Sizing a Standby Generator for Your Marin Home

It depends on what you want to power and what those loads actually draw. The first decision is scope: powering your whole home requires a larger generator than powering just essential circuits like refrigeration, some lighting, heating, a well pump, and connectivity. From there, a load calculation totals the running and startup demand of those loads to determine the correct size. There’s no single answer for every home — the right size is the one that matches your chosen loads with appropriate headroom, which an electrician determines through the calculation rather than a rule of thumb.

Both are valid; it’s a trade-off between cost and coverage. Whole-home backup means everything works during an outage but requires a larger, more expensive generator (and more fuel). Essential-circuit backup powers just what you truly need — keeping food cold, lights on, heat running, and critical devices working — with a smaller, more affordable generator. Many Marin homeowners find essential-circuit coverage meets their needs well, especially for PSPS outages. The right choice depends on your priorities and budget, and a transfer switch directs the backup power to the circuits you’ve chosen.

Because guessing leads to a generator that’s either too small to carry your loads or larger and costlier than necessary. A load calculation totals the actual electrical demand of the circuits you want to back up — crucially including the surge that motors (well pumps, AC compressors, refrigerators) draw at startup, which is much higher than their running draw. Missing that surge is a common sizing mistake that leaves a generator struggling. The calculation grounds the sizing in real numbers, ensuring the generator can start and run everything you’ve chosen reliably.

Fuel type affects runtime and practicality more than raw sizing, but they’re related. Natural gas, where available, provides a continuous supply so runtime isn’t limited by a tank — ideal for extended PSPS outages. Propane requires a tank sized for how long you want to run, so your expected outage duration influences the tank (and planning). Both fuels can power appropriately sized generators; the choice affects how long you can sustain backup and some installation considerations. An electrician factors your fuel availability and runtime goals into the overall recommendation.

Yes, if you’re likely to add them. Sizing a generator only for today’s loads can leave you undersized when you add an EV charger, heat pump, or other significant load in a year or two — potentially forcing an early replacement. Factoring reasonably anticipated future loads into the sizing now adds some capacity but avoids that costly scenario. It’s a balance: you don’t want to wildly oversize for loads you may never add, but accounting for concrete plans is prudent. Discuss your electrification plans with your electrician during sizing.

No — standby generator sizing and installation is a job for professionals. Sizing requires a proper load calculation, and installation involves connecting to your electrical system through a transfer switch, fuel connections, permits, and inspection, all done to code. An incorrectly sized or improperly installed generator is unreliable at best and dangerous at worst. A licensed electrician (working with the appropriate trades for fuel) sizes the generator to your needs, handles the permitted installation, and ensures the transfer equipment safely isolates your home from the grid. Given the stakes, professional handling is essential.