Sizing Methodology
Generator Sizing Guide: How to Right-Size a Whole-House Standby
Skip the “pick the biggest one” sales pitch. This is the load-calculation method a licensed electrician uses, the 125% design margin that gets misquoted as code, and where NEC Article 702 actually fits in.
Why sizing matters more than brand
An undersized generator runs at or near its rated output for hours during an outage, which shortens engine life and can trip breakers when a compressor kicks on. Oversizing wastes thousands of dollars up front and raises fuel consumption for no operational benefit. Between those two mistakes sits a right-sized unit, and the only way to land there is a real load calculation, not a square-footage rule of thumb.
The rest of this page walks that calculation, then covers the strategy choice (whole-house vs. managed loads), the code section that governs the install, and the fuel-supply sanity check that most homeowners find out about last.
The six-step load calculation
A licensed electrician performs a version of this on-site before quoting a generator. You can walk it yourself as a sanity check on any quote you receive.
| Step | What you do | Why it matters |
|---|---|---|
| 1. List every circuit that must run during an outage | HVAC (cooling and heating), well pump, sump, refrigerator, freezer, lighting, receptacles, medical equipment, network gear. | A whole-house design still needs an inventory — the transfer switch, ATS ampacity, and load management all depend on it. |
| 2. Record running (steady-state) watts for each load | Use nameplate data from the appliance label or the manufacturer's spec sheet. Do not guess. | Running watts sets the continuous demand the generator must sustain — the number nameplate ratings are calibrated for. |
| 3. Record starting (surge) watts for motor loads | AC compressors, well pumps, sump pumps, and large refrigerators surge to 2×–3× running watts for a fraction of a second at startup. | A generator sized only for running watts can trip on a compressor start if the surge column is ignored. |
| 4. Identify the single largest starting load | Typically the AC compressor or a well pump. Add its surge watts once — motors do not all start simultaneously in a properly configured system. | Standard practice is to size for the largest single motor start plus running load of everything else already online. |
| 5. Sum running load + largest surge — that is the peak demand | This peak, not the arithmetic sum of every surge, is the number a right-sized generator must cover. | Summing every appliance's surge overstates real demand and pushes homeowners into oversized (and overpriced) units. |
| 6. Apply the manufacturer 125% design margin (see note) | Manufacturer sizing tools commonly recommend selecting a unit rated at ~125% of calculated peak demand. | This margin absorbs measurement error, coincident starts, altitude/temperature derate, and long-term motor wear — see the NEC callout below. |
We deliberately don't publish a table of “a 2,500 sq ft home = 22 kW” targets. Two homes with the same square footage can have wildly different peak demand — one on an air-source heat pump with electric resistance backup, one on a gas furnace with a 3-ton AC. Sizing is per-home, not per-square-foot.
About that 125% figure: it's manufacturer guidance, not NEC
You'll see quotes that describe a 125% oversize as “code.” It isn't. The 125% design margin is a widely used manufacturer and design guideline — it appears in Generac and Kohler sizing tools and in electrician rules of thumb — because it absorbs measurement error, coincident motor starts, altitude and temperature derate, and long-term motor wear.
The National Electrical Code section that governs residential standby generators is NEC Article 702, Optional Standby Systems. Article 702 covers things like transfer equipment, wiring methods, and the marking of the disconnect — nota mandatory 125% oversize on the generator you buy. If a quote cites “NEC 125%,” ask which article. There isn't one.
The 125% margin is still a reasonable design target. Just call it what it is: engineering practice, not code.
Whole-house vs. managed loads
You have two legitimate ways to back a modern home. Which one is right depends on your critical-load list, your panel, and — honestly — your budget.
| Dimension | Whole-house | Managed / smart-panel |
|---|---|---|
| What it powers | Every circuit on the panel, subject to the generator's output. | A prioritized subset — critical circuits always on, discretionary loads shed when demand rises. |
| Transfer switch | Service-entrance-rated ATS sized to the full main. | ATS paired with load-management modules (or a smart panel) that drop non-essential loads. |
| Generator size | Sized to peak demand + design margin for the whole home. | Can be materially smaller because the panel sheds load before the generator overloads. |
| Cost driver | Larger generator, larger ATS, larger gas/LP supply. | Smaller generator, but load-management hardware and wiring add cost of their own. |
| Failure mode | Overload if actual demand exceeds design (rare with proper sizing). | Non-essential circuits drop when priority loads run — expected behavior, not a fault. |
The fuel-supply sanity check
Right-sizing the generator is only half the job. A properly sized unit will still stumble if the natural-gas supply can't deliver its full BTU demand — the standby-generator equivalent of a fuel-starved engine. Sizing the pipe from the meter to the pad, and the meter itself, is a separate calculation from sizing the generator. On natural gas that calculation drives whether the utility has to swap the meter for a higher-capacity one. On LP it drives the tank size and vaporization rate — a tank that's technically large enough by gallons can still fail to vaporize fast enough in cold weather.
For fuel-choice fundamentals — running costs, storage, and cold-weather vaporization — see the natural gas vs. propane comparison.
What NEC Article 702 actually covers
NEC Article 702 — Optional Standby Systems is the code section your inspector will apply to a residential standby generator install. It governs the interconnection: the transfer equipment (manual or automatic transfer switch), the wiring methods on the load and source side, the ampacity of the tap or feeder to the ATS, and the required marking of the standby disconnect at the service.
What Article 702 does notdo: it does not tell you what kW to buy. That stays a design decision driven by the load calculation above. If a contractor cites “the NEC” as the reason for a specific generator size, ask them which article and section — the honest answer is Article 220 (load calculation methodology) or Article 702 (installation requirements), not a code-mandated kW figure.
Keep going
Once you have a target kW, compare the two dominant residential platforms in the Generac vs. Kohler comparison. For what a real install costs — transfer switch, pad, gas, labor, permits — see the cost guide. And once it's installed, the maintenance and exercise schedule is what keeps the warranty intact.
Ready for a real load calculation?
Tell us about your home. We connect you with established local installers who will do the on-site load calc — not a national call center pushing a square-footage guess.