Power You Can Trust



What size generator do I need? How to decide what size generator I need? These are such common questions, and one not easily answered. Finding the right petrol or diesel generator, which you can be sure will handle all your power requirements, can be a complex and difficult decision to make, especially if your knowledge of this subject is left wanting. To help as much as we can, WB Power Services have written this article give you an understanding of the fundamentals, how the sizing process works and some key things that must be considered, helping you decide what size generator you need.

Whether you work in the commercial, manufacturing, utilities or any other industry, there are several things which you’ll need to consider before making a definite decision on your generator size. Whilst we would never recommend ‘going it alone’ and buying without expert consultation (for that, contact us!). Instead, this article has been put together to provide the reader with a few simple tips and resources and give you enough detail to go to an expert, with some basic knowledge, which will help get you started.

Before we walk you most important things to consider, we think it is worth highlighting some of the key advantages of selecting the right generator:

  • Opportunity to expand your maximum output
  • No unexpected shutdowns due to overloading
  • No damage to connected appliances
  • Increased longevity
  • Reliable performance
  • Increased fuel-efficiency
  • Less requirement for frequent maintenance


It’s a very common misconception that smaller generators can be used for standby power because they are not running all the time. This is simply not the case and can lead to all sorts of detrimental problems. Generator under sizing is one of the most common mistakes which new buyers fall into. By under sizing your generator, you risk damaging your new generators, other appliances connected to it and create hazardous situations. The key lesson to remember here is make sure you know exactly what your generator is going to be doing on a day-to-day basis, whether it’s for a standby, prime or continuous application. Understanding this will go a long way to helping you decide what generator you will need. We’ll explain these applications in a little more detail later…


Generators come in a huge array of sizes and configurations, so this step is key to deciding what size generator you need. Try to make a list of all devices you intend on running off your new generator. This should not just be confined to key pieces of equipment, make sure you include small devices such as kitchen appliances. They may seem insignificant, but if all these devices are not factored in, it could make a difference to your requirements and leave you under powered. For each item, note the:

  • Running Wattage (energy needed to operate it)
  • Starting Wattage (energy needed to turn it on)

You will usually find these values on the devices themselves. If not, they should be in the user manual. If you cannot find the right information on either of these places, we recommend visiting the manufacturers website. You will then need to convert this kW value in kVA using the below formulas:

kVA x POWER FACTOR (0.8) = kW


Now, back to understanding the generators application. For prime applications (which means you’re going to be using the generator as your main power supply), you cannot run your generator at an average greater than 70% in a 24-hour period. As a result of this, you’ll need to size for 70% capacity.

If you’re set is going to run as a standby application, the same principle will apply, you cannot run your generator at an average greater than 70% in a 24-hour period. There may also be time implications e.g., not being able to run your standby set for more than one hour in a 24-hour period. All these standards are set out in ISO 8528-1:2018, put in place to stop the set becoming overheated and/or damaged. It is so important to adhere to these standards, deviating from them could have incredibly expensive consequences!


You should now have a clearer idea of your power requirements and able to go to a professional with a more detailed information. But another integral factor to consider is your premises itself, and whether your exact power requirements are feasible given the space, conditions, and location of your site. Key questions include:

  • Do you have an idea where your generator is going to go? Do you have limited space, and if yes, is your generator going to fit, does room need to be made?
  • If your environment safe for a generator, do you need to consider civil works such as a concrete base and fencing prior to installation?
  • Do you have environmental factors to consider, such as noise restrictions? If so, you may need to install the set in a bespoke acoustic enclosure.
  • How are you going to keep the generator fuelled? Will the generators internal fuel tank be enough, or do you need to consider an additional fuel solution?

There is certainly a huge amount to consider, and this part can be especially confusing for people with limited knowledge. When it comes to sizing your generator or helping to build a solution to fit your requirements, WB Power Services can help. We have an experienced team of Project Sales Managers who will be able to consult with you, assess your size requirements, visit your site and help you decide what size generator you need.


WBPS is a leading generator specialist that provides solutions, maintenance, generator sales and hire to customers across the UK. We can ensure that your operations can continue in the event of mains power failure. For all our new and existing customers, here is just some reasons why you can rely on WB Power Services.

  • Over 35 years of professional experience selling generators as standalone products or as part of a power solution
  • A mobile sales team who can discuss your requirements, offer advice, and visit your site to define your exact needs
  • We are the largest European Kohler authorised distributor, meaning we can offer the industry leading Kohler generator range at an affordable price, delivered on quick lead times
  • We have a nationwide team of generator maintenance engineers. If your generator ever has any problems, we will always be on-hand to visit you and fix the issue.
  • We can offer a 4 hour on-site guarantee for our generator maintenance customers