Generator Sizing Calculator
Calculate the minimum generator size (kVA) needed to power your essential appliances during an outage. Enter the running watts and surge watts of each device, and get the total power requirement.
Published by:
isCalculator Editorial Team
Was this calculator helpful?
How to use this tool?
Follow these steps to calculate the minimum generator size for your appliances:
- Add an appliance – Select a preset from the dropdown, or enter custom Running Watts and Surge Watts manually.
- Click “Add Appliance” – The appliance appears in the list on the right. You can remove any item by clicking the icon.
- Repeat for all appliances you want to power simultaneously.
- View results – The calculator shows:
- Total Running Watts – sum of all running watts.
- Highest Surge Watts – the largest surge value among your appliances.
- Minimum Generator Size (kVA) – the larger of the two values divided by 1000 (assumes unity power factor).
- With 20% safety margin – recommended size for reliable operation.
- Clear all – Use the “Clear All” button to reset the list.
Note: All values are in watts (W). Surge watts represent the momentary startup power. For safety, always use the generator size with the 20% margin.
Previous Results
| Result | Inputs | Date |
|---|
Frequently Asked Questions
Running watts are the continuous power needed to keep an appliance running. Surge watts (or starting watts) are the extra power required to start motor-driven appliances like refrigerators or pumps. Generators must handle both.
Adding a 20% margin ensures the generator is not running at full capacity continuously, which can cause overheating and reduce lifespan. It also provides extra capacity for unexpected loads.
This calculator is designed for single-phase generators. For three-phase, the calculation is different (kVA = volts x amps x 1.732 / 1000). Consult an electrician for three-phase sizing.