The generator salesman's math looks clean on paper: add up the wattage stamped on your appliances, round up, buy a box. Then the well pump kicks on for the first time during a real outage, the engine bogs down, a breaker trips, and the house goes dark again — this time with a five-figure invoice already signed. The stamped running wattage was never the number that mattered. What actually sizes a whole-house generator is the split-second spike a motor pulls the instant it starts, not the number it settles into once it's spinning. Get that one distinction right and the rest of the job is arithmetic.
Running Watts vs. Starting Watts: The Number the Nameplate Doesn't Tell You
Any motor-driven load — a compressor, a pump, a blower — pulls several times its running current for a fraction of a second while the rotor accelerates from a dead stop. Depending on whose spec sheet you're reading, that spike is called starting watts, surge watts, or locked-rotor watts. Resistive loads (a water heater element, an oven coil, a dryer's heating element) don't do this; they draw the same number the instant they're switched on. Motors are the entire sizing problem.
A 1 HP submersible well pump is the textbook case of how badly this gets underestimated. Running, on its 240V circuit, it draws roughly 1,000W — small enough to write off. Starting, for well under a second, it can pull 3,500–4,000W while the impeller spins up against a full column of standing water. Size a generator to the running number and that pump will brown out the panel, stall the engine, or trip the generator's own overload protection every single time it cycles — which, on a well, is dozens of times a day.
| Load | Running watts | Starting (surge) watts |
|---|---|---|
| Central AC, 3-ton (240V) | 3,500–4,500W | 9,000–12,000W |
| Well pump, 1 HP (240V) | ~1,000W | 3,500–4,000W |
| Sump pump, 1/2 HP | 800–1,050W | 1,300–2,150W |
| Furnace blower motor | 600–800W | 1,500–2,350W |
| Refrigerator / freezer | 150–400W | 800–1,200W |
| Electric water heater | 4,500W | 4,500W (resistive — no surge) |
| Electric clothes dryer | ~5,000W | ~5,000W (resistive, small motor add) |
| Microwave | 1,000–1,500W | 1,500–2,000W |
Add It Up the Right Way: Running Total, Plus the Worst-Case Surge
The mistake in the other direction is summing every load's starting watts, which grossly oversizes the generator and the invoice with it. The rule installers actually use: total the running wattage of everything that could plausibly be on at once, then swap out the running number of the single largest motor and replace it with that motor's starting number. That's the worst moment a generator has to survive — everything else already spinning at steady state, one big motor slamming on top of it.
Worked example, a 2,600 sq ft home on a private well with gas heat and central air:
| Circuit | Running watts |
|---|---|
| Lighting & general receptacles | 1,800W |
| Refrigerator | 200W |
| Furnace blower | 700W |
| Well pump | 1,000W |
| Sump pump | 800W |
| Microwave | 1,300W |
| TV, electronics, misc. | 600W |
| Central AC | 4,000W |
| Running total | 10,400W (10.4kW) |
The AC has the biggest surge of anything in the house, so it's the worst-case trigger: swap its 4,000W running number for a mid-range 10,500W starting number, and the panel needs to survive 10,400 − 4,000 + 10,500 = 16,900W — call it 17kW — for the instant the compressor kicks on, on top of a steady 10.4kW running load. That gap between "runs fine all day" and "survives the worst instant" is the whole sizing exercise.
Why You Don't Need to Buy a 30kW Generator for a 2,600 sq ft House
You'd think a 17kW instantaneous hit means shopping for a 30kW machine with room to spare. In practice, whole-house standby units solve this in software, not brute-force iron. A load management module (Generac calls theirs a Smart Management Module) sits between the transfer switch and the panel and staggers starts: it holds the AC compressor off for a couple of seconds while the well pump starts, or briefly sheds the dryer while the AC cycles, then restores everything once the surge has passed. That's how a 10.4kW running / ~17kW peak load profile like the one above lands comfortably on an 18–20kW standby unit with a management module, rather than a 30kW unit sized to survive every motor starting at once — a scenario the module makes sure never actually happens.
| Home profile | Typical Generac-class size |
|---|---|
| Small home, gas heat, municipal water, one small AC | 10–14kW |
| Mid suburban home, one central AC, municipal water | 16–18kW |
| Well pump + central AC + normal appliance load (the classic "whole house" case) | 20–22kW |
| Larger home, two AC zones, well, electric range, hot tub | 24–26kW |
| Large home/estate, dual-zone AC, well, pool, EV charger, workshop | 32–48kW, often three-phase |
22kW and 24kW are the sizes Generac itself markets as "whole house" for a reason: that's the sweet spot for the well-pump-plus-central-AC profile above, which describes a very large share of suburban housing stock.
Sizing the Transfer Switch (This Is Not Optional)
You cannot extension-cord a generator into a wall outlet and call it backup power. Without a transfer switch physically isolating the house wiring from the utility line, your generator back-feeds through the meter and re-energizes the "dead" line outside — the exact line a utility lineworker may be handling a few blocks away, believing it's de-energized. That is not a paperwork technicality; it is the reason transfer switches are mandatory everywhere, not an upsell.
The Switch Is Sized to Your Service, Not Your Generator
An automatic transfer switch (ATS) is rated in amps to match the home's electrical service — not the generator's kW rating. A 22kW unit on a 200A service typically pairs with a 200A, service-entrance-rated ATS, which can double as the main disconnect and host the load-management logic. On a smaller 100A service, that same 22kW generator only needs a 100A-rated switch — and can back up literally everything the panel carries.
In the US, NEC Article 702 governs optional standby systems and requires listed transfer equipment on every installation, full stop — no unlisted knife switches or backfeed cords wired around code. Expect a permit and a local AHJ inspection. All-in, a straightforward 22kW installation — unit, transfer switch, pad, gas line, permit and labor — typically runs $10,000–$16,000, occasionally $20,000+ if the gas line needs a long trench or the panel needs an upgrade. The generator itself, at roughly $6,500–$8,000 MSRP, is usually only around half of that final number. See the full line-item cost breakdown for exactly where the rest of that money goes.
In Europe, the safety principle is identical — an interlocked change-over switch is required under national wiring regulations (Germany's VDE 0100, France's NF C 15-100, and equivalents elsewhere) — but the starting electrical layout is different. Many homes run on a 230V single-phase supply fused around 25–63A, roughly 6–14kW of connected capacity: plenty for a single-phase standby generator to cover completely. Newer builds, and any home with a heat pump, induction range, or EV charger, are commonly wired 400V three-phase instead, often fused at 3×25A — √3 × 400V × 25A ≈ 17.3kVA of connected capacity split across three separate legs. A single-phase generator physically cannot power all three legs at once. Either the home needs a three-phase-capable unit, or the owner accepts partial backup — one leg, roughly a third of the panel — rather than the whole house. That's the single most common surprise for a European homeowner shopping a US-style "whole house" spec sheet.
Key number
On a typical 22kW installation, the generator itself is often only around half — sometimes less — of the $10,000–$16,000+ installed price. The rest is the transfer switch, gas line, concrete pad, electrical work, and permit. Budget the installation, not just the unit.
Generator vs. Battery: Which Actually Backs Up a Whole House?
The other decision buyers stall on is fuel vs. stored charge. Both back up a house; they fail differently. For the deeper trade-offs beyond the summary below, see our full generator vs. battery comparison.
| Standby generator (Generac-class) | Whole-home battery (Powerwall-class) | |
|---|---|---|
| Runtime on an outage | Unlimited on a natural gas line; days on a large propane tank | Hours on stored charge alone — a 13.5kWh unit covers modest loads for roughly a day; a well pump plus AC drains it in hours |
| Refuels itself? | Yes on natural gas; propane needs a delivery for very long outages | Only if paired with solar — otherwise it's a fixed tank of electrons |
| Starting a well pump or AC | Handles the surge easily once correctly sized | Needs its own oversized inverter to survive the same motor-start surge |
| Noise & footprint | Audible engine noise, combustion exhaust, needs window clearance | Silent, no exhaust, mounts flush against the house |
| Upfront cost at whole-home scale | Lower per kW of backup capacity | Higher per kWh of storage; often needs multiple units for full-house coverage |
| Maintenance | Annual service, periodic oil changes, weekly self-test cycle | Effectively none |
For most suburban buyers weighing the two, the honest answer comes down to how long outages actually run in your area and whether solar is already in the picture:
- Outages are short — hours, not days — and you already have solar: a battery alone can cover you quietly, no fuel involved.
- You're on a well, run central AC, or outages regularly run past a day: a standby generator is the only thing that doesn't run out.
- You want silence for daily solar use and security for the once-a-year multi-day event: increasingly, buyers spec both — battery for the everyday, generator for the storm.
Run your own numbers before you spec anything against your actual panel — the Backup Power Sizing Calculator walks appliance-by-appliance and outputs a running kW, a worst-case surge kW, and a matching transfer-switch amperage in about five minutes.
Open the Backup Power Sizing Calculator →
Once you know your running kW and surge kW, matching it to hardware is the easy part. Here's what that looks like where you are:
Frequently Asked
Can a 22kW generator run a whole house?
For most homes matching the profile above — a well pump, one central AC zone, and normal appliance load — yes, especially paired with a load management module. It gets tight if you also run electric-resistance heat, a second AC zone, a hot tub, or an EV charger at the same time; those loads usually push the right answer to 26kW or larger, or call for a module that sheds a lower-priority circuit during startup.
How many watts do I need to run a well pump and central air at the same time?
Using the worked example above: roughly 10.4kW of continuous running load across a typical suburban house, with a worst-case instantaneous peak near 17kW the moment the AC compressor starts on top of everything else already running. That combination of running-plus-surge is what a correctly sized 20–22kW standby unit is built to survive.
What size transfer switch do I need for a whole-house generator?
The switch is sized to your electrical service, not the generator. In the US, that means a 100A or 200A automatic transfer switch matching your panel's main breaker, regardless of whether the generator itself is 14kW or 26kW. In Europe, the equivalent is an interlocked change-over switch rated to your supply fuse, sized per single-phase or three-phase configuration.
Do I need a permit for a whole-house generator?
Almost everywhere, yes. In the US, the local Authority Having Jurisdiction inspects the gas line, the electrical work, and the transfer switch under NEC Article 702. In Europe, national wiring regulations require a registered electrician's sign-off on the interlock. A vetted local installer already knows the paperwork for your specific address — it's one more reason not to DIY this one.