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Solar & battery

Solar Inverter Load Calculator

Add up continuous and surge loads to check an inverter can actually run them.

Your numbers

Result

Continuous load 1,500 Watts
Worst-case surge 2,950 Watts
Inverter surge capability 10,000 Watts
Continuous headroom 70.0%
Surge margin 70.5%
Continuous load is 1,500 W with 70.0% headroom, and the worst surge of 2,950 W sits inside the inverter's 10,000 W surge capability.

Only one motor is assumed to start at a time, which is the normal design case. Adding every surge together sizes an inverter far larger than reality needs. Surge ratings are also time-limited, typically a few seconds, so a load that surges repeatedly needs a soft starter rather than a bigger inverter.

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Questions

About this calculator

Why do motors surge so much?

A stationary motor has almost no back-EMF, so it draws locked-rotor current until it spins up. Three to seven times the running current is normal for a second or two.

What is a soft starter?

A device that ramps a motor up rather than switching it on hard. On air conditioners it can cut starting current enough to let a much smaller inverter or generator run them.

The formula

worst case = continuous load + the largest single starting surge

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