What this calculator does
An inverter that is too small clips the array output; one that is too large wastes money. This calculator sizes the continuous rating from your load with a margin, then checks whether your worst starting load fits within the surge capability you are likely to need.
How to use it
- Enter the largest simultaneous continuous load, in watts.
- Select the surge factor: 1 for resistive loads, 3 for motors, 4 for compressors and 2 for a mixed load.
- Set the margin, 25 % being a common allowance for future loads and inverter derating at high temperature.
- Size the inverter for whichever is larger — the continuous requirement or the surge requirement.
Governing formula
P_cont = P_load × (1 + m) ; P_surge = P_load × f_surge Where:
-
m= Design margin [–] -
f_surge= Surge factor for the load type [–]
Worked example — 3,500 W of loads with motors
The figures below were produced by running this page's own engine with the example inputs (Largest continuous load (W), Largest starting load, Design margin (%)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Continuous rating required | 4375 W |
|---|---|
| Surge rating required | 7000 W |
| Assessment | Size the inverter for ≥ 4375 W continuous and ≥ 7000 W surge (motors start at 3–5× running load for a few seconds). |
- Continuous rating with a 25 % margin is 4,375 W.
- A surge factor of 2 requires 7,000 W of surge capability.
- Select the smallest inverter meeting both — for this load, around 4,500 W continuous with at least 7 kW surge.
Assumptions & limitations
Engineering calculation. Continuous sizing with a margin plus a surge requirement derived from a load-type factor. It does not model efficiency curves, parallel units or grid-tie export limits.
- Surge capability is quoted for a few seconds only; large motor starts that take longer need a soft starter or a larger unit.
- Inverter efficiency varies with load, so a heavily oversized unit runs less efficiently at low load.
- Does not consider battery voltage window, grid-tie export limitation or parallel inverter configurations.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- IEC 62109 — safety of inverter converters for photovoltaic systems.
- Manufacturer datasheets for surge duration and continuous rating.