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Check everything you want powered at the same time during an outage. The tool sums running watts and adds the single largest motor-start surge, the way an electrician sizes it.
Essentials-class standby (or a large portable). Includes ~20 % headroom. A licensed electrician confirms with a formal load calculation before any quote.
The two numbers that size every generator
Every appliance has a running wattage (what it draws once going) and some have a starting surge: motors briefly pull two to four times their running draw in the first fraction of a second. A refrigerator runs at about 200 W but starts at over 1,000 W. A 3-ton central AC runs near 3,500 W and can demand 8,000 W at compressor start.
A generator only has to survive the worst single instant: everything running, plus the one biggest motor starting. That is why the method is "sum of running watts + largest single surge", not the sum of all surges. Two motors almost never start in the exact same instant, and modern standby controllers stagger them deliberately.
Reading your result: the size classes
Under 8 kW of calculated need puts you in essentials territory: a 10 kW standby unit or a large portable covers refrigeration, heat (gas furnace fan), a well or sump pump, lights and electronics. Between 8 and 16 kW is the mid-range where most selective-coverage homes land: 13 to 18 kW units. Past 16 kW you are sizing for whole-home comfort with air conditioning, and the 22 – 26 kW class earns its position as the market default. Those classes line up with installed whole-house generator pricing.
Two situations push the math up a class: all-electric homes (heat pump, electric water heater, electric range can stack 12 kW+ of running load on their own) and EV charging during outages (a Level 2 charger adds 7,000 – 11,500 W). Two pull it down: load-shedding modules that shed non-critical circuits for seconds when demand peaks, and honest triage about what an outage actually requires.
Where homeowners get sizing wrong
The classic overshoot is sizing for everything the panel could theoretically feed instead of what an outage realistically needs, paying thousands extra up front plus higher fuel burn at idle. The classic undershoot is forgetting the starting surge: a generator that handles every running load but stalls when the AC kicks is sized wrong by exactly one number.
The subtle one is the well pump. Submersible pumps hide a hard starting surge (a 1 HP pump can demand 3,000 W to start), they are non-negotiable for rural homes, and they often share outage time with sump pumps in the same storm. If your water comes from a well, size with the pump checked and read our generator-for-well-pump guide for the details.
From this number to a real quote
A licensed electrician confirms sizing with a formal load calculation (NEC Article 220) before any standby installation: it is part of every serious quote and the permit file. Expect the conversation to cover fuel type and meter distance, panel condition, and whether load management makes a smaller unit behave like a bigger one. The calculator result gives you the vocabulary to sanity-check what you hear.
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