Generator Sizing and Selection Guide

Buying a generator for either business or personal use is a significant investment so it is important to get it right first time. There are a few factors to consider when buying a generator so this will help you choose the right unit – the right fuel, the right type and the right size of generator for your needs.

What will you use your generator for?

  • Leisure, Camping or hobby use generators – these tend to the smaller units and normally run at 3000rpm. These are for occasional use and not for using a ‘prime’ power supply for long periods. These units can be used for short term ‘standby’ in the event of mains power loss by running extension leads into the property to run essential items such as freezers and lights.


    • What is an Invertor Generator? – Invertor generators are the latest technology in smaller petrol and gas generator – rather than the engine running at a fixed speed the engine revs raise and lower automatically on the load being applied. The energy from the engine is then fed to an ‘inverter’ to convert it to AC current. This means lower noise and lower fuel consumption it lower loads – these are the best choice for leisure use.


    • Fixed Speed Petrol Generator – The older design fixed speed petrol generators are still a good choice for several applications – sudden higher loads (vacuum cleaners, pressure washers etc) and when a more rugged and durable generator is needed – for professional use or on construction site for example.


  • Residential Use – there are two main types of residential generator installation:-


    • Off Grid Living – when a person decides to have an independent power supply rather than connect to the electricity grid. It may be that the property is too far away from the nearest power lines or it is too expensive to install mains power. It is best to consider a generator as part of the system, perhaps with solar panels and batteries.


    • Residential Standby Generators – Some properties are prone to power cuts and this can be a problem for modern ‘smart’ houses or where a family relies on power for health reasons. These will be smaller versions of the business standby generators and will normally be permanently installed. 3000 and 1500rpm generators are used for this application.


  • Business Use – again this can be split into two sections


    • Prime Power Use – when mains power is not available or does not have the capacity needed to run your business. Sometimes businesses need more power to run a special machine or process and a generator can be used for this purpose.
    • Standby Use – when the business normally runs on mains electricity but needs to have a reliable electricity supply. If the power fails then the generator will start and supply power automatically. An ATS (automatic Transfer Switch) is installed to monitor the mains, start the generator and apply the load.

Businesses will normally use 1500rpm generators and choose a good quality manufacturer with excellent aftersales and parts availability.

What Fuel is best for a generator?

  • Petrol Generators – These are the most economical to buy but if you are using it for long periods then the fuel consumption can be high. Petrol generators are also light and transportable meaning they can be easily stored and moved into position when needed.

The petrol can deteriorate over a relatively short time so they are best used often or stored without fuel or fuel ‘stabiliser’ added to increase the time.

Camping and leisure generators are mainly petrol but can also be LPG (gas) – it is rare for a diesel generator to be used for leisure purposes.

All petrol generators run at 3000rpm (for 50hz applications) to ensure they are compact, lightweight and portable.

  • Diesel Generators – These are more expensive but are more suited to longer running applications and use less fuel so can work out cheaper overall. They are generally noisier then a similar output petrol or gas generator so are not suitable for sensitive applications. They are heavier and more difficult to transport so are normally used for permanent installation and applications where they will be in position for longer periods.

Diesel generators can be 3000rpm or 1500rpm:-

3000rpm Diesel Generators are cheaper and more compact but can be noisy, have higher fuel consumption and shorter service life.

1500rpm Diesel Generators are more expensive and larger but are much quieter, lower fuel consumption and have a longer service life.

The noise of a diesel generator can be reduced with special acoustic enclosures which are designed to suit the project. A standard enclosure will normally provide a noise level of around 85dB(A) at 1m.

  • Rechargeable ‘Fuel-less’ Generators – portable power stations – if noise, fumes and messing about with petrol or diesel is putting you off then take a look a our range of portable power stations – no fuel, low noise, no emissions whatsoever. These are ideal for camping or leisure trips and for keeping in the home for when that power cut happens.

Sizing a Commercial Standby Generator

A standby generator for your business is an important aspect of your business continuity and disaster preparedness and recovery plan. The size or rating can be calculated from the equipment already installed or it can be measured over time using a meter on the incoming power supply.

Motors loadings are particularly important for generator sizing as these can place unusual and high loads on the generator for very short periods which need to be taken into account. Motors are found in lifts, air-conditioning units, pumps, fans etc so it is worth spending some time researching the requirements of your building.

Use our motor starting calculator to estimate the size of generator required.

Motor Starting is based on 400V, 3 phase with typical motor performance.

These calculators are based on standard conditions and your installation and application will be different. Please ensure any generator purchased is suitable for your application.

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Need expert advice?

Let us know if you have any questions or queries and we will be happy to give you expert advice.
Call us on 01329 722390 to discuss special pricing, installation, service agreements or special projects.​

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