Standby Generator Size & Cost Calculator

It comes from your connected load, not your square footage alone: add up the running watts of everything you want backed up, then add the starting surge of whichever single motor draws hardest — usually the AC compressor or the well pump, both of which briefly pull around three times their running watts for the first second or two. A generator sized to the running total alone will run the house fine until that motor starts, then stall or trip. This calculator runs both numbers and sizes to the peak, with a small safety margin on top.

Because a generator has to survive the worst single moment, not the average draw. Two homes with identical running loads can need different generator sizes if one has a 2 HP well pump and the other doesn't, since that pump's locked-rotor surge briefly stacks on top of everything else already running. Only the single largest motor's surge counts — the AC and the well pump don't start in the same instant — which is why this calculator takes the maximum, not the sum, of every motor's extra surge watts.

It depends on the generator's fuel demand in BTU per hour, whatever else already shares that gas line, and the actual routed distance from the meter or tank — longer runs lose more pressure to friction and need a bigger pipe to carry the same load. A 22 kW natural-gas generator thirty feet from the meter might run comfortably on 3/4 inch pipe, while the same unit a hundred and fifty feet away can need a full inch or more. Propane systems typically run at a higher regulated pressure, so the same nominal pipe carries meaningfully more BTU/hr on propane than on natural gas over an identical run.

A whole-house switch is rated to your electrical service — 100 to 400 amps — and hands the generator every circuit automatically, which only makes sense if the generator is sized to run the whole panel at once. A selected-circuits switch is sized to the generator's own output current instead and only energizes the handful of circuits deliberately wired into it, which is cheaper hardware but means picking your priority loads up front. A load-management module can let you add more selected circuits than the generator could run simultaneously by automatically shedding lower-priority ones under peak demand.

In nearly every jurisdiction, yes, for both trades. The transfer switch's connection into your electrical service needs a permit, an inspection, and in most areas a licensed electrician to make that connection. The gas line tie-in is the same story with a licensed gas fitter or plumber, since an undersized or leaking line is a real safety hazard. Setting the pad, running conduit, and prepping the site are reasonable DIY tasks, but don't energize or pressurize anything until both connections have passed inspection.

Natural gas runs as long as the utility keeps delivering it, which is usually reliable through an outage since the gas main is separate from the power grid, but it's not an option if you're not already on the gas line. Propane needs an on-site tank, which is finite fuel you have to plan and monitor, but it works anywhere and a large enough tank can run a generator for days. Propane generators also burn somewhat less fuel per kW than natural-gas versions of the same model, though tank vaporization rate becomes a real constraint on large units in cold weather.