generator sizing calculator
Stop guessing. Every "rule of thumb" sizing guide on the internet is wrong for your house because your house isn't a rule of thumb. Enter what you actually need to run, and I'll tell you exactly how many watts you need and which generators can handle it.
Check the boxes for every appliance you want to run during an outage. The calculator accounts for starting watts, adds a 20% safety margin, and recommends a generator size. Takes about 60 seconds.
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how this calculator works
The math is straightforward. Every appliance has two wattage numbers: running watts (what it draws continuously) and starting watts (the surge it needs for the first 1-3 seconds when the motor kicks on). Most sizing guides only show you running watts, which is how people end up with a generator that trips its breaker the first time the AC cycles on.
Here's what this calculator does:
- Adds up all your running watts. Everything you checked gets totaled.
- Finds your biggest surge load. The appliance with the highest starting watts gets its surge added on top of the running total. In practice, your appliances don't all start at the same instant — the worst case is your biggest motor kicking on while everything else is already running.
- Adds a 20% safety margin. This gives you headroom for loads you forgot, prevents the generator from running at 100% all day (which kills longevity), and covers minor efficiency losses.
The result is your minimum generator size in kilowatts. I round up to the nearest standard generator size because manufacturers don't sell a 13.7 kW unit.
what this calculator doesn't account for
No calculator is perfect. Here's what this one leaves out:
- Simultaneous startup. I assume your biggest motor starts while everything else is running steadily. If your AC and well pump kick on at the exact same second, the surge is higher than what I show. A transfer switch with load management handles this — most modern installs include one.
- Altitude derating. Generators lose roughly 3.5% of their rated output per 1,000 feet above sea level. If you're in Denver, knock 12-15% off the generator's rating, not your load calculation.
- Temperature derating. Extreme heat reduces output too. If you're sizing for a generator that will run in 110-degree summers, add 10% to your minimum size.
- Power factor. Some loads (especially motors) have a power factor less than 1.0, meaning they draw more apparent power than real power. The watt figures I use already account for typical power factors, but unusual equipment might differ.
If this calculator tells you 14 kW and you're choosing between a 16 kW and a 20 kW, buy the 20. Generators last longest when they run at 50-75% load. The price difference between one size up is almost always worth it for the extra headroom and the longer engine life. The only exception is if you're on a hard budget — in that case, the right-sized generator you actually buy beats the oversized one you keep thinking about.
related
- what size generator do I need? (the complete guide)
- best whole-home generators
- how much does a whole-house generator cost?
frequently asked questions
How do I know what size generator I need for my house?
Add up the running watts of every appliance you need to power during an outage, then add the starting watts of the largest motor-driven load (like your AC or well pump). Add a 20% safety margin to that total and you have your minimum generator size. The calculator above does all of that math for you.
Do I need to account for starting watts when sizing a generator?
Yes. Motor-driven appliances like air conditioners, well pumps, and sump pumps draw 2-3x their running watts for the first few seconds at startup. If your generator can't handle that surge, the breaker trips or the generator stalls. This calculator accounts for starting watts automatically.
What is a good safety margin for generator sizing?
20% is the standard safety margin most electricians and manufacturers recommend. It gives you headroom for loads you forgot to count, prevents the generator from running at 100% capacity continuously (which shortens its lifespan), and accounts for minor efficiency losses.