Inverter Sizing Calculator

How the inverter sizing works

The continuous load is grossed up by the safety margin, then rounded up to the nearest standard inverter size stocked worldwide. The surge figure is compared separately, because motors briefly draw several times their running power at start-up and the inverter must ride that surge without shutting down.

What you enter

Enter the continuous load, the peak surge and a safety margin; the calculator recommends the nearest standard inverter size. Every field is editable, and the result recalculates the moment you change a value.

Reading the result

The recommended size is the smallest standard inverter that covers your continuous load plus margin. Always confirm the inverter’s own surge rating exceeds the surge figure shown.

Safe by design

This calculator does the arithmetic of sizing — quantities, geometry and cost. It deliberately stops at the line a certified installer must sign off. It will never tell you a cable or fuse size, an earthing arrangement, a battery charge voltage, or a roof structural load: those are regulated calculations where a wrong number causes fire, electrocution or collapse. All electrical work must be carried out by a qualified, certified installer, and this page is not an installation guide.

Assumptions and limits

This sizes the inverter’s power rating only. It does not select the DC input configuration, protective devices or wiring — those are the installer’s regulated calculations. A single very large motor can dominate the surge requirement even when the continuous load is small.

Using it with the rest of BuildMetricLab

This calculator is one step in planning a system. Pair it with the other solar sizing, geometry and cost tools to go from an energy figure to a panel count, a layout and a costed bill of materials. Every BuildMetricLab tool runs entirely in your browser — no sign-up, nothing sent to a server, and the formula is shown on the page so the maths can be audited.