Almost every bad generator purchase comes from the same mistake: buying to a number on a box instead of to a list of things you actually need to run. Sizing is not complicated, but it does have one trap, and once you understand that trap the rest is arithmetic you can do on a napkin.
Running watts vs starting watts
Every appliance has two numbers. Running watts (sometimes called rated watts) is what it draws once it has settled into normal operation. Starting watts is the brief surge an electric motor pulls as it spins up from a dead stop, and it can be three to seven times the running figure. This surge is called inrush current, and it is the reason a generator that looks big enough on paper still stalls when the well pump kicks on.
Anything with a motor or a compressor has a meaningful surge: refrigerators, freezers, air conditioners, well pumps, sump pumps, furnace blowers, garage door openers, power tools. Anything purely resistive or electronic does not: LED lights, laptops, phone chargers, most televisions, a coffee maker's heating element.
The four-step sizing method
- List what has to run. Not what you own. What has to run during an outage. Be honest, and separate "must have" from "nice to have".
- Write down each item's running watts. Use the data plate on the appliance rather than a chart when you can. If the plate lists amps instead of watts, multiply amps by volts (120 for standard outlets, 240 for large appliances).
- Add the running watts together. That total is your continuous load.
- Add the single largest starting surge. Not all of them. Motors rarely start at the same instant, so the accepted practice is to take your running total and add only the biggest individual surge on the list.
A worked example
A typical "keep the house livable" load list looks like this:
| Appliance | Running watts | Starting surge |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Chest freezer | 500 | 1,500 |
| Furnace blower (1/2 hp) | 800 | 2,400 |
| Sump pump (1/3 hp) | 800 | 2,600 |
| Lights, router, phones, laptop | 400 | 0 |
| Microwave | 1,000 | 0 |
Running total: 4,200 watts. Largest single surge: the sump pump at 2,600, which is 1,800 watts above its running draw. So the peak demand is roughly 4,200 + 1,800 = 6,000 watts. Add 20 percent headroom and you want a generator rated around 7,000 to 7,500 starting watts, with a continuous rating near 6,000. That is a large portable, not a whole-home standby.
Notice what happened: the microwave and the sump pump both being on at once is what sets the size. Choosing not to microwave during an outage would take a full kilowatt off the requirement.
Whole-house sizing is a different calculation
If you want everything to work exactly as it does on utility power, including central air and an electric range, you are no longer adding appliances. You are sizing to the service. A licensed electrician performs a load calculation based on the National Electrical Code, using the square footage of the home, the connected appliance loads and demand factors that account for the fact that not everything runs at once. Typical results land between 14 kW and 26 kW for an average single-family home, and the calculation is part of what you are paying for when you get an installation quote.
Context helps here: the U.S. Energy Information Administration reports the average American home uses about 10,500 kilowatt-hours of electricity per year, which averages under 1.2 kW of continuous draw. Generators are sized for peaks, not averages, which is why a 20 kW unit spends most of an outage loafing.
Bigger is not automatically better
An oversized generator costs more to buy, more to install and more to run. Engines are least efficient and dirtiest at light load, and a diesel or gas engine run for long stretches at a small fraction of its rating can suffer from wet stacking and carbon buildup. The sweet spot is a unit that sits between 40 and 80 percent of its rated output during a typical outage.
Managed loads: the cheaper way to go big
Modern standby systems can be paired with a smart management module that sheds or staggers large loads. Instead of buying a 26 kW generator so the air conditioner and the electric dryer can run at the same time, you buy an 18 kW unit and let the controller decide that the dryer waits while the compressor runs. Load management usually costs a few hundred dollars and routinely saves several thousand on the generator itself. Ask about it by name when you collect quotes.
How the units relate
You will see watts, kilowatts, amps and kilowatt-hours used interchangeably by salespeople who should know better. A watt is a rate of energy flow; a kilowatt is a thousand of them; a kilowatt-hour is what you get when a kilowatt flows for an hour. A generator is rated in watts because it delivers a rate. A battery is rated in watt-hours because it holds a quantity. Confusing the two is why people ask how many watts a battery holds, which is a bit like asking how many miles per hour are in a gas tank.
Before you buy
- Measure, do not guess. A clamp meter or a plug-in energy monitor on your biggest appliances will beat any online chart.
- Check whether anything you own needs 240 volts. Well pumps, electric ranges, dryers and central air usually do, and a small portable that only offers 120 volt outlets cannot run them at all.
- Check whether anything you own is sensitive. Furnace control boards, modern televisions and computers prefer clean power; that is an argument for an inverter generator or for running those items from a battery unit.
- Decide how the power gets into the house. A pile of extension cords is a different project from a transfer switch, and the transfer switch has to be sized too.
Do the arithmetic once, write it down, and take the list with you when you shop or when you meet an installer. It turns a sales conversation into a specification.