Generator sizing
Generator Sizing Guide & Calculator
Most homeowners size a generator by adding up running watts. That's how they end up with a 5,000W unit that browns every time the fridge compressor kicks in. The NEC 702 rule is: running watts × 1.25 + the largest single motor's starting surge. That single formula prevents 90% of sizing mistakes.
Did you know? NEC 702 doesn’t require you to add every motor surge — only the largest. Adding all of them pushes you to a 25kW standby when a 12kW portable covers the same load. The math is conservative, not paranoid.
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Generator sizing FAQ
How do I size a generator for my house?
Add up the running watts of everything you want to power simultaneously, then add a 25-30% buffer for startup surges. Motors (fridge, AC, well pump) draw 3-7× their running wattage at startup. Essential-circuit coverage typically needs 5,000-7,500 watts; whole-house coverage needs 15,000-25,000 watts. Use our generator calculator for an exact recommendation.
What can a 5,000-watt generator run?
A 5,000-watt generator typically runs: refrigerator (700W), furnace fan (800W), lights (200W), phone chargers (50W), sump pump (1,000W), and a microwave (1,000W) — but not all at once. With smart load management, you can cover essential circuits during a 6-8 hour outage. Total peak surge capacity: ~6,500 watts for brief motor starts.
Portable or standby generator?
Portable generators ($500-2,000) need manual setup, run on gasoline (8-12 hour runtime per tank), and provide 3,000-10,000 watts. Standby generators ($5,000-15,000 installed) auto-start during outages, run on natural gas or propane (unlimited runtime), provide 8,000-25,000 watts, and require professional installation with a transfer switch per NEC 702.
What size generator for central AC?
Central AC (3-5 ton) needs 5,000-10,000 running watts plus a 3-5× startup surge. A 3-ton unit typically needs a 10,000-12,000 watt generator. Window ACs (5,000-12,000 BTU) only need 1,000-2,500 watts. Mini splits (9,000-24,000 BTU) need 1,500-4,500 watts — much more efficient than central AC on generator power.
How long does a generator run on a tank of gas?
Runtime depends on tank size and load. A typical 5,000W portable with a 6-gallon tank runs 8-12 hours at 50% load, 6-8 hours at full load. Standby generators on natural gas have unlimited runtime; on propane, a 500-gallon tank provides ~7-10 days at typical household load. Diesel portables get 20-30% better fuel economy than gasoline.
Do I need a transfer switch?
Yes — for any generator connected to your home's electrical panel. A transfer switch prevents backfeed that can electrocute utility workers and is required by NEC 702. Manual transfer switches ($300-500) work for portables; automatic transfer switches ($1,000-3,000) are built into standby generators. Never use a "suicide cord" (male-to-male plug) — it's illegal and dangerous.
Standards used: NEC 702 (NFPA 70) sizing guidance • DOE Appliance Energy Use Chart • Manufacturer spec sheets (Generac, Honda, Champion) • Editorial policy • Methodology
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