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Inverter Size Calculator

Determine the continuous and surge power rating an inverter needs to safely run a given set of loads, accounting for the high starting current of motors and compressors.

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These calculators provide estimates and educational results. Always verify critical engineering calculations against applicable standards and qualified professional review.

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What Is It

Inverters have two ratings: continuous power (what they can supply indefinitely) and surge power (a brief higher output for starting motors). Undersizing either rating causes the inverter to shut down or fail to start motor-driven equipment like refrigerators, pumps and power tools.

How to Calculate

Enter the total continuous load in watts, select the load type (which sets a typical surge multiplier), and a safety margin. The calculator returns a recommended standard continuous rating and the expected surge demand your inverter must briefly handle.

Example

An 800W continuous load that's motor-heavy (refrigerator compressor plus other equipment) with a 20% safety margin: recommended continuous rating rounds up to 1,000W, while surge demand at a 3x motor-starting factor reaches 2,400W - meaning the inverter needs a surge rating well above its continuous rating, not just the continuous figure alone.

Recommendations

Always check an inverter's surge (peak) rating specification separately from its continuous rating - a 1,000W continuous inverter with only a 1,200W surge rating will fail to start a refrigerator or well pump even though the continuous load fits comfortably.

Frequently Asked Questions

Why do motors need so much starting power?

Induction motors draw 3-8 times their running current for a fraction of a second at startup to overcome initial mechanical inertia and reach operating speed, which is why motor-heavy loads need a much higher surge rating than their steady-state wattage suggests.

What safety margin should I use?

20% is a reasonable general-purpose margin for headroom and future load growth; systems with frequent motor starts or critical backup power applications often use 25-30% for extra reliability.

Can I run multiple motor loads on one inverter?

You need to account for the largest single motor's surge demand added to the continuous running load of everything else already operating, since multiple motors are unlikely to start at exactly the same instant but their combined running loads do add up continuously.

Last reviewed: 2026-09-12