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.
In short
- Generator size is set by simultaneous load — what runs at the same moment — not by square footage and not by brand.
- The 125% design margin is manufacturer guidance and engineering practice. It is not an NEC requirement, though it is often quoted as one.
- Headroom is where 14 kW fails: a 4-ton AC compressor can draw up to 8,500W at startup, and adding a furnace blower (~800W) plus a refrigerator (~700W) leaves only about 4 kW spare on a 14 kW unit.
- A managed-load setup sheds large loads automatically, which lets a smaller generator protect a whole panel.
Read this if
- You are choosing a size, or checking one an installer has already proposed.
- A quote names a kW figure and you want to know how that number was reached.
- You are weighing a smaller unit plus load management against a larger one.
Skip it if
- You already hold a written load calculation from a licensed electrician — that beats any guide, including this one.
- You have settled the size and are choosing between brands. Go to Generac vs. Kohler.
- Your generator is already installed and will not start. Go to troubleshooting.
What to confirm for your own home
- A written load calculation for your actual panel and appliances — not a size quoted from your square footage.
- Your AC tonnage, and whether a soft-starter is being included to cut startup draw.
- Whether the quote is whole-home or managed-load, and exactly which circuits shed if demand spikes.
- Whether the unit being proposed is air-cooled or liquid-cooled, and why that was chosen.
What this page does not do
It cannot size your generator. Only a load calculation against your actual panel, appliances and service can do that, and in Tennessee that is licensed electrical work. Use this page to check that the number you are given was arrived at properly.
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 — not a 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.
A worked example: why 14 kW runs out of headroom
For a typical 2,500 sq ft home with a forced-air gas furnace and a 3–4 ton central AC, the right answer is usually 22kW. Here is why the 14kW that looks tempting in the specs almost always disappoints during a summer heat wave.
The load headroom problem — most installers don't show you the math
Generac's own published spec for the 14kW Guardian rates it at 14kW on natural gas — the same as LP. But a 4-ton AC compressor draws up to 8,500W at startup. Add a furnace blower (~800W running) and a refrigerator (~700W), and you're over 10,000W in concurrent load during a hot day — with only ~4kW of headroom left on a 14kW unit. One additional appliance cycling at startup and you are at the limit. The 22kW Guardian is rated at 21kW on natural gas (per Generac's published spec) — comfortable margin for a 4-ton home.
Sizing by Scenario
| Your Home Setup | Minimum | Recommended |
|---|---|---|
| All-gas appliances, 3-ton AC | 14kW (tight on NG) | 18–22kW |
| All-gas appliances, 4-ton AC | 18kW | 22kW |
| Add electric water heater | 22kW | 22kW |
| Add Level 2 EV charger | 22kW | 26kW |
| Essentials only (no AC) | 10–14kW | 14kW |
14kW is enough when:
- ✓All-gas appliances + 3-ton or smaller AC
- ✓You add a soft-start kit (~$300) to the AC compressor
- ✓Home under ~2,000 sq ft
- ✓Fine managing loads manually during an outage
22kW is necessary when:
- →4-ton or larger AC (common in 2,500+ sq ft homes)
- →Electric water heater (4,500W continuous load)
- →Whole-home coverage with no load management
- →Natural gas, no soft-start on the AC compressor
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.
As a concrete figure: a 22kW Generac on natural gas demands roughly 327,000 BTU/hr at full load — enough that an undersized gas pipe creates a pressure drop that starves the furnace during an outage. That is why the gas line to the pad is sized to the generator's full BTU demand, not just run to the nearest tap.
For fuel-choice fundamentals — running costs, storage, and cold-weather vaporization — see the natural gas vs. propane comparison.
Air-cooled vs. liquid-cooled
Liquid-cooled generators run at 1,800 RPM vs. 3,600 RPM for air-cooled — dramatically less wear per hour. They make sense when you expect week-long outages, need over 26kW, or have a hard noise constraint. For most suburban installs — where outages run days, not weeks — air-cooled is the right call. The $6,000–$12,000 installed premium is hard to justify for comfort backup in a subdivision.
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 not do: 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.
Five ways homeowners get sizing wrong
Each of these turns up in real quotes. None requires an electrician to spot — only the number the quote is built on.
Sizing by square footage.
InsteadSquare footage does not draw current; appliances do. Size from the six-step load calculation above — an inventory of what must run, its running watts, and its starting watts.
Treating the 125% margin as code.
InsteadIt is manufacturer design guidance and engineering practice. The code section that actually governs residential standby is NEC Article 702 (optional standby systems), and it mandates no particular kW. An installer who calls 125% a code requirement is wrong on a checkable point.
Adding up the starting watts of every motor in the house.
InsteadMotors do not all start at once in a properly configured system. Peak demand is running load plus the single largest starting load. Summing every surge overstates demand and sells a bigger unit than the home needs.
Reading the nameplate as the whole answer and ignoring headroom.
InsteadCheck what is left over. A 4-ton AC compressor can draw up to 8,500W at startup; add a furnace blower (~800W) and a refrigerator (~700W) and a 14 kW unit has roughly 4 kW spare. One more appliance cycling on and it is at its limit.
Assuming the advertised kW is what you get on your fuel.
InsteadRatings are per model and per fuel. Generac rates the 14 kW Guardian at 14 kW on natural gas, but that does not generalise — confirm the figure for the specific model you are quoted, on the fuel you will actually run. See natural gas vs. propane.
Keep going
Already own a generator? The load calculation matters most at replacement — houses gain heat pumps, EV chargers and finished basements, and re-buying the old nameplate by default is the most expensive way to get sizing wrong twice. Availability of the size you land on is a scheduling question rather than a sizing one; when to install covers what that does to the calendar.
A target kW is only half the decision. The other half is the transfer switch and load management — whole-home or managed-load is what actually decides which circuits stay on, and it is the choice that lets a smaller unit cover a whole panel. Then compare the two dominant residential platforms in the Generac vs. Kohler comparison. If you are still deciding whether to buy one at all, is a whole-house generator worth it here puts the cost against the local outage record. For what a real install costs — transfer switch, pad, gas, labor, permits — see the cost guide, and when the proposals come back, how to compare generator quotes is the line-item checklist that makes two numbers comparable. 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 read every request against what installers are actually covering here, and tell you what that coverage looks like. What you want is an installer who does the on-site load calc — not a national call center pushing a square-footage guess.
Prefer to send the full picture up front? Send a Generator Project Brief — coverage objective, fuel, priority loads, and timeline, so the installer's first call starts at the proposal. “Not sure” is an accepted answer throughout.