What Size Generator Do I Need?
Sizing a generator correctly is a math problem, not a guess. Buy too small and it can't handle your appliances; buy too big and you're paying for capacity, fuel burn, and noise you don't need.
Step 1: List what you want to power
Start with the essentials: refrigerator, a few lights, phone chargers, maybe a sump pump or window AC unit. Full whole-home backup means listing everything on your electrical panel, which usually calls for a much larger generator and a licensed electrician's help.
Step 2: Add up running watts
Every appliance has a running (or 'rated') wattage — check the nameplate or manual. Add these together for everything you plan to run simultaneously.
Step 3: Account for starting watts
Motors — refrigerators, well pumps, sump pumps, AC units — briefly draw significantly more power to start than to run, sometimes 2-3x their running wattage. Find the single largest starting-watt draw among your appliances and add it on top of your total running watts. That combined number is your minimum generator size.
Step 4: Add a buffer
Generators lose some output over their lifespan and perform worse in extreme heat or at altitude. Sizing 10-20% above your calculated minimum gives headroom and reduces strain on the generator.
Typical sizing ranges
Camping and electronics: 1,000-2,500 watts is usually enough for lights, phones, and a small cooler.
Partial home backup (fridge, lights, a few circuits): 3,000-5,000 watts covers most households.
Larger home backup (adding a well pump or window AC): 5,000-7,500 watts is a common range.
Whole-home backup: often 7,500+ watts, and best planned with an electrician who can review your panel.
Frequently Asked Questions
- How many watts does a refrigerator need?
- Most refrigerators run at 100-400 running watts but can briefly spike to 1,200+ starting watts, which is why sizing for starting watts matters even if the fridge is 'small.'
- Can I just buy the biggest generator I can afford?
- You can, but you'll pay more upfront and burn more fuel per hour than necessary. Sizing to your actual needs plus a buffer is usually the better value.
- Do I need an electrician to use a portable generator?
- Not for basic extension-cord use with a few appliances. If you want the generator to power your home's wiring and circuits, yes — a licensed electrician should install a transfer switch or interlock kit. Never backfeed power through a wall outlet. See our best generator transfer switches roundup for how the hardware options compare.
- What if noise matters more to me than total output?
- If you're mainly powering camping gear, an RV, or a few electronics and want to keep noise down, a quiet inverter generator is usually a better fit than sizing up a conventional model. See our best quiet portable generators roundup for picks built specifically to run quiet.
- My generator is sized correctly but won't start — what's wrong?
- Sizing and starting are separate problems. If it's properly sized for your load but won't fire up at all, see our generator won't start troubleshooting guide, which covers stale fuel, switch positions, and the low-oil sensor — the most common causes.