If you've shopped for a generator, you've probably seen two numbers on the box: something like "9,000 running watts / 12,000 starting watts." That gap isn't a marketing trick — it's the single most important thing to understand before you size a generator, and getting it wrong is the most common reason people end up with a unit that can't actually run what they need.
Running Watts: The Steady-State Number
Running watts (sometimes called rated watts) is the continuous power an appliance draws once it's up and operating normally. A refrigerator humming along, a light bulb glowing, a TV playing — these are all running-watt loads, and they stay fairly constant the whole time the appliance is on.
Starting Watts: The Hidden Spike
Anything with an electric motor — a refrigerator compressor, a well pump, a central air conditioner, a furnace blower — needs a brief surge of extra power the instant it starts spinning from a dead stop. Once the motor reaches full speed, power draw drops back down to the running watt level. That brief spike is the starting watt (or surge watt) number, and it's typically 2 to 3 times higher than the running watts.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 700W | 2,200W |
| Well Pump (1/2 HP) | 1,000W | 3,000W |
| Central AC (3-ton) | 3,500W | 10,500W |
| Sump Pump (1/2 HP) | 800W | 2,400W |
Why This Trips People Up
A generator rated for "8,500 running watts" sounds like plenty if your total running-watt load is only 5,000 watts. But if your central AC's compressor needs a 10,500-watt surge to start — even for a fraction of a second — a generator that can't momentarily deliver that surge will stall, trip a breaker, or shut down entirely. This is the single most common reason a "big enough" generator fails the first time it's actually tested during a real outage.
The Formula That Actually Works
You don't need to add up the starting watts of every single appliance — in real use, you stagger startups so motors don't all kick on at the exact same instant. The formula that manufacturers and electricians actually use is:
Generator Size = (Total Running Watts + Largest Single Starting-Watt Surge) × 1.20
The 1.20 multiplier — a 20% safety margin — keeps your generator from running flat-out at its rated maximum, which extends its life and improves fuel efficiency.
A Worked Example
Say you want to run a refrigerator (700W run / 2,200W start), a well pump (1,000W run / 3,000W start), a few lights and a TV (300W run), and a sump pump (800W run / 2,400W start) during an outage.
Total running watts: 700 + 1,000 + 300 + 800 = 2,800W
Largest single surge: the well pump's is highest at 2,000W extra (3,000 − 1,000)
Peak demand: 2,800 + 2,000 = 4,800W
With 20% margin: 4,800 × 1.20 = 5,760W → round up to a 6,500W generator
How Manufacturers List These Numbers
Generator listings and boxes typically show two numbers side by side, like "9,000 running watts / 12,000 starting watts" or "9,000W rated / 12,000W peak." The larger number is almost always the more prominent one because it's the more impressive figure — but it's the smaller, "running" or "rated" number that tells you what the unit can actually sustain continuously. Appliance labels work in reverse: most only print the running-watt figure on the nameplate, and starting watts for motor-driven appliances often isn't listed at all, which is why generator sizing guides (including this one) provide standard starting-watt estimates for common appliance categories.
When you're comparing two generators, always compare rated/running watts to rated/running watts, and peak/starting watts to peak/starting watts — mixing a generator's peak number against a competitor's running number will make the second generator look artificially larger than it is.
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Use the Free Wattage Sizing Calculator →Frequently Asked Questions
Electric motors draw a large surge of current the instant they start spinning from a standstill, because they haven't yet built up rotational momentum. Once the motor is up to speed, current draw drops to the running watt level. This surge can be 2–3× the running wattage.
Anything with a compressor or large motor: central air conditioners, well pumps, sump pumps, and refrigerators/freezers. A 3-ton central AC unit can surge to 10,000+ watts on startup even though it only draws 3,500 watts running.
No. In practice you stagger startups, so only the single largest motor surge needs to be added on top of your total running watts.
An undersized generator will stall, trip its breaker, or shut down when a large motor tries to start, and running the unit at or above its rated capacity for extended periods causes overheating and premature failure.