Solar Sizing
Inverter Sizing Calculator
Pick an inverter kW rating from your running and surge loads
Updated 12 July 2026 · Live
What this tool does
Sizes a solar inverter from your continuous running load and a safety margin, and displays your peak surge load so you can check it against the chosen inverter's surge rating.
Enter the continuous load ({continuous_w} W), the peak surge ({surge_w} W) and a safety margin ({margin_pct}%); the calculator recommends the nearest standard inverter size: {result}.
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Formula Used
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.
Sources & methodology
Sizes a solar inverter from your continuous running load and a safety margin, and checks the surge load against the running figure. Every result is calculated from the values you enter, all inputs are editable, and no time-sensitive prices or tariffs are embedded.
Frequently asked questions
What is the difference between continuous and surge?
Continuous is the steady load the inverter carries indefinitely; surge is the brief spike when a motor starts. Inverters quote both, and both must be covered.
What safety margin is sensible?
A 20–30% margin above the continuous load gives headroom for extra appliances and avoids running the inverter flat out. Larger margins add cost.
Why round to a standard size?
Inverters are sold in fixed ratings. The tool picks the smallest standard rating that meets your load plus margin so you can shop for a real product.
Does a bigger inverter waste energy?
A heavily oversized inverter has higher idle draw. Matching the size to your load plus a sensible margin keeps standby losses down.
Spotted something off?
Calculations or display — let us know.