What Size Generator Do I Need? Sizing Guide

The power goes out, and suddenly the question isn't "do I want a generator" — it's "will the one I bought actually run my fridge without choking?" A generator that's too small stalls the moment your well pump kicks on. One that's too big means you paid hundreds of dollars, and burn extra fuel, to run a couple of lights and a phone charger. Neither is where you want to be at 2 a.m. in a storm.

The good news: sizing a generator isn't guesswork, and it isn't about horsepower or engine cc's. It comes down to two numbers — running watts and starting watts — and a single rule for adding them up. Get that right and everything else (fuel, noise, outlets) is just picking the flavor you like. This guide walks you through the math with a real example, then covers what actually separates a good generator from a regrettable one.

How to size a generator: the two-number rule

Every generator has two power ratings, and confusing them is the number-one reason people buy the wrong size. Running watts (sometimes called rated or continuous watts) is what the generator can supply hour after hour. Starting watts (also called surge or peak watts) is the brief burst it can deliver for a second or two when a motor spins up. Anything with a motor or compressor — a fridge, a well pump, a sump pump, an air conditioner, a power tool — draws two to three times its running watts for that first instant, then settles down.

Here's the rule that ties it together: add up the running watts of everything you'll run at the same time, then add the single largest starting surge on top of that total. Your generator's starting-watt rating has to clear that peak, and its running-watt rating has to cover the steady load. You only add one surge — not every appliance's surge — because in the real world your fridge and your furnace almost never happen to kick on in the exact same fraction of a second.

Let's work a real example. Say during an outage you want to run: a fridge (700 running watts), a few LED lights and a modem (200 watts total), a furnace blower (600 running watts), and a phone/laptop charger (100 watts). Your running total is 700 + 200 + 600 + 100 = 1,600 watts. Now find the biggest single surge. The fridge compressor is the culprit here — it briefly pulls roughly 2,200 starting watts (about 3× its running number). So your peak demand is 1,600 (everything running) minus the fridge's own 700 running watts, plus its 2,200 surge — which lands around 3,100 watts at the worst instant. In practice, the clean way to shop is: buy a generator with at least ~1,600 running watts and a starting rating comfortably above 3,100. A unit rated 2,200 running / 3,500 starting would cover this, though I'd bump up a size for breathing room. Which brings us to the honest advice: pad your total by 20–25%. Appliances draw more as they age, and running a generator flat-out all night wears it out fast.

The specs that matter after size

Once you know the wattage you need, a few other numbers decide whether you'll actually enjoy owning the thing. Fuel type is the big one. Gasoline is cheapest and everywhere, but it goes stale in months and stores poorly. Propane runs cleaner, stores for years, and is great for occasional backup — but you get slightly fewer watts from the same engine. Dual-fuel models take either, and honestly, for a home backup unit that might sit unused for six months, that flexibility is worth the small premium.

Inverter vs. conventional is the next fork. Inverter generators produce 'clean' power (low harmonic distortion) that won't fry sensitive electronics — laptops, modern TVs, medical devices, anything with a delicate circuit board. They're also dramatically quieter and sip fuel by throttling the engine to match the load. Conventional open-frame generators are louder and produce dirtier power, but you get more watts per dollar, which matters if you're mostly running pumps, tools, and lights that don't care about waveform quality.

Then check the boring-but-critical stuff: runtime at 50% load (a good number is 8+ hours, so it survives a night without a refuel), noise in decibels (under ~60 dB is neighbor-friendly; open-frame units near 70+ dB are genuinely loud), and the outlet panel. Make sure it has the receptacles you actually need — and if you ever want to power circuits in your house rather than run extension cords, look for a 30-amp 120/240V outlet so you can add a transfer switch. That single feature separates 'camp generator' from 'home backup.'

Choosing between the main types

Most buyers land in one of three camps, and the right pick follows straight from your wattage total and how you'll use it.

If your sized number is small — say under ~2,000 running watts for a fridge, some lights, and devices — a portable inverter generator is the sweet spot. Quiet, fuel-efficient, light enough to carry, and clean power for electronics. Perfect for short outages, tailgating, and RVs. If your number is larger, in the ~3,000–8,000 watt range, you're looking at a bigger portable (often open-frame or a larger inverter), ideally dual-fuel and with a 240V outlet so it can feed a transfer switch and power much of the house. This is the practical choice for most homeowners who want real outage coverage without a permanent installation.

At the top end sits the standby generator — a permanently installed unit wired to your electrical panel and fueled by natural gas or a large propane tank. It starts automatically the second the grid drops and can run your whole home indefinitely. It's also a professional install that costs several times what a portable does. That's the right call if outages are frequent and long where you live, or you have medical equipment that can't skip a beat. For everyone else, a well-sized portable delivers 90% of the benefit for a fraction of the money — and I'd steer most first-time buyers there.

Placement, setup, and running it efficiently

The single most important rule with any fuel-burning generator: never run it indoors, in a garage, or near open windows — not even for a minute. Generators produce carbon monoxide, which is odorless and deadly, and CO poisoning during outages kills people every year. Run it outside, at least 15–20 feet from the house, with the exhaust pointed away from doors and windows. Look for a model with a built-in CO shutoff sensor; it's a genuine lifesaver and worth prioritizing.

For connecting it, resist the temptation to 'backfeed' by plugging the generator into a wall outlet — it's dangerous and can electrocute utility workers. Either run heavy-gauge extension cords to individual appliances, or have an electrician install a transfer switch or interlock kit so you can safely power household circuits. A transfer switch turns a decent portable into something close to whole-home backup and is the best upgrade most owners make.

On efficiency: a generator runs most economically and lasts longest at around 50–75% of its rated load, which is exactly why padding your size estimate (rather than buying the smallest unit that technically fits) pays off. Use fresh fuel or add stabilizer, and run the unit for a few minutes every month or two so it starts when you need it. Keep it dry and elevated during storms, and store fuel safely in approved containers away from living spaces.