Choosing the right generator size is a question we get all the time at Spitfire Engineering. Go too small, and you risk power outages and equipment damage. Too large, and you’re wasting money and fuel. Here’s how you can work it out for yourself—no jargon, just a simple formula and a real-world example.

 

The Simple Generator Sizing Formula

To calculate the generator size you need, follow these steps:

1. List all the equipment you want to run at the same time.
2. Find the running power for each item (usually in kW, or convert from amps and volts: Amps × Volts ÷ 1,000 = kW).
3. Check if any items have a high starting load (motors, compressors, etc.). If so, double their running power to account for the surge.
4. Add up all the running powers (including surges).
5. Add a safety margin—multiply your total by 1.2 (for 20% extra capacity).

Worked Example

Let’s say you need to power the following equipment on your site:

• Deck screen: 7.5 kW (motor, high starting load)
• Conveyor: 3.0 kW (motor, high starting load)
• Lighting: 1.2 kW (no high starting load)
• Office cabin: 2.0 kW (no high starting load)

Here’s how you work it out:

1. Double the running power of items with a high starting load:
   - Deck screen: 7.5 kW × 2 = 15.0 kW
   - Conveyor: 3.0 kW × 2 = 6.0 kW
2. Add in the rest:
   - Lighting: 1.2 kW
   - Office cabin: 2.0 kW
3. Total load = 15.0 + 6.0 + 1.2 + 2.0 = 24.2 kW
4. Add a 20% safety margin:
   24.2 kW × 1.2 = 29 kW (rounded up)



Recommended generator size: 29 kW

Key Takeaways

• List all your equipment and their running power.
• Double the kW for anything with a high starting load.
• Add everything up and multiply by 1.2 for your safety margin.
• Round up to the next available generator size.

Still unsure? Every site is different—if you want peace of mind, contact Spitfire Engineering. We’ll help you get it right, first time. If it’s to be, then it’s up to us to help you choose the right generator for your needs.