What Size Generator Do I Need? A Wattage Sizing Walkthrough

Running watts vs starting watts — the number that trips everyone up

Every generator wears two numbers: running (rated) watts it can supply continuously, and starting (surge) watts it can deliver for a few seconds. Appliances have the same split. Anything with a motor or compressor — fridge, sump pump, well pump, A/C — draws two to four times its running wattage for the second or two it takes the motor to spin up. Resistive loads like lights, space heaters, and electronics draw the same watts starting and running.

This is why a '2,200-watt' generator can run a 700-watt fridge with room to spare but stall trying to start a 1,200-watt well pump: the pump wants 4,000 watts for the first second. Size for the surge, not just the steady state, and you'll never be surprised.

Step 1: list what actually has to stay on

Skip the fantasy of powering the whole house and write down what an outage actually threatens. For most homes the real list is short: refrigerator, freezer, some lights, phone and laptop charging, internet, and the furnace blower in winter or a window A/C in summer. Households on a well or with a basement add the well pump and sump pump — usually the two biggest surges on the list. Everything else (dishwasher, dryer, oven) can wait a few hours.

Check the nameplate sticker on anything you're unsure about — it lists watts, or amps you can multiply by 120 volts. Where the nameplate only gives running amps, assume motors surge two to three times higher.

What common appliances really draw

These are typical figures — your nameplate wins any disagreement. Notice the pattern: electronics are rounding errors, resistive heat is a steady hog, and motors are cheap to run but expensive to start.

Step 2: do the math (it's two additions)

First addition: sum the running watts of everything on your must-run list — that's the continuous load your generator carries all day. Second addition: find the single item with the largest gap between its starting and running watts, and add that gap on top. Motors don't all start at the same instant (and you can stagger big ones by a few seconds yourself), so you only budget surge for the worst one.

Then add 20 percent headroom. Generators are happiest — quietest, most fuel-efficient, longest-lived — loaded to 50-80 percent of rating, and you'll inevitably plug in one thing you forgot.

A worked example

A winter outage kit: refrigerator (200W running), gas furnace blower (800W), sump pump (1,050W), lights and charging (150W), internet (20W). Running total: 2,220 watts.

Largest surge gap: the furnace blower needs 2,350W to start — 1,550W above its running draw (the sump pump's gap is 1,150W, so the furnace wins). Peak requirement: 2,220 + 1,550 = 3,770 watts.

With 20 percent headroom you're shopping for roughly 4,500 starting watts — a midsize 3,500-4,000 running-watt inverter generator, not the 2,200W suitcase and not a 10kW monster. That one calculation just saved you either a stalled generator or a thousand wasted dollars.

What each generator class actually runs

2,000-2,400W inverter (the Honda EU2200i class): fridge, lights, internet, charging, CPAP, a fan or small microwave — the essentials, quietly. Not enough to start a well pump or any A/C bigger than a small window unit.

3,500-5,000W: the sweet spot for most storm kits. Everything above plus the furnace blower, sump pump, and a window A/C, with surge to spare.

7,500-12,000W: adds a well pump, a small central A/C (with a soft-starter), or an electric water heater — this is 'the house mostly works' territory, usually fed through a transfer switch or interlock.

18-26kW standby: the whole house, automatically, including central air — no load list required, which is exactly what you're paying for.

Why bigger isn't better

An oversized generator burns meaningfully more fuel at idle loads, runs louder, costs more to buy and maintain, and — for open-frame units — can even carbon-foul running long hours at 10 percent load. Fuel is the real constraint in a long outage: a right-sized 4,000W unit sipping half a gallon an hour outlasts a thirsty 10kW unit on the same stored fuel. Buy the size your load list says, not the one the storm panic suggests.