Plan reliable home backup
Inverter Size Calculator
Estimate planning inverter capacity from your connected load, safety margin and power factor.
Smart result
Your inverter estimate will appear here
We will turn the inputs into a clear answer, explain what drives it, and show what to do next.
Quick answer
This calculator estimates the minimum inverter size, in VA, for the appliances you want running together during a power cut. Add up their running watts, keep some headroom and the calculator converts that load into the VA rating printed on inverters. It sizes the inverter only; use the battery calculator for backup hours.
Real-world example
Suppose the fans, lights, TV and Wi-Fi router you want during a power cut add up to 750 W, with the default 25% headroom and a 0.8 power factor.
- Running load
- 750 W
- Headroom
- 25%
- Power factor
- 0.8
Look for an inverter rated at least 1,200 VA.
750 W plus 25% headroom is 938 W; dividing by 0.8 and rounding up to the next 50 VA gives 1,200 VA.
Illustrative load. Add the running watts of your own appliances from their labels.
How it is calculated
Headroom is added to the running load, the result is divided by the power factor to convert watts into VA, and the answer is rounded up in 50 VA steps so it is never below the calculation.
Inverter VA = running watts × (1 + headroom %) ÷ power factor, rounded up to the next 50 VA
Results are planning estimates, not engineering designs, quotations or eligibility decisions.
Assumptions
- Every appliance you list may run at the same time.
- The 25% headroom and 0.8 power factor are editable general defaults, not product ratings.
- Motor starting surge (fridges, pumps, mixers) is not calculated and needs a separate manufacturer or installer check.
What can change the result?
- What you run together
- Only loads used at the same time count. Leaving the iron or geyser off keeps the size down.
- Motor loads
- Fridges, pumps and mixers can draw several times their running watts for a moment when they start.
- Power factor
- A lower power factor needs a larger VA rating for the same watts.
- Future appliances
- If you plan to add loads later, more headroom avoids an early upgrade.
What should I do next?
- Next: Estimate the battery size for your backup hours →The inverter handles the load; the battery decides how long it lasts.
- Check the watts and running cost of each appliance →Find realistic wattages for the loads you want to back up.
- Compare on-grid, hybrid and off-grid solar for backup →See whether a hybrid solar system could cover power cuts.
Frequently asked questions
How much safety margin should I use?+
The editable margin allows normal headroom. Motor starting surge and simultaneous peak loads may require separate professional sizing.
Why is inverter capacity shown in VA?+
Inverters are commonly rated in VA. The calculator converts design watts using the entered power factor.
Is this enough for motors or pumps?+
Not by itself. Confirm starting surge and manufacturer requirements for motor loads.