Generator Sizing Calculator

“Whole house” is the wrong question to start with

Generator shopping usually starts with “what size do I need for my whole house,” and that question doesn’t have a stable answer — it depends entirely on which circuits you actually want powered during an outage, which varies enormously between a household running a fridge and some lights and one also running a well pump, a sump pump and central air. The useful question is narrower and more answerable: given the specific list of things you want running, what’s the total load? This calculator works from a checklist of common household loads so you can build that list instead of guessing at a single headline wattage.

Running watts versus starting watts, the mistake that trips breakers

Every motor-driven appliance — refrigerators, sump pumps, well pumps, furnace blowers, window air conditioners — pulls a short, sharp surge of current at the instant it starts, well above what it draws once running steadily. A refrigerator that runs at 150 watts might spike to 1,200 watts for a fraction of a second at startup. A generator sized only for the sum of running watts will bog down, trip its breaker, or stall outright the moment one of these motors kicks on, which is exactly the moment you can least afford it to fail — usually right as the sump pump tries to start during a storm.

The standard, practical way to size around this — and what this calculator does — is to total the running watts of everything you want on at once, then add the single largest surge on top of that total, on the reasoning that in ordinary household use, motors don’t all start in the same instant. If your situation genuinely requires two large motors to start simultaneously (a well pump and a sump pump both kicking on together, say), size conservatively and consider that in the surge figure by hand.

What actually belongs on the “essential” list

The honest exercise here is triage, not aspiration. A refrigerator and freezer protect real food loss. A sump pump protects a basement from actual flood damage. A furnace blower (even without central air) keeps a house from freezing pipes in winter. Beyond that tier, most households are choosing comfort and convenience — lights, a microwave, phone charging, a TV — which matter but at much lower stakes than the first tier. Building the checklist honestly, rather than including everything in the house, is what keeps the generator size (and its cost) reasonable.

The 20% headroom, and why it’s not optional padding

This calculator adds 20% on top of the calculated peak load before recommending a generator size. That’s not conservative padding for its own sake — running a generator continuously at or near its rated maximum shortens its life, runs it hotter, and leaves zero margin if a motor’s actual surge runs higher than its nameplate figure, which happens. A generator you run at 75-80% of capacity during normal operation will last longer and behave more predictably than one you’re maxing out.

Portable versus standby, and the safety rule that actually matters

Portable generators are cheaper and don’t need a permanent installation, but they require manual setup during an outage — extension cords or a transfer switch, fuel refills every several hours, and critically, correct outdoor placement. Standby generators install permanently, run on natural gas or propane, and start automatically, at a meaningfully higher cost.

Whichever type, the one non-negotiable safety rule is ventilation: portable generator exhaust contains carbon monoxide, and it accumulates to lethal concentrations shockingly fast in a garage, breezeway, or any partially enclosed space — even with a door open. The U.S. Consumer Product Safety Commission’s guidance is specific about clearance distances from windows, doors and vents; read it before your first use, not during an emergency when you’re improvising. Combine this calculator’s wattage estimate with the HVAC BTU calculator and energy cost calculator if you’re planning broader home mechanical decisions at the same time.