Which Size Generator do I Need?

One of the most important considerations when purchasing a diesel generator is making sure it can cope with your requirements.

If the generator is not the right size of power output it can cause damage to both the equipment connected to it as well as the generator itself.

Prime, backup, stand-by, single phase, three phase, kW, kVA……. It can all get a bit confusing and make you wonder where to start!

Firstly, it is essential to talk to us about what you need and expect from making this large purchase.  We have years of experience and are well-equipped to help you and explain the various options available and make the relevant calculations.  Getting your calculation as accurate as possible will avoid system failures, capacity overload, a risk to personal safety.  There is also less chance of damage to tools or the generator itself, a better performance and longevity of the generator.  A well worthwhile exercise indeed!!

Your requirements

  • Compile a list of all equipment / devices that need to be powered by the generator. Include in this list the starting running wattage of each item.  (This is often found on the specification place of the device itself).
  • Calculate the total power required in kW or kVA.

This will give you an idea of the minimum power output you should be looking for.  In this instance more is definitely better than less!

Ampere – Wattage Conversion

Sometimes the power requirements of tools are stated in Amperes.  To convert the power requirement of a tool from Amperes to Watts use the following calculations:

For resistive load – Wattage = amperes x volts

For reactive load – (amperes x volts) x load factor

We also have a calculation tool in our Information Charts / Calculators section .

Occasionally, the power requirements of a tool / device cannot be found.  Obviously these still need to be taken into account when calculating your total power requirements.  Our Power Consumption chart in our Information Charts / Calculators section gives you typical wattages used for common tools and appliances.  If you are unsure about a tool, appliance or device, let us know and we can advise you.

Contact Eagle Generators today if you need any help deciding on which generator is best for you. Call us on freephone 0800 999 3303+44 (0) 121 5570270 or e-mail us on sales@kw1.com. today.


Eagle Generators Ltd – Specialists in used diesel generators for over 35 years.