Inverter Size Calculator
Size an inverter for continuous and surge loads with headroom.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Inverter Size Calculator does
An inverter must handle both the steady load and the brief surge when motors start. Surge demand often sets the size rather than the continuous figure, and the DC current it draws determines the battery cable size.
Formula
Continuous rating = Load × (1 + Headroom)Surge rating = max(Surge load, Continuous rating)DC current = Load ÷ Efficiency ÷ System voltageVA rating = Continuous rating ÷ Power factor
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Continuous load | W | Yes | Accepts more than 0. |
| Peak surge load | W | Yes | Motors draw 3–7× their rated power on startup. Accepts 0 or more. |
| Design headroom | % | Yes | Spare capacity for future loads and safety margin. |
| Inverter efficiency | % | Yes | — |
| Battery system voltage | V | Yes | Accepts more than 0. |
| Load power factor | number | Optional | Accepts 0.1 or more, up to 1. |
How to use it
- Enter Continuous load, Peak surge load, Design headroom and Inverter efficiency and 1 more.
- Optionally add Load power factor.
- Select Calculate.
Worked example
A 1,200 W continuous load with a 3,000 W surge, 25% headroom, 90% efficiency at 24 V.
- Continuous
- 1200 W
- Surge
- 3000 W
- Headroom
- 25
- Voltage
- 24 V
Continuous requirement 1,500 W → 1,500 W inverter. DC current 55.6 A running, 138.9 A on surge.
Frequently asked questions
Why does surge rating matter so much?
Induction motors draw several times their rated current at startup. An inverter that cannot supply that surge will shut down even though the running load is well within its rating.
Method and sources
Method. Continuous rating from total running load plus the headroom percentage entered, with surge capacity considered against the largest starting load.
Applies to. None applied — installation is governed by national wiring regulations
Assumptions
- The running watts entered represent simultaneous operation, and surge figures are available for motor loads.
Limitations
- The headroom percentage is a planning convention, not a standard. What margin is appropriate depends on the loads and how much simultaneity is realistic.
- Surge, not continuous rating, is what fails first with motor loads: a compressor can draw several times its running current for a moment, and an inverter sized on running watts alone will trip.
- This is a sizing estimate, not a system design. Wiring, protection and earthing are governed by regulations and are not addressed.
Sources
- The national wiring regulations, and the surge specifications of the equipment being powered — Varies by country and manufacturer. The installation requirements, and the surge figures that decide whether a given inverter will actually start the load.