Calculator
Inverter Size Calculator
List the AC devices your inverter needs to power at the same time. Enter the running and startup watts for each device from its manual, nameplate, or a measurement. The result separates continuous demand from two startup scenarios so you can compare each with your inverter's 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.
Inverter size estimate
How much inverter output you could need
Continuous AC output needed while the selected devices run 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 to run at the same time.
These are AC output estimates, not a recommendation for a specific inverter. Compare the results with the inverter's watt and VA ratings, surge duration, output voltage and waveform, and any temperature derating. The battery and wiring must also be able to supply the required DC input; this calculator does not check them.
Edit your inputsSee calculation details, assumptions and sources
Continuous W = sum of quantity × running W for devices operating together. Startup W is the total AC power a device draws while starting. 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, add quantity × startup W across all rows.
For example, one air conditioner at 1,200 W running and 2,400 W starting, plus two 100 W devices with no extra startup draw, need 1,400 W continuously. Both startup scenarios reach 2,600 W if the two smaller devices each have 100 W starting draw. With multiple surge loads, the two scenarios can differ.
Use real AC input watts, not DC battery draw, Wh/day, cooking output, or volts × amps without power factor. A device's starting peak may be specified as current or VA rather than W; do not treat those units as interchangeable. No generic surge multiplier or safety margin is added. Actual peak duration and the inverter's rated duration must be compared separately.
References: Victron Inverter VE.Direct 120V manual lists continuous W, time-limited output and peak W with durations; Victron inverter selection guidance notes simultaneous loads and battery support. See the site methodology.