SpecCalc Hub Guide
Inverter sizing: continuous load, surge, and margin
Why surge loads and power factor matter before buying an inverter.
Continuous vs surge
Motors, compressors, and pumps can need much more power at startup.
Not the full design
Battery current, wiring, fuses, and thermal limits still need professional checking.
Why it matters
A label such as 1000 VA is not a promise of 1000 W. Likewise, a brief peak rating is not a continuous rating. Using only the sum of nameplate watts can miss startup failure or a separate apparent-power limit.
Formula or method
Continuous design watts = running load × (1 + chosen margin); required continuous VA = design watts / combined load power factor. Total startup watts = the starting device’s startup demand + other devices still running. In this calculator the planning margin applies to continuous watts; the surge field already contains the total intended peak. It does not automatically add another surge margin.
Worked example
Illustrative case: 300 W of other running devices plus a 150 W motor gives 450 W continuous. If that motor requires 750 W at startup, total startup demand is 1050 W, not 750 W and not 1200 W. With 25% continuous margin and PF = 0.8, the model gives 562.5 W continuous, 703.125 VA continuous and 1050 W peak. Verify that the manufacturer permits that peak for the necessary startup interval.
How to use in practice
Enter 450 W as continuous load, 1050 W as total peak, 25% margin and 0.8 PF to reproduce the example. PF for the steady combined load must not be assumed to describe the motor’s startup current or waveform. Inverter temperature derating and the battery system remain independent checks.
Comparison table
| Topic | Value | Note |
|---|---|---|
| Continuous active | 562.5 W | 450 W with 25% margin |
| Continuous apparent | 703.125 VA | 562.5 W / 0.8 PF |
| Total startup | 1050 W | 750 W startup + 300 W other running loads; duration checked separately |
Common mistakes
- Adding the motor’s running watts again after its startup watts already replace them.
- Assuming all compressors start simultaneously, or assuming none do, without checking the operating sequence.
- Treating a short peak, continuous watts and continuous VA as interchangeable ratings.
Limitations
- The 750 W startup figure is an illustrative input, not a default for all 150 W motors. Obtain actual startup and duration information.
- This example does not choose cables, fuses, earthing, battery BMS limits, waveform compatibility or a specific inverter product.
When to use the calculator
Use the calculator to compare measured or documented loads and explicit margins. Choose the product only after its continuous W/VA limits and startup-duration curve satisfy the application.
FAQ
Should the whole running load be multiplied by a motor startup factor?
Not automatically. Calculate the loads actually starting at that instant and add the other running loads. Several simultaneous starts require a separate combined scenario.
Does dividing by PF turn watts into more watts?
No. Dividing active watts by power factor yields apparent power in VA in this model. Both limits must be checked against the inverter documentation.
How long must the surge rating last?
Long enough for the actual load to start under the operating conditions. There is no universal duration; compare the load requirement with the manufacturer’s surge curve.
What should I document before choosing an inverter?
Write down the continuous load, the devices that can start together, the peak duration required by each motor or compressor, and the product limits stated by the manufacturer. This keeps the calculator result tied to real equipment rather than a generic safety margin.
Related guides
Sources and equipment verification
Explanation updated: 2026-09-16