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. Never backfeed power through a wall outlet.