Size a generator by adding up the running watts of everything you want to power at once, then adding the single largest starting surge — because only one motor (the fridge, the well pump, the AC) spikes at a time, not all of them together. Add about 20–25% headroom so the generator isn't maxed out. A few essential circuits (fridge, sump, furnace, lights) often fit a 5,000–7,500 W portable; running a central AC or well pump pushes you toward 10,000 W or a whole-house standby. The nameplate on each appliance always beats a chart.
Sizing backup power? The generator size calculator adds your running loads plus the largest surge and applies a headroom factor — free, no signup. This guide explains running vs starting watts, why only one surge counts, and how to right-size without overbuying.
⚡ Running watts vs starting watts
Every load has a running (continuous) wattage. Anything with a motor — a refrigerator, a well pump, an air conditioner, a furnace blower — also has a much higher starting (surge) wattage for the split second the motor spins up, often two to three times its running draw. A fridge that runs at 150–700 W can surge to 2,000+ W at startup.
Purely resistive loads — incandescent lights, space heaters, electric water heaters, a range burner — have no surge: their starting and running watts are the same. The distinction only matters for motors and compressors.
➕ Why only the largest surge counts
Here's the key to not oversizing: motors don't all start at the same instant. The odds of the fridge, the well pump, and the AC compressor all surging in the same fraction of a second are negligible. So the correct peak demand is:
peak = (sum of all running watts) + (the single largest starting surge)
You add every appliance's running watts, then add just one surge — the biggest difference between a load's starting and running draw. Adding every surge together, as some charts do, badly overshoots and has people buying far more generator than they need.
📊 Headroom — don't run it flat out
A generator shouldn't run continuously at 100% of its rating. Industry guidance is to add roughly 20–25% headroom so it “loafs” at around 80% of capacity — that runs cooler, lasts longer, handles an unexpected extra load, and leaves margin for the fudge in every wattage estimate. The calculator applies this margin to the peak demand to land on a recommended size.
Bigger isn't free. An oversized generator costs more to buy and burns more fuel, and a lightly loaded one can “wet-stack” (run too cool and gum up). Right-sizing — enough for your real loads plus headroom, not double it — is the goal.
🏡 Essential circuits vs whole house
What you want to power drives the size. A essentials-only setup — refrigerator, sump pump, furnace blower, a few lights, and outlets for phones and a modem — commonly fits a 5,000–7,500 W portable feeding a transfer switch or interlock. Add a well pump or central air conditioner and the surge alone can be 4,000–8,500 W, pushing you to a 10,000 W+ unit or a permanently installed whole-house standby generator.
Big continuous loads change the math entirely: an electric water heater, an electric range, or an EV charger each draw thousands of watts continuously and are usually left off backup power unless you go whole-house.
✅ The bottom line
Add the running watts of everything you'll power at once, add the single largest starting surge, and add 20–25% headroom. Match the result to a generator — essentials on a 5,000–7,500 W portable, add a well pump or central AC and step up to 10,000 W or a standby unit. Use each appliance's nameplate, not a generic chart, and remember big continuous loads usually stay off backup power.
Wattage figures are sourced ranges for planning; your specific equipment governs, and a transfer switch or interlock must be installed to code. Estimate the size with the generator calculator, then confirm the connection with a licensed electrician.
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What size generator do you need? Free calculator from the loads you want to back up — running watts, motor-starting surges, and sizing headroom.
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