Enter your connected load, power factor, and surge requirement to get the recommended inverter VA/kVA rating — plus the nearest standard commercial size.
Fill in your total connected load and any motor surge requirement below.
— kVA continuous rating needed
Enter your values and hit calculate to see the nearest commercially available inverter rating.
An inverter converts stored DC battery power into the AC power your appliances need, and it has to be rated for two different things at once: the apparent power (VA) of everything running continuously, and the much higher surge power needed for a fraction of a second when a motor-based appliance — a fridge, pump, or air conditioner — starts up. Sizing for running watts alone is one of the most common reasons an inverter trips or fails shortly after installation.
Since most household and office loads aren't purely resistive, the total watt figure has to be converted to VA using the load's power factor, because the inverter is limited by the apparent power it can deliver, not just the real power: Apparent Power (VA) = Total Watts ÷ Power Factor. A safety margin is then added on top to account for voltage sag, future load growth, and the fact that no inverter should be run right at its rated limit continuously: Recommended Inverter (VA) = Apparent Power × (1 + Safety Margin%).
Separately, the surge rating must cover the single largest motor-driven appliance's startup current, since capacitor-start motors like pumps and compressors typically draw 2 to 7 times their running wattage for a second or two at switch-on — even if the inverter's continuous rating is otherwise more than enough: Surge Requirement (W) = Highest Motor Running Watts × Surge Multiplier. The chosen inverter's surge rating must always be greater than or equal to this figure.
Once the recommended VA is known, this calculator also matches it against the common commercial inverter sizes sold in the market — 650, 850, 1100, 1600, 2000 (2 kVA), 3000, 3500, 5000, 7500, and 10,000 VA — and suggests the smallest standard size that comfortably covers your calculated requirement, since inverters are rarely sold in arbitrary VA ratings.
Worked example: A home with 1,100 W of continuous load (lights, fans, TV, router) at a 0.8 power factor needs 1,100 ÷ 0.8 = 1,375 VA. Adding a 25% safety margin: 1,375 × 1.25 ≈ 1,719 VA, so a 2 kVA inverter would be selected. If the highest motor load is a 200 W fridge compressor with a 3x surge multiplier, the surge requirement is 200 × 3 = 600 W — comfortably within a 2 kVA inverter's surge capability, which is typically rated well above its continuous VA.
An inverter's output stage is limited by the total current and voltage it can supply — the apparent power — regardless of how much of that is converted into real, useful work by the load. Since most connected loads have a power factor below 1, the VA required is always higher than the watt figure, which is why inverters are rated in VA rather than watts.
Surge power is the brief spike in current a motor draws at the instant it starts, often 2–7 times its running wattage. An inverter that is correctly sized for continuous load can still fail to start a motor if its surge rating isn't high enough, so both figures need to be checked independently.
Manufacturers only produce inverters in a fixed set of VA ratings (650, 850, 1100, 1600, 2000 VA and so on). Rounding your calculated requirement up to the next available size ensures you can actually purchase a matching unit, rather than being stuck between two standard ratings.
No — this tool focuses purely on the inverter's VA and surge rating. For battery Ah sizing based on your desired backup duration, use the UPS Calculator or the battery backup section of the Solar Panel Size Calculator.