SIZE IT RIGHT, FIRST TIME
Inverter Sizing Calculator
List your appliances to get a recommended inverter size that covers both running load and motor startup surge.
Your appliances
0 items| Appliance | Qty | Watts | Surge | Remove |
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Surge is the extra startup power a motor or compressor briefly draws — pick 1× for lights and electronics, 2–3× for fans, pumps, fridges and ACs.
System settings
Sets a recommended safety margin below — you can still override it.
Extra headroom for appliances you might add later.
Typical pure sine inverters run 90–95% efficient.
Auto-filled from system type. Edit it if you want a tighter or wider buffer.
Advanced: power factor
Only matters if your inverter is rated in VA rather than watts. Leave at 1 for a standard resistive/mixed household load.
Recommended inverter size
How to use this calculator
- List every appliance you want the inverter to run — quantity, running wattage, and a surge multiplier for anything with a motor or compressor.
- Pick your system type. This sets a sensible default safety margin; commercial and industrial setups default a little higher than residential.
- Set expansion and efficiency. If you plan to add appliances later, raise the expansion margin. Leave efficiency at the default unless your inverter’s datasheet says otherwise.
- Read the recommendation. The result updates as you type, showing the nearest standard inverter size along with whether it’s a comfortable fit or a tight one.
- Check the surge warning. If one appliance’s startup surge dominates the sizing, the tool flags it — that’s often the real reason a “small” load needs a bigger inverter than expected.
How the numbers are calculated
- Total running load (W) = Σ (watts × quantity) across every appliance.
- Surge load (W) = running load + the single largest surge appliance’s extra startup draw. Motors rarely all start at the exact same instant, so this calculator only adds headroom for your biggest single surge event rather than summing every appliance’s surge together — see inrush current for background on why motors briefly draw more than their rated wattage.
- Required capacity (W) = surge load ÷ inverter efficiency × (1 + expansion margin) × (1 + safety margin).
- Recommended size = the nearest standard inverter rating at or above the required capacity.
- Utilization (%) = running load ÷ recommended inverter capacity — a lower number here means more everyday headroom, even if the surge briefly uses more.
Worked example
A home running a 150W fridge (3× surge), a 750W water pump (3× surge), 5× 10W LED bulbs (1× surge), and 2× 75W fans (2× surge) has a running load of 1,100W. The pump’s surge briefly adds 1,500W on top of that, for a surge load of 2,600W. After a 92% efficiency allowance, a 20% expansion margin, and a 25% residential safety margin, the required capacity comes to roughly 4,240W — rounding up to a 5kW inverter, even though everyday utilization sits at a comfortable 22%.
Frequently asked questions
Related tools
Use these alongside your inverter recommendation to plan the rest of your system.
Related articles
A few guides worth reading alongside your result:
Results are estimates. Verify your final inverter selection against the manufacturer’s specifications and with a qualified installer, especially for high-surge appliances such as air conditioners, refrigerators, and water pumps.