Generator Size Calculator
Size a portable generator from the loads you actually plan to run — with the NEC continuous-load rule and the power your engine loses to elevation both on the page.
Results
Enter hours for at least one appliance to size a generator. Nothing is assumed for you.
How it works
Same load calculation as the inverter tool — a generator's running watts are its continuous rating and its starting watts are its surge rating — plus one thing inverters do not care about: a combustion engine makes less power the higher you go.
nec_continuous_w = noncontinuous_w + continuous_running_w × 1.25
base_required_w = max(nec_continuous_w, running_w × 1.15)
altitude_derate = 1 − 0.035 × altitude_ft / 1000
required_running_w = base_required_w / altitude_derate
required_starting_w = peak_startup_w / altitude_derate
surge_required_w = required_starting_w / 1.25
recommended_size_w = smallest tier ≥ max(required_running_w, surge_required_w)
running_w- Running watts if everything you selected is on at once.
continuous_running_w- Running watts of the loads you ticked as on for 3+ hours at a stretch.
nec_continuous_w- NEC 210.19 mix: noncontinuous watts plus 125% of continuous watts.
base_required_w- max(NEC mix, running × 1.15) — the need at sea level.
altitude_derate- 1 − 0.035 × elevation / 1,000: share of rated power left.
required_running_w- base ÷ derate — the nameplate running watts to shop for.
required_starting_w- peak startup demand ÷ derate — the nameplate starting watts to shop for.
surge_required_w- starting watts ÷ 1.25 — the running rating a unit needs before it has the surge headroom to start this load. The size recommended is the smallest tier at or above both this and the running requirement.
Assumptions
- The 125% continuous factor is NEC 210.19(A)(1) / 210.20(A), and a continuous load is NEC Article 100's three-hours-at-a-stretch test — not three hours spread across a day. The rule is written for conductors and breakers; using it as generator headroom is the same convention, not a code listing.
- The 1.15 floor covers the fact that the appliance table holds representative watts, not your exact units, and that engines lose roughly 1% per 10 °F above 60 °F. It only decides the answer when continuous loads are under about 60% of the running total — above that, 125% already gives more headroom.
- 3.5% per 1,000 ft is what Honda and Generac both print in their owner's manuals, and it applies even after high-altitude carburetor jetting. Without that jetting the loss is larger. The figure is for naturally aspirated engines.
- Ambient temperature is not a separate input. Generac's ~1% per 10 °F above 60 °F is about 4% at the 104 °F top of a portable's rated range, which already sits inside the 1.15 floor.
- Startup surge is a demand estimate: roughly 3× running for motor and compressor loads, 1× for resistive and electronic ones. Your exact motors will differ. Converting that demand into a size uses 1.25× running watts, because that is where consumer generators tend to publish their starting rating — a real unit may publish more or less, so check the datasheet. Sizing on that ratio is the deliberate choice: a generator that carries the running load but cannot start it is the wrong recommendation, so the surge requirement moves the size up rather than only printing a warning.
- Inverter generators produce cleaner power and throttle down under light load, which makes them quieter and thriftier but usually caps them below 5,000 W. Open-frame units go bigger and cost less per watt, at constant engine speed and more distortion. Sizing is the same either way.
- Sizes are a rounded ladder of common portable ratings and are snapped up, never down. A size has to clear the running requirement and the startup requirement together — whichever is higher decides it. A need above 12,000 W on either count returns no recommendation instead of a number we cannot stand behind.
- Carbon monoxide from a portable generator kills people every year, and the exhaust is invisible and odourless. CPSC's rule is outdoors only, at least 20 ft from the house, exhaust pointed away from every opening. A transfer switch or interlock must be wired by a licensed electrician so you never backfeed the grid.
FAQ
What size generator do I need to run my house?
Add the running watts of everything you want on at once, add 25% for whatever stays on three hours or more at a stretch, then check the startup surge. A fridge plus a microwave comes to 1,984 W of running need — a 2,000 W unit on paper — but the fridge compressor's startup pushes the surge demand to 3,175 W, and a 2,000 W generator typically publishes only about 2,500 W of starting power. So the answer is a 3,000 W unit, not a 2,000 W one. Startup, not the running total, is what most calculators leave out.
Does elevation really change what generator I need?
Yes, and by more than people expect. Both Honda and Generac publish a loss of about 3.5% of rated power per 1,000 ft. At 5,000 ft a generator delivers roughly 82.5% of its nameplate, so the fridge-plus-microwave load that wanted 1,984 W of running capacity at sea level wants 2,405 W in Denver, and its 3,175 W startup demand becomes 3,848 W. Both climb enough to move the recommendation one tier up.
Can a battery power station replace a generator?
For the running watts, often yes — and it can run indoors, which no generator can. The two catches are surge and duration: check that the station's surge rating covers your startup demand, and that its kilowatt-hours cover how long the outage lasts. A generator keeps going as long as you keep feeding it fuel.
Sources
- NFPA 70 (NEC) 210.19(A)(1) / 210.20(A) and Article 100 — noncontinuous load plus 125% of continuous load; continuous means three hours or more at a stretch
- Honda Power Equipment — Carburetor jetting for high elevation: horsepower falls about 3.5% per 1,000 ft even with correct jetting
- Generac GP series portable generator owner's manual — maximum power decreases about 3.5% per 1,000 ft of elevation and about 1% per 10 °F above 60 °F
- U.S. CPSC Safety Alert — Portable Generator Hazards: outdoors only, at least 20 ft from the house, never in a garage or shed even with the doors open
- U.S. DOE Energy Saver — Estimating appliance and home electronic energy use (running watts behind the appliance table)
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