Sizing & Buying Guide
What Size Standby Generator Do You Need? A Homeowner's Sizing Guide
The right size isn't a guess from square footage. Here is the documented method, the two loads that actually decide the number, and why the final size should be confirmed by a professional load calculation.
How Sizing Actually Works
The documented whole-house method is straightforward: a professional records your home's peak kW demand continuously over about 30 days — while the home is occupied, and including the larger of your heating or cooling load — then multiplies that peak by 125% and sizes to the next standard generator size.
Important: the 125% rule is manufacturer guidance, not a code requirement
The 125%-and-next-size-up figure is Generac's recommended optimal practice — it is not a National Electrical Code requirement. Residential optional-standby generators fall under NEC Article 702(optional standby systems). Sizing above the measured peak is prudent engineering; the specific 125% figure comes from the manufacturer's sizing guide, not the code book.
The Two Numbers That Matter: Running vs. Starting Load
Two different loads decide the size, and they behave very differently:
- Running watts — what your appliances draw continuously, added up.
- Starting (surge) load — the brief spike when a large motor kicks on, such as a central AC compressor or a well pump.
Two manufacturer rules you have to respect when you do the math:
- Never add amps directly. Convert amps to kW first. Power factor and mixed voltages change the arithmetic, and adding raw amp numbers gives you the wrong answer.
- Check the single largest motor's starting surgeagainst the generator's surge capability — not just the total running watts.
The plain-language takeaway: an undersized unit can fail to start a big motor load even when the running-watt total looks fine. That is why the largest starting load — not the sum of running watts — often drives the size.
Load headroom — why the smaller unit disappoints
For most Naperville homes — 2,500 sq ft, forced-air gas furnace, 3–4 ton central AC — the right answer is 22kW. Here is why the 14kW that looks tempting in the specs almost always disappoints during a July 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— its published starting surge, per manufacturer compressor specs. 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
The most underused tool in sizing: a soft-start kit
A soft-start kit — the Micro-Air EasyStart is the common one, roughly $300–$400 installed — cuts a central-AC compressor's startup surge by about 65–70%. On a 4-ton compressor that turns an ~8,500W starting spike into roughly 3,000–4,000W. That one change is often what lets a well-chosen 14kW unit comfortably back up a home that looked like it needed a larger generator — and it is far cheaper than buying up a size. By installer consensus it is the most underutilized tool in residential generator sizing.
Surge reduction from installer sources (multiple, independent); the 8,500W starting figure is the verified 4-ton compressor spec (Kohler G18-122).
Don't forget the gas line
Sizing the generator is only half the job — the gas line has to feed it. A 22kW unit on natural gas can demand roughly 327,000 BTU/hr at full load and needs a minimum 1-1/4" black-iron line, so an undersized or shared line can starve the furnace during a cold-weather run. The permits & utility guide covers gas-line sizing and Nicor meter capacity in detail.
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 Naperville 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.
Sizing Ranges
The ranges below are general guidance, not a per-home answer. Your actual size depends on your specific loads and, for optimal sizing, a measured 30-day peak.
| Kilowatt range | Typical coverage |
|---|---|
| ~10–14 kW | Essentials — refrigerator, sump pump, furnace, some lights. |
| ~16–22 kW | Broader whole-home comfort loads. |
| 24 kW+ | Larger homes, heavy HVAC, electric heat, well pumps, or multiple high-demand appliances. |
General guidance based on manufacturer sizing literature. Confirm your home's correct size with a professional load calculation.
Essential Circuits vs. Whole-Home
There are two coverage philosophies, and the choice is really about what you want powered — which then decides the generator size and the transfer switch:
- Managed / essential-circuit backup. A pre-selected set of critical circuits (heat, refrigerator, sump, well, a few outlets). Pairs with a smaller unit and a managed-load transfer switch.
- Whole-home backup. Everything runs during an outage. Requires a larger unit and a whole-home transfer switch.
This choice ties directly to the transfer-switch scope on your quote and to the installed cost. It’s really a transfer-switch decision — our transfer switch & load-management guide shows how load-shedding lets a smaller generator back the whole house. For what changes the install price on either side, see the cost guide.
Plan for the Future
A common piece of installer guidance — professional judgment, not a hard rule — is to add roughly 10–20% headroom for planned future loads: an EV charger, a finished basement, a hot tub, an added heat pump. It is meaningfully cheaper to size up during the initial install than to replace an undersized unit a few years later.
Get It Confirmed
The final generator size should be confirmed by a professional load calculation — performed by an authorized dealer or a qualified electrician — and correlated with local codes and your authority having jurisdiction (AHJ). That is the step the manufacturer's own guidance points to, and it is the step that turns a reasonable range into the right size for your specific home.
The Bottom Line
The honest answer to “what size do I need?” is: size to your measured 30-day peak, respect the starting-surge of your largest motor, decide whether you want essential-circuit or whole-home coverage, add a little headroom for planned future loads, and confirm the final number with a professional load calculation.
Once you have a size, the other two decisions worth making are the fuel and the brand. See the natural gas vs. propane comparison, the Generac vs. Kohler comparison, and the maintenance guide before you commit.
Sources & methodology
Sizing methodology on this page is drawn from the manufacturer's published standby generator sizing guide and the 2023 National Electrical Code (Article 702 for optional standby systems), retrieved June 2026. The kilowatt ranges shown are general guidance; your home's correct size depends on a professional load calculation. Where a figure reflects manufacturer practice rather than a code requirement, we say so.
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