Generator Size Calculator: What Size Generator Do I Need?
Answers: what size generator do I need?
Home Systems Desk
Covers heating, cooling, generators and water heaters. We maintain the sizing calculators and the guides that explain what a number means before you call a contractor.
10 guides · 1 tools · reviewed by the Data & Methodology Desk
Our standards
- Every formula is shown next to its result.
- Defaults come from US DOE, NEC or manufacturer data, cited on the methodology page.
- We say plainly when a licensed professional is required.
- Safety warnings are never dismissible or softened.
Sources we rely on most
- US Department of Energy (Energy Saver)
- ACCA Manual J and Manual S principles
- National Electrical Code (NFPA 70)
- US Consumer Product Safety Commission
How a guide gets reviewed
- 1Research. Pull the source data and extension or agency guidance.
- 2Write. Answer first, then the reasons and the steps.
- 3Figure check. An editor re-derives every number from its source.
- 4Review. The Data & Methodology Desk signs off.
- 5Re-review. Scheduled each season; changes go in the change log.
Contributors
Named contributors with verifiable credentials will be listed here. This desk publishes as an organization until then.
Tools this desk maintains
Guides from this desk
Home Energy Guide
What Humidity Should a House Be in Winter? 30–40%, Lower on Cold Nights
About 30–40% relative humidity, inside EPA’s 30–50% range, and lower on the coldest nights. Our NOAA estimate of when windows fog in 20 US cities, plus signs of air that is too dry or too damp.
Home Energy Guide
Calculator
How Much Does It Cost to Run a Space Heater? Costs by State and City
A 1,500-watt space heater costs about 27¢ an hour at the 2026 US average electricity price. Costs by state and in 20 cities, and how its heat compares with a furnace or heat pump.
Home Energy Guide
What Temperature to Set Your Thermostat in Winter: 68°F, Then Lower at Night
About 68°F while you are awake and 7–10°F lower at night for a furnace or boiler; a heat pump stays steady. What a setback saves in 20 US cities, plus the 55°F and health floors.
Home Energy Guide
Calculator
At What Temperature Do Pipes Freeze? The 20°F Threshold and Where It Hits
Pipes in unheated spaces are most at risk at about 20°F outside, per a University of Illinois study. How often 20 US cities get there, and what to do before a freeze.
Home Energy Guide
What Temperature Should a Water Heater Be Set At? 120°F, and When to Go Higher
Most homes should run a water heater at 120°F. What DOE, CDC, CPSC, EPA and OSHA each say, how fast hotter water burns, and what a lower setting really saves.
Home Energy Guide
Calculator
What Size Generator to Run a Furnace? Watts for Gas, Oil and Electric Heat
A gas or propane furnace needs about 300–875 running and 1,600–2,350 starting watts. What oil furnaces, electric furnaces and heat pumps draw, from generator charts and furnace spec sheets, with the altitude derate for 15 cold-winter cities.
Home Energy Comparison
Tankless vs Tank Water Heater: Real Savings and Winter Flow Where You Live
Gas tankless beats a minimum-efficiency tank by $90–$210 a year but an ENERGY STAR tank by only $18–$42, and its flow drops with cold winter water. Estimates for 22 cities.
Home Energy Guide
Calculator
Portable Generator Safety: The Rules and the CPSC Data Behind Them
Outdoors only, 20 feet from the house, exhaust away, CO alarms on every level. CPSC records of 789 generator CO deaths show why each rule exists.
Home Energy Explainer
Do Heat Pumps Work in Cold Weather? What the Data Says
Cold-climate heat pumps keep at least 70% of their capacity at 5°F. How cold 21 US cities get, from NOAA normals and design temperatures, plus field results.
Home Energy Comparison
Heat Pump vs Furnace: Which Is Cheaper to Run Where You Live
A heat pump beats a gas furnace on running cost only when its COP tops a break-even set by your rates. Estimates for 20 cities, the formula, and a lookup table.