How the inverter sizing calculator works
Last updated 10 November 2026
An inverter that is too small trips when the load is high. One that is much too large costs more than needed. The right size follows from what you want to run at once.
How it works
- Running load = the sum of watts for appliances on together.
- Surge = the largest single extra start-up draw (assuming one starts at a time).
- Minimum size = the larger of the running load plus margin and the peak, divided by the power factor, in VA.
Worked example
With the starting figures:
- The running load is 1 750 W.
- The biggest start-up surge adds 300 W, a peak of 2 050 W.
- With the margin and power factor, the minimum size is about 2 700 VA.
Why size matters
An inverter that is too small trips when the load is high. One that is much too large costs more than needed. The right size follows from what you want to run at the same time and from the start-up surge of motors such as fridges and pumps.
How to use it
- List each appliance with its power in watts and how many you have.
- Mark which ones must run together, so you do not add up everything in the house.
- Mark appliances that surge when they start, such as fridges, freezers and pumps.
- Choose a safety margin.
How the result is worked out
The running load is the sum of the watts for appliances that are on together. The surge is the largest single extra draw at start-up, assuming one appliance starts at a time. The minimum size is the larger of the running load plus margin and the peak load, divided by the power factor, and shown in volt-amperes.
Common mistakes
- Adding every appliance, which gives a size far above what you need
- Forgetting heating appliances such as kettles and ovens, which are large loads, and which many people leave off the backup circuit
- Looking only at the headline VA rating and not at the continuous and surge ratings
- Ignoring the battery, which must also supply the load
An estimate, not a design
The result is a starting point for talking to an installer. It does not replace a site visit or electrical design. For how long a battery will run the load, see the battery runtime calculator. Nothing you enter leaves your browser.
Things to watch
- Count only what must run together, not everything in the house.
- Heating appliances such as kettles and ovens are large loads. Many people leave them off the backup circuit.
- Check the inverter’s continuous and surge ratings, not just the headline VA.
See also the battery runtime calculator. Read the full disclaimer.