What Size Generator Do I Need for a House During an Outage?
For essentials only — a refrigerator (~400-800 running watts), some lights, and one motor load like a furnace blower or sump pump — a portable generator in the roughly 3,000-5,000 running-watt class is a typical fit; backing up most of a panel, including a well pump or central air conditioning, pushes you into the 8,000-10,500 running-watt class connected through a transfer switch installed by a licensed electrician.
Start from your loads, not the box
Generators are marketed on starting (surge) watts — the brief spike a motor draws at power-on — while running (rated) watts is the smaller number that limits continuous use. Motor-driven appliances (fridges, well pumps, sump pumps, AC, furnace blowers) surge roughly 2-4x their running draw; resistive loads like lights barely surge. Add the running watts of everything you want on at once, then check the single largest starting-watt load against the generator's surge rating. The Startup Watt Index lists both numbers for 12 common appliances.
The three sizing tiers
- Essentials (~3,000-5,000 running watts): refrigerator, lights, phone charging, plus one mid-size motor load such as a furnace blower or sump pump — run on extension cords rated for the load.
- Well pump territory (~5,000-8,000 running watts): a 1/2 HP well pump needs about 900-1,500 running and 1,500-3,000 starting watts, and a deep-well submersible can need up to 6,000 starting watts on top of your other loads.
- Most-of-panel backup (~8,000-10,500 running watts): units like the 8,000-10,500 rated-watt class can carry several circuits at once through a manual transfer switch — and central AC's startup surge (roughly 7,000-18,000 watts depending on compressor LRA) is the deciding factor at this tier. See the central AC sizing guide for the LRA × voltage × 1.2 formula.
Our home backup buying guide compares specific units in the top tier by their rated watts. You can also browse current home-backup portable generators on Amazon once you've worked out your running-watt total.
Safety is part of sizing
At the whole-panel tier, a licensed electrician and a transfer switch are the safe, code-compliant way to connect a generator to home wiring — never back-feed a panel through a regular outlet. And at every size, gasoline generators produce carbon monoxide, an odorless gas that can be fatal in minutes: run the unit fully outdoors, away from windows, doors, and vents, never in a garage even with the door open. See how far a generator should be from a house.
This page gives typical planning ranges compiled from published sizing guidance, not a guaranteed figure for your home — check each appliance's nameplate and size with an electrician for panel-level backup.
Frequently Asked Questions
For essentials only — a refrigerator (~400-800 running watts), some lights, and one motor load like a furnace blower or sump pump — a portable generator in the roughly 3,000-5,000 running-watt class is a typical fit. Backing up most of a panel, including a well pump or central air conditioning, pushes you into the 8,000-10,500 running-watt class connected through a transfer switch installed by a licensed electrician.
The big number is starting (surge) watts — the brief spike a motor draws for a fraction of a second at power-on. Running (rated) watts is the smaller number that limits what you can power continuously, and it's usually buried in the fine print. Size your continuous load against running watts and your worst single motor start against starting watts.
Only the largest portable units. Central AC typically needs about 3,500-6,000 running watts and a startup surge of roughly 7,000-18,000 watts depending on the compressor's Locked Rotor Amps (LRA). The documented sizing formula is starting watts ≈ LRA × voltage × 1.2 — a 60A LRA compressor at 240V needs roughly 17,000 surge watts, which rules out most portables.
No. Add the running watts of everything you want on at once for your continuous total, then size surge capacity to the single largest starting-watt load, since motors don't typically hit their startup instant at the exact same moment. Staggering what you switch on first reduces instantaneous demand further.
Through a manual transfer switch installed by a licensed electrician — never by back-feeding a panel through a regular outlet, which is dangerous and not code-compliant. And any gasoline generator must run fully outdoors, away from windows, doors, and vents, never in a garage even with the door open, because of carbon monoxide risk.