How generator sizing works
Every appliance uses two numbers: running watts (power to keep it going) and starting watts (the short surge a motor needs to kick on). A refrigerator might run on 150 watts but surge to 1,200 watts for a second when the compressor starts. Your generator has to survive the biggest surge on top of everything else already running — that's the number cheap calculators get wrong, and it's why people buy generators that trip out on the first night of an outage.
Generator sizes at a glance
- 2,000–3,500W (portable inverter): fridge, lights, phones, TV, CPAP. Quiet and fuel-sipping.
- 4,500–7,500W (portable): the essentials plus a well pump, furnace fan, or window AC. The sweet spot for most homes.
- 9,500–14,000W (large portable / small standby): adds central AC and an electric water heater with careful load management.
- 18,000W+ (whole-home standby): runs nearly everything at once, starts automatically, costs the most.
Frequently asked questions
Will a 7,500 watt generator run a house?
It will run the essentials of most homes — refrigerator, furnace fan, well pump, lights, and outlets — as long as large loads like central AC and electric dryers aren't all on at once.
What size generator do I need for a 3-ton AC?
A 3-ton central AC uses roughly 3,500 running watts but can surge past 10,000 watts at startup. Plan on at least a 12,000–14,000W generator for the AC plus basics, or fit a soft-start kit to cut the surge dramatically.
Is a bigger generator always better?
No. Oversized portables burn more fuel, cost more, and wear faster at light loads. Size for what you'll actually run, plus a 20% buffer — which is exactly what this calculator does.