Generator Size Calculator: What Size Generator Do I Need?
Answers: what size generator do I need?
Home Energy
Heating costs, cold-weather heat pump performance, tankless vs tank water heaters, generator sizing and safety, worked out for the climate you live in.
- 10 guides
- 1 tool
- 3 topics
- Reviewed Oct 3, 2026
Set your ZIP to see dates for your area.
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Home Energy, one topic at a time
Heating & cooling
What to set the thermostat to, heat pump or furnace costs where you live, what a space heater… 6 guides
Generators
Portable and standby generator sizing, running vs starting watts, what a furnace needs, and safe placement outdoors. 2 guides
Water heaters
How tank, tankless and heat pump water heaters compare on cost and efficiency, how cold winter tap water… 2 guides
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Home Energy tools
The home energy year
What to do, month by month
Typical timing for most of the US, starting this month. Set your ZIP and the guides shift every date to your frost dates.
- OctNow Check the water heater setting Most homes: 120°F, the setting DOE’s FEMP, ENERGY STAR and CPSC name.
- Nov Test CO alarms as heating starts Where alarm status was known, 83% of generator CO deaths had no alarm (CPSC).
- Dec Ask for capacity at your design temperature Not the nameplate. Contractors size with ACCA Manual J.
- Jan Watch your heat pump’s backup heat In a DOE field test, backup heat ran about a quarter of the time at −10°F to 0°F.
- Feb Tankless flow is lowest now Tap water is coldest in February: a 5.8 gpm gas unit gives about 4.8 gpm in Minneapolis.
- Mar Size a backup generator Add running watts, then the single biggest starting surge.
- Jun Size a generator before storm season Know your running and starting watts ahead of time.
- Jul Check your generator’s altitude loss Engines lose about 3.5% of their power per 1,000 ft.
- Aug Hurricane-season generator check Run it outside only, at least 20 feet from the house.
- Sep Heat pump or furnace this winter? At state-average prices, gas was cheaper in 13 of the 20 cities we modeled.
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This month for your home energy
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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.
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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.
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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.
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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.
About this section
How we cover home energy
Energy advice goes wrong when it ignores where the house is. A heat pump that is the cheaper choice in Atlanta can lose to a gas furnace in Detroit, and a generator that is big enough at sea level can come up short in Denver, where thin air cuts its output by almost a fifth. Every guide and calculator here starts from your local climate, then the loads you actually run.
We use NOAA 1991–2020 normals for the station nearest your ZIP, U.S. Energy Information Administration prices, Department of Energy and national-lab field data, CPSC safety data and manufacturers’ own manuals and spec sheets, cite each one, and say plainly where a licensed contractor or electrician has to take over. Our estimates help you ask better questions; they are not a Manual J load calculation or a permit.
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