Sizing an inverter when motors start, not just run
Continuous rating versus surge capability, and the load types that decide which governs.
The engineering question
The inverter trips every time the compressor starts. Is it too small, or is the surge the real constraint?
Worked example
The figures below were produced by running the Inverter Sizing Calculator with the same inputs — during the build, from its authoritative engine.
| Continuous rating required | 4375 W |
|---|---|
| Surge rating required | 14000 W |
| Assessment | Size the inverter for ≥ 4375 W continuous and ≥ 14000 W surge (motors start at 3–5× running load for a few seconds). |
- Continuous rating is the largest simultaneous load plus a design margin.
- Surge rating is that load multiplied by the starting factor for the load type.
- The inverter must satisfy whichever requirement is larger.
Sanity check
Compressor and motor starting can be three to five times running load, but only for a few seconds. A soft starter may be cheaper than a larger inverter.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- IEC 62109 — safety of photovoltaic inverters
- Manufacturer surge duration data
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