A clear, step-by-step walkthrough — with a full worked example — so you understand exactly what you need and why.
You've seen generator listings with numbers like "7,500W / 9,375W starting." You've read that motors need "surge" power. But how does it actually add up to a number you can shop against? This guide walks through the math from scratch — no engineering background required.
If you want the answer without the math, our free generator size calculator does all of this for you in seconds. But understanding the calculation means you'll never be surprised by an undersized generator or talked into a unit you don't need.
Key Concept: Your generator must meet two separate requirements simultaneously: (1) sustain your total running watts continuously, and (2) be capable of supplying a brief peak of starting watts whenever a motor starts. Fail either requirement and the generator trips its breaker, bogs down, or damages the appliance trying to start.
Start with your use case. "During a power outage, what absolutely must stay on?" Common answers:
Be honest about simultaneity. The furnace fan and AC don't usually run at the same time — include one or the other, not both, unless your setup requires it. Use our wattage chart to look up the running and starting watts for each appliance.
For each appliance, you need both numbers. For appliances without motors (toasters, heaters, lights), starting watts equal running watts — no surge exists. For anything with a motor (refrigerators, pumps, AC units), starting watts will be higher than running watts.
Where to find wattage data: Check the appliance nameplate label (usually on the back or inside the door), the owner's manual, or use our reference chart for typical values.
Sum every appliance's running wattage. This is your total continuous load — the sustained output your generator must produce without overheating.
Formula: Total Running Watts = Σ (Running Watts for each appliance)
Here's the key insight most people miss: you do not add all the starting watts together. Motors don't start simultaneously. Only one motor starts at a time — the refrigerator compressor, then 20 minutes later the sump pump, then an hour later the AC cycles on.
Instead, find the appliance with the largest surge — the biggest difference between its starting and running watts. That's the extra load you'll need to support.
Formula: Largest Surge = max(Starting Watts − Running Watts) across all motor appliances
Your peak starting demand is the moment when every other appliance is already running and the highest-surge motor kicks on. This is the number your generator's starting watt capacity must meet.
Formula: Peak Starting Demand = Total Running Watts + Largest Surge
Your generator's stated starting watts (also called surge watts or peak watts) must be greater than or equal to this number.
Running a generator at exactly 100% capacity is bad practice. It runs hot, reduces generator life, and leaves no room for any unexpected load. Most experts recommend sizing so your continuous load stays at 80% or less of rated output.
Rule of thumb: Divide your Total Running Watts by 0.8 to find the minimum rated output for healthy operation.
Then round up to the nearest standard generator size: 2,000 / 3,500 / 4,000 / 5,000 / 6,500 / 7,500 / 10,000 / 12,000W. Also verify that the chosen generator's starting watts exceed your Peak Starting Demand.
Let's size a generator for a typical suburban home during a summer power outage. The homeowner wants to keep running: refrigerator, sump pump (1/2 HP), window AC (10,000 BTU), LED lights, TV, and phone chargers.
| Appliance | Running Watts | Starting Watts | Has Motor? |
|---|---|---|---|
| Refrigerator | 150 W | 600 W | Yes |
| Sump Pump (1/2 HP) | 1,050 W | 2,150 W | Yes |
| Window AC (10,000 BTU) | 1,000 W | 2,000 W | Yes |
| LED Lights (10 bulbs) | 100 W | 100 W | No |
| TV (55") | 100 W | 100 W | No |
| Phone Chargers (×3) | 30 W | 30 W | No |
150 + 1,050 + 1,000 + 100 + 100 + 30 = 2,430 W total running
Largest single surge = 1,100 W (sump pump)
Total Running (2,430 W) + Largest Surge (1,100 W) = 3,530 W peak starting demand
This means: when the sump pump kicks on with everything else already running, the generator needs to supply 3,530 W for about 2–3 seconds.
Running requirement: 2,430 W ÷ 0.80 = 3,037 W minimum rated output for healthy operation.
Starting requirement: Generator's surge rating must be ≥ 3,530 W.
Standard size to choose: 4,000W generator — the next standard size above our 3,530W peak demand. A 3,500W generator would fall 30W short; always round up, never down.
For even more headroom, a 5,000W unit is a comfortable choice — enough room to occasionally run a microwave or coffee maker during the outage too. A 4,000W generator (e.g., DuroMax XP4850EH at 4,850W running / 3,850W starting surge) handles this load comfortably.
The most common error. If someone adds up the starting watts of every appliance on their list, they might calculate a "peak demand" of 9,000W when a 4,000W generator would actually handle everything fine. Motors don't start at the same time. Only one starts at a time, and you only plan for the worst-case single startup event.
These are often the highest-surge appliances in a home and the most critical during storms. A 1/2 HP well pump has a 2,100W starting surge — enough to trip a generator that's undersized for everything else. Don't leave them off the list because they're "out of sight."
If you size a generator purely for running watts without accounting for starting surges, it will frequently trip its overload protection. The motor tries to start, demands 2–3× its running wattage for 2 seconds, the generator's breaker trips, and everything shuts off. Always check the starting watt requirement.
Generator manufacturers rate their machines at peak output, but sustained operation at 90–100% load causes overheating, faster wear, and shorter engine life. Plan for 80% or less of the rated output as your continuous load ceiling.
Electric dryers (5,400W running) and electric ranges (up to 12,000W for the full stove) are among the largest loads in any home. Most portable generators — even 7,500W units — can't handle these on top of other loads. If you plan to use them during an outage, factor them in explicitly and be prepared to stagger their use (run one at a time, never simultaneously with other heavy loads).
A generator that's the right wattage but connected with a thin 16-gauge extension cord will lose voltage over long runs, causing motor damage and potentially starting a fire. Use 12-gauge or 10-gauge cords for high-draw appliances and keep runs as short as possible. Better yet, install a transfer switch and use your home's wiring.
Total Running Watts = sum of all running watts for selected appliances
Largest Surge = max(Starting Watts − Running Watts) for any single motor appliance
Peak Starting Demand = Total Running Watts + Largest Surge
Minimum Rated Running Watts = Total Running Watts ÷ 0.8 (80% load rule)
Generator Must Have:
— Running Watts ≥ Minimum Rated Running Watts
— Starting Watts ≥ Peak Starting Demand
Round your result up to the nearest standard size:
2,000W → 3,500W → 4,000W → 5,000W → 6,500W → 7,500W → 10,000W → 12,000W → Standby
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