A standby generator is sized to the loads you actually intend to run — and to the split-second surge when their motors start. Before you shop by kilowatts, you decide what stays on, add up the watts, and pick a fuel and a transfer switch to match. Here's how to do each step honestly.
Skip a step and you either overpay for capacity you never use or trip the unit the first time the AC and well pump start together. Work through them in sequence.
Decide whether the generator carries the entire panel or a curated set of essential circuits. This single choice moves the target size — and the price — more than anything else below.
Total the continuous draw of everything that can run at once, then add the biggest motor-starting surge on top. Motors — compressors, pumps — pull two to three times their running watts for a moment at startup.
Confirm your fuel supply can actually feed the unit at full load, and account for the small power derate natural gas carries versus propane. Fuel choice affects the deliverable kilowatts, not just convenience.
A transfer switch isolates your home from the grid so the generator can never backfeed the utility line. Match its type and amperage to the scope you chose in step one.
Neither is "better." Whole-home buys convenience; critical-loads buys a smaller, cheaper unit. Load management sits in between — it lets a mid-size generator behave like a whole-home unit by staggering big loads.
Powers a chosen subset through an emergency sub-panel: heat, refrigeration, water, and key outlets. Smaller footprint, lower cost, less fuel burned per hour.
Carries the full service so nothing changes during an outage — including central air conditioning, which is usually the largest single load in the house.
Tip: a soft-start kit on the AC compressor and a load-shedding module can let a smaller generator do whole-home work by preventing two big motors from starting at the same instant.
Check what you want to keep running. The ledger sums continuous watts, then adds the single largest starting surge — the way a generator actually experiences peak demand. Figures are typical planning estimates; always verify against your equipment's nameplate.
Items marked SURGE contain a motor that spikes at startup.
Select the loads you plan to back up. The suggestion rounds peak demand up to a common residential size and leaves headroom.
Most home standby units run on either fuel, and many are field-convertible. The trade-off is supply independence versus refueling — plus a real difference in energy density that shows up as deliverable power.
| Consideration | Natural gas | Propane (LP) |
|---|---|---|
| Supply | Piped continuously from the utility. No tank to fill or monitor. | Stored on-site in your own tank. Independent of any utility. |
| Energy density | Lower per cubic foot (~1,000 BTU/ft³). | Higher per unit (~2,500 BTU/ft³), so it carries more energy for a given volume. |
| Deliverable power | Same engine usually produces slightly less rated output on natural gas than on propane, because of the lower density. | Typically the higher of the two ratings on a dual-fuel unit's spec sheet. |
| Runtime limit | Effectively unlimited while the gas line stays pressurized. | Finite — set by tank size and burn rate. Full-load hours drop fast, so size the tank deliberately. |
| Watch out for | The gas meter, regulator, and pipe must be sized to feed the generator and the home at once. An undersized line is a common cause of a unit that won't hold full load. | Cold weather lowers a tank's vaporization rate; a small tank may not keep up in deep cold at high demand. |
Because the natural-gas rating is the lower one, always size against the fuel you'll actually use — a unit that's ample on propane can fall short on gas.
The transfer switch is the one non-negotiable safety component. It guarantees the house is connected to exactly one source at a time — utility or generator, never both.
Detects a power loss, signals the generator to start, transfers the load once voltage is stable, and switches back when the utility returns — all unattended. Standard on permanently installed standby systems.
Cheaper and simple, but requires someone present to transfer the load by hand. More common with portable generators than with automatic standby units.
Why it exists: connecting a generator to your wiring without a proper transfer switch can push power back onto the utility line — a lethal hazard to line workers and a code violation. A transfer switch physically prevents that backfeed.
Size the switch to your scope: a whole-home service-entrance switch handles the full panel; an emergency sub-panel switch feeds only the circuits you chose in step one.
Large motors rarely start at the exact same instant, and controls or a load manager can stagger them. The generator's worst realistic moment is its full continuous load with one big motor starting — so peak demand is continuous watts plus the largest single starting surge, not every surge summed.
Air-cooled units cover most homes and are common up through the mid-20s of kilowatts. Above that, liquid-cooled engines take over for larger whole-home demand and longer continuous run times. Scope and total watts decide which category you're even shopping in.
Often, yes — with help. A soft-start device cuts the compressor's inrush surge dramatically, and a load-shedding module drops the AC for a beat if another big load is starting. Together they can bring central air within reach of a mid-size unit instead of forcing a jump to the largest models.
No — the ledger uses typical planning estimates so you can see how the math behaves. Real draws vary by model, age, and efficiency. Read the nameplate on your own equipment, and have a licensed electrician perform a formal load calculation before you buy or install.
A permanently installed standby generator ties into your service, gas or propane line, and grounding. That work is governed by electrical and fuel codes and generally requires a licensed installer and a permit. It's also how you get the transfer switch and fuel line sized correctly the first time.