size Calculator to land on a number you can trust.
⚠️ Disclaimer
This article is for informational purposes only and does not constitute electrical or safety advice. Generators produce deadly carbon monoxide and must never be run indoors or in a garage. Have a licensed electrician install any transfer switch or inlet, and always check your appliance nameplates for exact wattage.
What Is the Difference Between Running and Starting Watts?
Running watts (also called rated watts) are the continuous power an appliance draws once it is operating, while starting watts (surge watts) are the brief spike a motor or compressor pulls for a second or two when it switches on. That spike is commonly two to three times the running figure, because a stopped motor has to overcome inertia and build magnetic fields from zero.
Heaters, toasters and incandescent bulbs are resistive loads: they need the same power to start as to run. Anything with a motor or compressor — refrigerators, sump pumps, furnace blowers, air conditioners, well pumps — is an inductive load with a big startup kick.
Here are typical figures for the loads people most often want during an outage:
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Furnace fan (1/2 HP) | 875 | 2,300 |
| Central A/C (3 ton) | 3,500 | 7,000 |
| Electric dryer | 5,400 | 6,750 |
The U.S. Consumer Product Safety Commission (CPSC) attributes roughly 70 carbon monoxide deaths a year in the United States to portable generators, which is why sizing is only half the job — placement at least 20 feet from the house matters just as much.
Key Takeaway
A generator has to survive the single worst moment — the instant its biggest motor starts — not just the average load. My neighbour's 2,600 running watts were fine; the fridge's 2,200-watt startup surge on top of everything else was not.
How Do You Size a Generator?
Generator sizing is the process of adding up the running watts of every appliance you want powered at once, then adding the largest single starting surge, to find the minimum output a generator needs. You only add one surge, not all of them, because motors almost never start in the exact same second.
The method used by most manufacturer sizing guides, and by our calculator, is:
"Surge extra" means starting watts minus running watts for that one appliance. The margin (10–25%, with 20% a common default) keeps the generator at about 80% load, which improves fuel economy and reduces wear. Round up to the next common size, usually the nearest 500 watts.
The Department of Energy and most generator makers recommend working from a written list of loads rather than guessing, and recommend never running a unit continuously at its absolute maximum.
A Worked Example: Essentials vs Whole Home
Let's size a generator for a typical storm-prep list. I'll use the figures from the table above plus 150 W for ten LED bulbs, 400 W for a TV, router and phone chargers, and a 1,000 W microwave.
Scenario A — essentials:
| Load | Running W |
|---|---|
| Refrigerator | 700 |
| Sump pump | 800 |
| Furnace fan | 875 |
| LED lights | 150 |
| TV, router, chargers | 400 |
| Microwave | 1,000 |
| Total running | 3,925 |
- Largest surge extra: the fridge, 2,200 − 700 = 1,500 W
- Peak load: 3,925 + 1,500 = 5,425 W
- With a 20% margin: 3,925 × 1.2 = 4,710 W
- The larger figure is the peak, so round up: 5,500 watts
Notice the surge, not the margin, decided the answer. Skip the microwave and the same list drops to 2,925 running, 4,425 peak, so a 4,500-watt unit works. That single appliance costs you 1,000 watts of generator.
Scenario B — add central air conditioning:
- Running: 3,925 + 3,500 = 7,425 W
- Largest surge extra is now the A/C: 7,000 − 3,500 = 3,500 W
- Peak: 7,425 + 3,500 = 10,925 W, so roughly an 11,000-watt generator
Adding one air conditioner doubled the generator size. That's why many people run a window unit instead: a 10,000 BTU window A/C is 1,200 running and 3,600 starting watts, a far smaller step. (To size the cooling side itself, see the BTU Calculator.)
💡 Pro Tip
You don't have to run everything at once. Staggering big loads — fridge on, then microwave only when the furnace fan is idle — lets a smaller generator do the job. Size for what runs simultaneously, not for your whole house.
Watts, Amps and Volts: US, UK and Australia
Amps matter because your generator's outlets and cords have current limits. The relationship is simple:
Our 5,500 W result works out as follows:
| Region | Voltage | Amps at 5,500 W |
|---|---|---|
| US / Canada (outlet) | 120 V | 45.8 A |
| US (240 V circuit) | 240 V | 22.9 A |
| UK / Australia | 230 V | 23.9 A |
A US portable generator typically splits its output between several 20-amp 120 V outlets, so 5,500 W total doesn't mean you can pull 45 A through one socket. Use the Watts to Amps Calculator to check any single appliance, and the Voltage Drop Calculator if your extension cord run is long, since thin cords over 50 feet (15 m) can starve a motor of voltage at startup.
Mistakes That Overload a Generator
I've watched people make the same handful of errors:
- Sizing to running watts only. The story I opened with.
- Trusting a "typical" number for an old appliance. A 20-year-old fridge or a pump with a worn motor can draw far more at startup. Read the nameplate.
- Forgetting resistive hogs. Electric water heaters (4,000 W), space heaters (1,500 W) and dryers (5,400 W) eat capacity fast. Gas appliances are much kinder.
- Running at 100% for days. Leave that 20% margin.
- Plugging into a wall outlet ("backfeeding"). It can electrocute utility workers. Use a properly installed transfer switch or interlock.
⚠️ Note
If you're comparing against a battery power station instead, the maths is different: capacity is in watt-hours, not watts. The Battery Charge Time Calculator and Amp-Hour Calculator help you estimate runtime.
Frequently Asked Questions
What size generator do I need to run my house?
For essentials only (fridge, sump pump, furnace fan, lights, electronics) 3,500–5,500 watts is usually enough. Whole-home backup including central air conditioning typically needs 7,500–12,000 watts.
What can a 5,000-watt generator run?
Usually a fridge, sump pump, furnace fan, lights and electronics together, with a surge ceiling of around 6,250 watts. It will not handle central A/C or an electric dryer at the same time.
Do I add up all the starting watts?
No. Add all running watts, then add only the largest single surge extra (starting watts minus running watts), because motors rarely start at the same instant.
Is bigger always better?
Not necessarily. An oversized generator costs more, burns more fuel at low load and is heavier to move. Aim for roughly 80% load at your realistic peak.
How far from the house should a generator run?
The U.S. Department of Energy and CPSC advise at least 20 feet (6 m) from the home, with the exhaust pointed away from windows and doors, and a battery-powered CO alarm indoors.
Try It Yourself
The lesson from that 2am ice-storm fiasco is simple: size for the worst second, not the average one. Add your appliances to the Generator Size Calculator to get running watts, peak watts and a recommended size in seconds. Then check cord runs with the Voltage Drop Calculator, and see what a long outage might cost in fuel and power with the Electricity Cost Calculator.
Do it this weekend, while the weather is still kind.
