Quick answer
Size a standby generator from your critical load list — heat, refrigeration, sump pump, lights, well pump, medical equipment — not from your home's square footage. Essential-circuit systems in the low-teens kW cover survival loads for most Puget Sound homes; whole-home systems in the 20+ kW class carry everything, provided the gas supply can feed them. The load list, the surge math for motor startups, and the fuel supply are the three decisions everything else hangs on.
- Start with the load list, not the brochure: what must run during a three-day November outage, and what can wait?
- Motor loads (heat pump compressors, well pumps, sump pumps) briefly draw several times their running watts at startup — the surge allowance is where undersizing bites.
- Essential-circuit coverage costs meaningfully less than whole-home and covers what most families actually miss during an outage.
- Natural gas means indefinite runtime on PSE's supply; propane needs a tank sized for multi-day outages.
- Install queues fill every fall — the cheapest, calmest time to size and install is spring and summer.
Use this guide when
Last winter's windstorm left you dark for days, you work from home or rely on medical equipment, you're on a well (no power = no water), or your neighborhood sits on the end of a feeder that PSE restores last. Plan before wind season: fall install queues fill fast, and sizing decisions made during an outage are made badly.
What actually drives the sizing
Two lists and one calculation: the loads that must run (with their running watts), the motor loads among them (with their startup surge), and the largest combination that can hit simultaneously. A generator that handles everything running steady but can't absorb the well pump kicking on while the furnace blower runs will trip exactly when you need it — surge math is the whole game.
Compare your options
Essential-circuits system
A moderately sized standby unit feeding a critical-loads subpanel: heat, refrigerator and freezer, sump pump, well pump, some lights and outlets, internet. Lower equipment and fuel cost, and honestly what most households need — an outage with heat, running water, cold food, and Wi-Fi is an inconvenience, not an emergency.
Whole-home system
A larger unit (20+ kW class) behind an automatic transfer switch carrying the entire panel — nothing to choose, nothing off-limits, including electric ranges and EV charging. It demands more: higher equipment cost, a gas meter and line that can actually deliver the BTUs at full load (often a PSE meter upsize), and more physical space with code clearances.
Load-managed middle path
Modern transfer switches and load-shedding modules let a mid-size generator behave like a bigger one: heavy loads (dryer, range, EV charger, second HVAC stage) get shed or staggered automatically when the generator approaches capacity. It's often the best value in the comparison — whole-home convenience at essential-circuits fuel burn. A battery or SPAN panel can play the same role; see our backup power comparison.
Key terms and context
This guide is written for electrical decisions in the Puget Sound. It uses the same terminology you'll hear from inspectors, technicians, and permit offices.
Undersizing the surge
Running watts add up neatly on paper; startup surges don't. A heat pump compressor or well pump can briefly demand several times its running draw, and if two motors start simultaneously during an outage, an undersized unit trips offline — usually at 2 a.m. in a storm. Sizing software accounts for locked-rotor amps and staggered starts; square-footage rules don't.
Ignoring the fuel supply
A whole-home generator rated beyond what the gas meter and line can deliver starves under full load — voltage sags, the engine bogs, and protection shuts it down. Natural-gas installs need the meter and line verified (and sometimes upsized by PSE) as part of the project; propane installs need tank capacity sized for realistic multi-day outages, not the smallest tank that fits.
The unexercised generator
Standby units that never run don't start when called — fuel systems gum, batteries die, controls drift. A weekly self-exercise cycle and an annual service (oil, filters, battery, transfer-switch test under load) are part of ownership. A generator without a maintenance plan is a lawn ornament with a warranty.
How we build this guidance
- Sizing comes from your load list and surge math, not the biggest unit on the truck.
- One licensed team handles the electrical, the transfer switch, and the gas line — no coordinating three contractors.
- We'll tell you when essential-circuits (or a battery) covers your real needs for less.
Methodology: Load listing and sizing per manufacturer sizing software with locked-rotor (surge) allowances for motor loads; fuel-supply verification against meter and line capacity; transfer-switch requirements per NEC and Washington electrical code.
Last updated: 2026-07-05
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Common questions
Natural gas or propane for a Puget Sound standby generator?
Homes with PSE gas service almost always choose natural gas: indefinite runtime, no tank, no refueling in a storm. The check that matters is meter and line capacity at full generator load — sometimes PSE needs to upsize the meter. No gas service means propane, with the tank sized for realistic multi-day outages; we size that as part of the design.
What size generator does a typical Puget Sound home need?
It depends entirely on the load list, which is why we won't quote a size over the phone. As a shape: essential circuits (heat, refrigeration, sump or well pump, lights, internet) usually lands in the low-teens kW; carrying the whole panel including big electric appliances pushes into the 20+ kW class. Motor surge allowances move the answer more than square footage does.
Can a standby generator run my heat pump?
Yes, with honest sizing — the compressor's startup surge is the constraint, and modern variable-speed heat pumps with soft-start behavior are much friendlier to generators than old single-stage units. Some designs add a soft-start kit or configure load management to stagger the compressor against other big loads. It's a design question we answer with the surge math, not a yes/no.
How long can a standby generator actually run?
On natural gas, effectively as long as the outage lasts — multi-day runs are routine, with the weekly-exercise and oil-service intervals as the practical limits. On propane, runtime is your tank: a properly sized tank covers several days at realistic loads. Either way, load-managing the big appliances stretches fuel meaningfully.