How This Calculator Works
Running vs. Starting Watts
Running watts is continuous draw. Starting (surge) watts is the brief spike — often 2–3× running watts — that motors need to kick on. Your generator must handle the highest surge of any single load, on top of everything else already running.
The Formula
Generator Size = (Total Running Watts + Largest Single Surge) × 1.20. The 20% margin keeps the unit from running flat-out, which extends its life and improves fuel efficiency.
Why Only One Surge Counts
You don't start every motor at the same instant. Staggering startups — letting the fridge settle before starting the well pump — means only the single largest surge needs to be covered on top of steady-state running watts.
Standby vs. Portable
Once you have your target wattage, decide between a portable unit (cheaper, manual) or a standby unit (automatic, pricier). Compare both with our Standby vs. Portable Calculator.
A Worked Example
Say a household wants to keep the refrigerator, a well pump, a sump pump, and basic lighting running during a multi-day outage:
Refrigerator: 700W running / 2,200W starting
Well pump (1/2 HP): 1,000W running / 3,000W starting
Sump pump (1/2 HP): 800W running / 2,400W starting
Lights + electronics: 300W running (no meaningful surge)
Total running watts: 700 + 1,000 + 800 + 300 = 2,800W
Largest single surge: the well pump's, at 2,000W above its own running watts (3,000 − 1,000)
Peak demand: 2,800 + 2,000 = 4,800W
With the 20% margin: 4,800 × 1.20 = 5,760W, which rounds up to the next available class — a 6,500W generator.
What Could Make This Result Wrong for Your Home
A calculator can only work with the loads you tell it about. The most common reasons a homeowner's actual need ends up higher than an online estimate come down to loads that are easy to forget or hard to estimate accurately:
- Electric resistance heat or heat pumps — these draw far more than gas or oil heat and are frequently left out of a first pass
- Central air conditioning — has one of the largest single starting-watt surges in a typical home
- Well pumps and sewage ejector pumps — often deeper or larger-horsepower than the default assumption, especially on rural properties
- Multiple refrigerators or freezers — a second fridge or chest freezer in a garage or basement is easy to forget
- Electric water heaters or electric ranges — large continuous or near-continuous loads that some homeowners assume are gas
- EV charging — a Level 2 charger alone can rival a small home's entire essential-circuit load
- Shop equipment and power tools — welders, air compressors, and table saws have some of the largest surges of any household load
- Incomplete appliance information — nameplate wattage isn't always visible or accurate; when in doubt, size up rather than down
This calculator's number is a starting point for a conversation with a licensed electrician or generator installer — not a substitute for one. A final sizing decision should also confirm your home's electrical service capacity, available fuel supply, and any local code or permit requirements.
Understanding Generator Size Classes
Generators aren't sold in every possible wattage — manufacturers build them in a set of standard size classes (roughly 2,000W, 3,000W, 4,000W, 5,000W, 6,500W, 7,500W, 8,500W, 10,000W, and upward in larger steps for whole-house standby units). That's why this calculator's recommendation always rounds up to the next available class rather than giving you an oddly specific number like "5,760 watts" to shop for. If your peak demand lands close to the boundary between two classes, it's worth deciding in advance whether you'd rather pay a bit more for headroom or trim a lower-priority load to fit comfortably in the smaller class — both are reasonable choices depending on your budget and how much margin for error you want.
Frequently Asked Questions
No. Generator sizes are sold in fixed classes (5,000W, 7,500W, 10,000W, and so on), and this calculator always rounds your peak demand up to the next available class, never down. Rounding down means the unit may stall or trip when your largest motor tries to start.
In real use you don't start every motor at the exact same instant — you stagger startups. Because of that, only the single largest surge needs to be layered on top of your steady running-watt total. If you know two large motors genuinely need to start simultaneously (for example, a well pump and central AC on the same automatic cycle), size up manually beyond this calculator's recommendation.
The wattage math works the same for both — running watts and starting-watt surge don't change based on which type of generator delivers them. If you're specifically trying to size a whole-house standby unit from your electrical panel instead of an appliance list, our Whole-House Sizing Calculator is built for that starting point.
Use the Custom Loads section to add it directly. Check the appliance's nameplate or owner's manual for its running-watt rating; if it has a motor or compressor and no starting-watt figure is listed, a reasonable placeholder is 2 to 3 times the running watts, consistent with the appliances already in this calculator's database.
Retail generator boxes often lead with the peak/surge wattage number because it's the bigger, more attractive figure — not the continuous running wattage the unit can sustain. Compare this calculator's recommended running-watt capacity against a model's actual rated running watts, not its peak watts, when shopping.
Yes, to a degree this calculator can't measure. Extreme heat can reduce a generator's output, and an older or poorly maintained unit may deliver less than its rated capacity. The standard 20% margin used here helps absorb some of that variance, but it isn't a substitute for regular maintenance.