Calculator
Generator Size Calculator
List the AC devices you plan to power from a generator at the same time. Enter each device's running watts and startup watts. The result separates continuous load from two startup scenarios so you can compare them with a generator's running and starting watt ratings.
Not sure what to enter? Read the step-by-step guide.
Your inputs
AC device
Startup power is the device's total power draw while starting, not extra watts to add to its running power. If the device has no startup surge, enter its running watts in both fields.
Generator size estimate
How much generator power you could need
Continuous AC output for devices running together.
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Startup scenarios
Peak if one device starts at a time
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Peak if all devices start together
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Enter the AC devices you expect this generator to power at the same time.
These are load estimates, not a generator model recommendation or a guarantee that your RV setup will operate correctly. Check the generator's running and starting watt ratings under your expected operating conditions, along with surge duration, voltage, frequency, receptacle and circuit limits, and any derating. This tool does not evaluate wiring, transfer equipment, fuel use, or exhaust safety.
Edit your inputsSee calculation details, assumptions and sources
Continuous W = sum of quantity × running W for devices used together. For one device starting while all other devices are already running, add the largest startup W − running W of a single device to continuous W. If all devices start together, sum each device type's quantity × startup W. Startup W is the device's total draw while starting, not extra watts to add twice.
For example, one 1,200 W running / 2,400 W starting air conditioner, two 100 W running / 350 W starting pumps, and a 75 W TV with no surge total 1,475 W running. The one-at-a-time startup peak is 2,675 W; if all devices start together, the peak is 3,175 W. The two peaks describe different assumptions, not two ratings that every generator provides.
Enter AC input watts, not DC battery draw, Wh/day, appliance output ratings, or VA treated as watts. If you only have VA, you need the device's power factor to determine watts. Use startup data from each device's manual or measurement. No generic multiplier or margin is added. The one-at-a-time figure assumes only one device starts while all other devices are already running. Actual starts may overlap. Compare the scenario you expect with the generator's continuous and startup/surge ratings, including their stated conditions and duration. If you already have generator ratings, use Can I Run This on My Generator? for the W comparison.
References: Generac portable generator owner’s manual describes adding the largest motor's starting watts to other running loads; Champion explanation of running and starting watts distinguishes sustained output from short surge output. See the site methodology.