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Top 5 Mistakes

When Buying a Used Generator

Below are the top 5 most common mistakes people make when buying a used diesel generator. This helpful guide could help save you time and money and ensure you buy the right generator for you.

1. Buying on Price:

The old adage of you get what you pay for is certainly true when buying used generators. We often here customers say that the generator is only going to be used for standby power less than 5 hours per year and as such they do not want to pay much money for the generator.  The purpose of buying a standby generator is that it will replace your mains when the mains fails i.e. it needs to work and is critical for your facility. What is the point of buying a cheap generator that when the mains fails also fails and leaves you without power? As with all industries there are many different grades of used generator company – please buy from one that fully tests their products and shoes evidence of this through full test reports. At FW Power we undertake a 75 point service inspection check and a full load test so that you can be confident of the equipment we supply.

2. Generator Not Fully Tested:

That brings us on nicely to the second mistake – buying a generator that is not fully tested. Too many times we see used generators that have been washed and ran for no more than 10 minutes and are put up for sale. This is nonsense !! The only way to know if a generators works correctly is to run it under full load using something known as a load bank. At FW Power all of our generators have a full load test running through the various load steps of the generator and then are finally loaded to 50% of the rated load in one step to mimic the high impact loads seen in industrial facilities. We also complete our 75 point service check which ensures that all parts of the generator work correctly and the vital shutdowns are there to protect the generator in the event of a fault. That is why it takes us 3-4 days minimum to run all of our tests on a used generator. All our tests are certified and you the customer get a copy of these certs which shows all of the work we have done – make certain that you get this information from whoever you buy a used generator from – it is the only way to know that it will work correctly and give you peace of mind.

3. Relying Solely on Age & Hours.

The age and hours play an important part when deciding what generator to buy but they do not give the whole story. A poorly maintained generator or one that is ran on low quality fuel  can have significant issues within 500 run hours. We once had a generator with less than 50 run hours where the fuel injection pump and injectors required replacing due to fuel issues. On the flip side we maintained a generator that finally gave up after 68,000 run hours but had been maintained perfectly throughout its life. In general, the lower the hours on a well maintained generator should give a generator with the least amount of wear – the exceptions to this are those mentioned above and when a new generator is ran on loads of less than 30% of its maximum which will cause glazing of the bores. Equally with age, a newer generator will have less time dependent failures such as rusting, dry seals, hoses etc. Normally an older generator with higher hours will sell for a reduced price and this can be a great cost saving for a standby generator – but please make certain that it has been well maintained and fully tested prior to buying it !!

4. The Generator Has Been Installed Incorrectly:

Through a 27 year career working with generators I have had countless “warranty” calls only to visit site and seen some horrendous installations !! The most common problems we see are:

1. Air inlets and outlets are restricted not allowing sufficient cooling air to flow.

2. Remote fuel tanks have been used where either the pipe work or the fuel tank is not suitable.

3 .Battery charger and jacket water heater auxiliary supply either not connected or connected incorrectly.

4. Incorrect phase rotation.

5. Incorrect earthing.

6. Poor or inadequate power cable connections.

7. Remote start incorrectly installed.

8. Incorrect exhaust systems causing back pressure issues and ultimately engine failure.

9. Incorrectly sized generators (see below).

Generator engineers are highly trained and highly skilled – I would personally not allow an engineer to undertake an install on their own with less than 3 years of experience but more usually at least 5 years. Yet, we regularly encounter people with zero experience installing generators and then they wonder why there is an issue. If you want your generator to stand the test of time then please install it correctly and as a minimum have a competent generator engineer to check over and commission  the generator before use.

5. The Generator is Incorrectly Sized:

This is much more of an issue than you might imagine and covers the more common under sizing of a generator but also the over sizing.

Let’s address the simple over sizing first of all. Usually encountered because an engineer has done a good job of looking at the inrush currents of the site but failed to realise that a generator is designed to take usually 2.0 to 2.5 its running current as an in rush current to assist with motor starting. In this case the client spends too much money on a generator, may use unnecessary fuel and may have a low loading problem.

The more common problem is that the client fails to realise the size of the inrush currents which on a Direct On Line motor can be up to 9 times the running current. It does not take too much thought to see how a generator will quickly be unable to cope with such loads and either the engine will stall or the control panel will see an under frequency / under voltage issue and shut the generator down. Sometimes the generator just about copes with the inrush current but when the load comes off quickly as the motor enters its steady running state then the engine begins to speed up, the AVR in the alternator is working hard and the generator shuts down on over voltage or over speed.

Please read the FW Power Guide to Generator Selection and Installation.


FW Power is here to assist you in solving these problems and finding the right Diesel Generator for your application.

FW Power – The Home of Diesel Generators



New & used Diesel Generators ranging from 10 kVA up to 2900 kVA. Working with leading brands to ensure a reliable power source. Each used generator undergoes our thorough 75 point service inspection & load bank test.


FW Power will be able to guide and advise what size ATS you need for your application.  With new or used available, our ATS’s will ensure a smooth transition from mains to  generator  power and back again.


Our skilled generator engineers are on hand to install and commission any sized diesel generator. We have experience on a wide variety of complex installations.


FW Power service and maintain both prime power and standby generators across the UK. We have our own fleet of load banks and a fully stocked parts warehouse. We offer many different packages and can tailor them to suit your needs.


Offered to our contracted customers we offer a direct number to one of our  experienced engineers who can normally get you up and running over the phone until a full repair can be undertaken.