I Tried Three Ways to Size a Backup Generator, Only One Skipped the Guesswork
Every time a big storm shows up in the forecast, generator sales spike and so does the number of people who buy the wrong size and find out the hard way. I wanted to see how three common sizing approaches actually compared, so I ran the same hypothetical household load through all three.
I've bought two generators in my life, undersized the first one badly, and wanted to actually understand the math before buying a third. So this was less a scientific test and more me working through the same hypothetical numbers three different ways and seeing where they diverged.
Method 1: The Retailer's Rule of Thumb
The salesperson approach is usually some version of "add up the wattage of what you want to run and buy something a bit bigger." For a household running a fridge, a sump pump, some lights, and basic electronics, that came out to just over 1,000 watts, so the recommendation was a 1,500 to 2,000 watt unit.
The problem showed up immediately once I looked at starting watts instead of just running watts. A sump pump's motor can spike to well over a thousand watts for a second when it kicks on, and that number never enters into the retailer's rule of thumb at all. This method optimizes for a sales conversation, not for whether the unit survives its first real storm.
Method 2: The Internet Forum Consensus
Home improvement forums tend to converge on "just buy the biggest one you can afford," which avoids the sizing math entirely by throwing money at the problem. It's not bad advice exactly, oversizing is safer than undersizing, but it also means paying for capacity and fuel consumption you don't need, and bigger generators are louder and heavier to move around, which matters if you're setting one up alone in bad weather.
This approach works if budget genuinely isn't a constraint. For everyone else, it's an expensive way to avoid doing five minutes of arithmetic. I get the appeal, nobody wants to be the person whose generator trips at 2am during a storm, and "buy way more than you need" feels like a foolproof way to avoid that outcome. But foolproof and cost-effective aren't the same thing, and the forum consensus rarely acknowledges the tradeoff it's actually making.
Method 3: Actually Calculating Running Plus Starting Load
The third approach was adding up running watts for everything I'd want powered simultaneously, then adding just the single largest motor-starting spike on top, rather than every motor's spike stacked together. For the same hypothetical household, that landed around 2,200 to 2,400 watts, comfortably between the retailer's undersized recommendation and the forum's "just go huge" advice.
Doing that math by hand across several appliances with different nameplate numbers got tedious fast, so I ran it through a free electrical power calculator instead, which has a dedicated motor-starting mode built specifically for this exact problem. It converts between watts, amps, and volts too, which matters once you're checking whether your extension cords and outlets can actually handle the load you calculated.
A Detail I Almost Missed
While comparing the three methods, I nearly made the mistake of adding up starting watts for every motor-driven appliance on the list instead of just the single largest one. Refrigerators, sump pumps, and window AC units all technically have a starting spike, and stacking all three together produces a number that's both unrealistically high and expensive to actually buy toward.
In practice, appliances rarely start at the exact same instant. A refrigerator's compressor cycling on while a sump pump is already running is a real scenario worth planning for, but three separate motors starting simultaneously is not something a generator realistically needs to be sized for. Adding just the largest single spike to the running total gets you a number that's realistic without being wastefully oversized.
This is the kind of detail that's obvious once someone points it out and easy to get wrong on your own the first time you sit down and try to do the math from scratch. It's also exactly the kind of mistake that pushes a shopper toward a bigger, more expensive unit than they actually need, without ever fixing the underlying misunderstanding about how motor starting load actually works.
I'd have made this exact mistake myself if I hadn't double-checked the math against the calculator before writing this up, which is a good reminder that "I understand the concept" and "I got the arithmetic right on the first try" aren't always the same thing, even for something that sounds this straightforward on paper.
What I'd Actually Recommend
Skip the retailer's rule of thumb entirely, it systematically underestimates anything with a motor in the load. The "buy huge" forum advice isn't wrong, but it's an expensive way to avoid five minutes of math that isn't actually that hard once you know what running watts, starting watts, amps, and volts mean in relation to each other.
There's a longer breakdown of the running-versus-starting watt math, plus a worked example with real numbers, in How to Size a Backup Generator Before the Next Outage, which is worth reading in full before you actually buy anything. It also covers the amperage and voltage checks that matter once you've settled on a wattage target, which none of the three sizing methods above touch on at all.
The Cost of Getting It Wrong in Either Direction
Undersizing has the obvious cost: a tripped generator during the exact moment you needed it, plus the risk of appliance damage from a unit straining at the edge of its rating for hours at a time. That's the mistake my first generator made, and finding out during an actual outage rather than in a calm moment is a genuinely bad way to learn a lesson, especially with a basement slowly filling with water because the sump pump kept cutting out.
Oversizing has a quieter cost that's easy to ignore in the moment. A bigger generator burns more fuel per hour even at partial load, costs more upfront, and is heavier and louder to deal with, which matters if you're the one hauling it out of a garage and starting it alone in bad weather. Neither extreme is free, which is exactly why the middle approach, calculating a realistic peak instead of guessing low or guessing high, is worth the ten minutes it takes.
Sources Worth Checking Before You Buy
Consumer Reports independently tests generator wattage delivery and noise levels, which is a useful cross-check against manufacturer claims. Ready.gov covers generator placement and carbon monoxide safety, which matters just as much as sizing. The U.S. Department of Energy has consumer guidance comparing portable, standby, and battery backup options if you're not sure a portable unit is even the right category for your situation.
Sizing math isn't hard once someone shows you which two numbers actually matter. It's just rarely the number printed in the biggest font on the box, and retailers have very little incentive to point that out to you while you're standing in front of a pallet of units they'd like to sell.
