How it works
Running W = Σ(running × qty). Peak demand = running total + the single largest starting (surge) watts. Recommended kW = peak demand × (1 + headroom%) ÷ altitude factor.
Running watts is every appliance added up — that is the continuous load the generator carries. Peak demand is the worst instant: all running loads plus the one biggest starting surge, because in practice only one motor inrushes at a time (two compressors do not start in the same millisecond), so adding *every* surge — as most competitors do — wildly oversizes the unit. Headroom (20–25% typical) keeps the generator off its red-line, and the altitude factor reduces rated output by about 3% per 1000 ft above 500 ft, since a naturally aspirated engine makes less power in thinner air. The kVA figure divides kW by the power factor, because generators are nameplated in both kW and kVA and the kVA ceiling can bind first on motor-heavy loads. If you size up the conductors for a long feeder, also check the run for voltage drop on the circuit.
Code references
- Generators — scope, marking, output terminals NEC 2023, Article 445 — Generators
- Output conductor ampacity — at least 115% of nameplate current NEC 2023, 445.13(A)
- Disconnecting means for generators NEC 2023, 445.18
- Optional standby systems NEC 2023, Article 702
- Transfer equipment required for permanent standby systems NEC 2023, 702.5
- Manual transfer switch / interlock for portable connection NEC 2023, 702.12
FAQ
What is the difference between running watts and starting watts?
Running watts (also called rated or continuous watts) is the power a device draws while it runs steadily. Starting watts (surge watts) is the much larger, momentary spike a motor needs to overcome inertia at start-up — typically two to three times running watts for a fridge, sump pump, well pump or air conditioner. A generator must supply the running watts of everything that is on, plus the starting surge of whatever turns on next. Size only for running watts and the generator stalls when the first motor starts.
Do I need a transfer switch? NEC 702 explained
Yes, if you connect the generator to your home’s wiring. A permanently installed standby generator requires automatic transfer equipment that disconnects from the utility before the generator energizes the panel (NEC 702.5). A portable generator may use a manual transfer switch or a listed breaker-interlock kit (NEC 702.12). What you must never do is back-feed through a dryer outlet with a double-male "suicide cord" — it energizes the utility line and can kill a lineman. Powering devices through extension cords from the generator outlets needs no transfer switch.
How do I size the wire from my generator? NEC 445.13 rule
NEC 445.13(A) requires the conductors from the generator output terminals to the first overcurrent device to have an ampacity of at least 115% of the generator’s nameplate current rating. Enter the nameplate amps in the optional field and the calculator returns that minimum ampacity; then pick a wire size with the wire ampacity chart. The 115% factor exists because a generator can run continuously at full output, so its output conductors get the same continuous-duty uplift as a continuous load.
How much does altitude affect generator output?
Naturally aspirated engines lose roughly 3% of their rated output for every 1000 feet above about 500 feet (and more in high heat), because thinner air means less oxygen for combustion. A 7 kW generator rated at sea level may only deliver around 6 kW at 5000 feet. Enter your site altitude and the calculator increases the recommended size to compensate. This is the EGSA / manufacturer rule of thumb; check the specific engine’s derating curve for a permanent install.
What size generator do I need to run a 3-ton AC unit?
A 3-ton (36,000 BTU) central AC compressor runs around 3500–5000 watts and surges to roughly 12,000–16,000 watts on a hard start — which is why a soft-start kit is often added to make it generator-friendly. On its own a 3-ton unit needs a generator in the 12–16 kW range for the surge unless a soft starter cuts the inrush. Add the unit as a row with its running and starting watts, plus anything else you run at the same time, and the calculator gives the combined size.
Why does the tool only add the largest starting surge, not all of them?
Because motors do not all start in the same instant. The realistic worst case is the full running load with one motor inrushing — the biggest one. Adding every appliance’s surge together (as many sizing forms do) assumes a simultaneity that never happens and pushes you toward a needlessly large, expensive generator. If you genuinely start two big motors together, list the combined load as a single row.
This calculator provides a sizing estimate only. Appliance watt values vary by model — use your equipment nameplates — and final generator selection, transfer-switch type, conductor sizing and grounding (NEC 250.30 for separately derived systems) must be verified against the current NEC edition and local code with a licensed electrician.