Home / Electrical / Generator Sizing Calculator

Generator Sizing Calculator

Enter your total running load and your largest motor's starting surge — get the recommended standard generator size, with a safety margin.

Whole-house or essential-circuits backup? Decide this first

Before running any numbers, decide the scope — it changes the whole calculation, not just the size at the end. A whole-house generator has to cover everything on your panel, sized and installed by an electrician through a whole-house transfer switch. An essential-circuits setup only backs up what's wired into a smaller sub-panel — fridge, well pump, a few lights and outlets, maybe one AC zone — which is both cheaper to install and meaningfully cheaper to fuel over a multi-day outage.

Once you know the scope, total the running watts for only what's in that scope — a whole-house calculation and an essential-circuits calculation on the same home will legitimately produce very different generator sizes.

How to use this calculator

Add up the running watts of everything within your chosen scope that you want the generator to power simultaneously, then enter your single largest motor's running and starting watts separately (from its nameplate) — usually a central AC compressor, well pump, or furnace blower. The calculator sizes for your steady running load plus the extra surge that motor draws for a moment when it kicks on while everything else is already running, since that's the real peak demand a generator has to survive.

Worked example

8,000W of total running load, with a 3,500W AC compressor that surges to 9,000W on startup, 20% margin: the surge requirement is 8,000 + (9,000 − 3,500) = 13,500W — that's the real peak, higher than the 8,000W running total alone. With a 20% margin, the target is 16,200W, which rounds up to a standard 17,000W generator. Looking at the table above, that lands this example in "mid-size / partial AC" territory, not whole-house — a useful sanity check that the scope and the number actually agree with each other.

Why the starting surge matters more than the running total

A generator only needs to be as big as its largest single moment of demand, not the sum of every appliance's peak — but that moment is usually a motor starting, not steady-state running. Motors draw 2-3x their running watts for a fraction of a second on startup. A generator sized only for total running watts can stall or shut down the instant that motor kicks on, even though it handles the steady load fine once everything is running.

Frequently asked questions

Do I need to size for every appliance's starting surge, or just one?

Just the single largest one, assuming your transfer switch or panel staggers large loads rather than starting everything at once — which is standard practice. Two large motors starting at the exact same instant is a real but less common scenario; if that's a concern for your setup, size for the combined surge of whichever loads could realistically start together.

What's the difference between running watts and starting watts?

Running watts is what a device draws once it's operating steadily. Starting (surge) watts is the brief spike when a motor first kicks on — only motor-driven equipment (compressors, pumps, some power tools) has a meaningfully higher starting value; resistive loads like heaters and lighting don't surge.

Can I upgrade from essential circuits to whole-house later?

Often yes with a portable or mid-size generator paired with a manual transfer switch — but a permanently installed standby system is usually sized and wired for its scope at install time, and upsizing later can mean a new transfer switch or larger fuel supply line, not just a bigger unit. Worth discussing future plans with your electrician at install, not after.

Reference only — actual generator sizing should also account for your specific transfer switch setup and manufacturer specifications. Consult a licensed electrician before installing backup power equipment.