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What is a Petrol Generator

What is a Petrol Generator and How Does it Work?

If you are buying a generator you need a reliable source of power that might be for working out doors but it might be a good deal more important that – it could be a power fall back – something you rely on in the event of a power-cut.  Of course, they are commonly used on building sites, catering vans, fairground entertainment, anywhere you need a reliable source of power.  Maybe it’s even the main source of power – either way better to know what’s what.

Seems like I was looking too comfortable sipping my tea and sneaking in a chocolate biscuit – the boss spotted me and the next thing I knew I had a reflective vest in my hand and I was pushed out into the inhospitable deep space of the warehouse. I spent a bit of time with one of our techies and was pretty surprised to find that generators don’t actually create electricity!  I mean, that’s what actually happens on the outside right - petrol in and electricity out? In essence yes, but let’s take a moment to delve a little deeper and find out how a standard domestic generator actually works. It could well be worth knowing!

I was booted out into the void knowing nothing and now, after just a little while, I feel like Spock from Star Trek – the roving reporter version! Even more so, I’ve learned the techies really know their stuff.  Alright, so - you put the fuel in one end and pull the starter a couple of times and you’re fired up and electricity is flowing out of the other end. But how exactly does that happen?

Chemical to Mechanical

From the get go, the chemical energy in the petrol is immediately converted to mechanical energy by powering a four-stroke engine. The engine turns a crank shaft inside the machine. Okay. The genius (thanks to Mr Edison) is in the fact that this then produces energy by way of electromagnetic induction. Wow!

Mechanical to Electromagnetic

Okay, this is where we drill down to the detail.  What happens is that the shaft drives an alternator that in turn generates an EMF (Electro Magnetic Field) by spinning a magnet in a coil made of wire. In a quality generator the coil is usually made of copper or aluminium.

The spinning motion between the magnetic and electric fields builds a movement of electrons. This is generally known as the current. This electricity in its ‘raw’ state is then converted to electricity that you can put to use in infinite ways. This current is known as AC (alternating current). Most generators are fitted with an AVR (Automatic Voltage Regulator) which keeps the output voltage stable and helps to prevent damage to your electrical equipment.


This is the bit that your reporter is most concerned about, let me tell you.  You know why? Because this is the bit I know will concern YOU the most.  It's double underlined in my note book – got to be important. It’s a world of on-line choice and you’ll need to know that you’re getting what you want and need. Size matters apparently – the size of the output in this case.

Let’s keep it simple. By far the most important part of any generator is the power it gives you, and how much... after all it’s what you want it for! So remember - output of any generator is given in voltage and kVa (or kW).

You can check out more about this in our handy guide - Which Generator is for You? Written by your roving report who now has more oil on his hands than ever before!  Why didn’t I get the brief that included the jolly to London? Luckless but increasingly into engineering – that’s me!


Your reporter can inform you that voltage is super important. The voltage of an appliance is given in volts (or V for short). Here in the UK when you plug something into the wall at home the voltage that is being delivered by our electricity network is 230V.

230V - 16A Socket 230V - 16A Socket

Some power tools and other electrical equipment require other voltage though. If you plug an appliance that requires anything other than 230v into a 230v circuit it simply won't work and in most cases will cause damage to both the appliance and your generator.

If we was to describe what "voltage" is - it is the fuel which powers anything electric. Think of a car - every car needs fuel, either petrol, diesel or LPG. Voltage is the fuel anything electrical needs to function.

Some generators can output both voltages – but not at same time of course.  That’s like having a shower in the bath. Confusing and wrong. No, you simply switch down from 230v to 110v - it’s one or the other. They can also output multiples of each - example 2 x 230V and 1 x 110V giving you the flexibility to operate two 230V appliances at the same time (Kettle and Toaster.... nobody likes cold toast with a hot cup of tea).

Another point to consider is the way in which you take delivery of that voltage can also be somewhat confusing. You will see that most petrol generators do not have have the same socket as you find at home. You know the nice white one you have (or Brushed / Polished Chrome if you're really posh) with the three rectangular shaped openings (One at the top and two at the bottom).

They are instead fitted with industrial type socket which are capable of carrying the same voltage as your home but with a lot more current (amperage). Without going into much of the technicality behind why; your home plug socket are designed to carry a maximum of 13 amps (and the reason why the largest fuse you can fit inside one is 13A). Most petrol generators they can usually carry a lot more load and are fitted with an industrial type socket which can carry that additional load (Usually 16/32a).

The added benefit of having this type of plug is that they are usually a lot more resistant to dust and water ingress (handy for when you leave the generator outside).

The disadvantages of having this type of plug is that if you choose to run your home appliances with your generator you will either need to rewire a new plug, which isn't ideal if you're going to be powering it from both your home and generator.  Or indeed you can opt for the easier option and buy/make an adaptor to convert the plug from the white three pin plug you have at home (13A) to the larger 16A type shown above.

This type of adaptor is often referred to as a "Fly Lead" and are available to buy from most online retail stores. It is important to note that whilst it is perfectly safe to use a fly lead to get your 13amp plug into a 16amp socket - it is not recommend to fit a 16amp plug in a 13amp socket (It would be like trying to fit a size 10 foot into a size 8 shoe).

Finally - the colour of the electric socket on your generator will also tell you what voltage it is giving out. Handy eh?

  • Yellow = 110V
  • Blue = 230V
  • Red = 415V

Each coloured socket is a different style too. This will prevent you or your apprentice from plugging a yellow (110v) plug into a red electrical socket (415v).


All generators will produce a rated and maximum kW or kVa.  In order to simplify the complex understanding of kVa vs kW see the below examples:

  • 1kW = 1.25kVa (1 / 0.8)
  • 2kVa = 1.6kW (2 * 0.8)

We either multiple or divide the kW/kVa by 0.8 (the power factor).

Label shown on Appliances

We could really drill down into all that put, honestly, I’ll be including that elsewhere.  At present all we need to know is that knowing the capacity of your generator is super important – the last thing any one wants is you being in possession of a generator that won’t meet the demand of the tool. Like the terrible caffeine free coffee they always leave me. Useless.

Ask yourself this... before you choose the petrol generator you definitely need to understand what specification you require. To do this simply look at the appliance you want to power.

Most appliances will have a label affixed to give you the technical parameters in which it operates (if they don't - check the product literature).

This lamp for example uses 230v and draws 40w of electricity to run. Our PPG-2800 outputs a maximum of 2.8kVa (or 2,200 watts), so will comfortably run this lamp.

If you are running a number of appliances at the same time, then you need to combine the total wattage to ensure it is still under the maximum output of your generator. Also remember that some appliances do not draw power all the time. Your fridge for example will only need the electricity when it is running the compressor, or powering the light when you open the door.

You also need to ensure you have enough headroom. Generators will have both a rated and maximum output. It is not recommended that your generator outputs electricity at its maximum output for prolonged periods of time. It would be like driving your car down the motorway in 1st gear. Eventually something is going to give... and you'll end up on that hard shoulder waiting for the AA.

You will also need some headroom. Appliances fitted with an induction motor will draw more current on startup. General rule of thumb, multiply the wattage by 2.5 to get the wattage required on startup.

Example: A vacuum cleaner (which will be fitted with an induction motor) is rated at 600w. You will therefore need a generator capable of producing at least 1,500w (600 x 2.5) or 1.9kVa (1.5/0.8).

What voltage do I require?

This is the part we can't tell you - as we simply do not know. Voltage output from generators varies a great deal.  They go from 110v to a whopping 440v.

The two most common voltage requirements in the UK are 110 and 230V. Homes in the UK work on an 230v AC, so anything with a standard 3 pin plug that you plug in the wall at home will operate on 230v.

Power tools can work on 110 volts with large industrial tools and machines working up to 440v. Make sure that you check the output guys – it really is essential.

All power equipment you plan  to operate will have the voltage and wattage requirement on the specification label. Check it carefully before making your choice on generator.

A Final Word

Also, did you know?  The casing/frame around the generator is not just for stability and protection; it also helps to earth the whole object.  I feel better for knowing that somehow.

News just in though – portable versions exist! These are light, come in a range of outputs (for example 1,000 watts) and while they won’t power the Star ship Enterprise, they are pretty nifty for smaller load requirements.

But hold up!  No need to get lost in the maths – there are loads of converters on-line. These machines are essential to so many situations that’s it’s well worth having an idea of how they work. For a simple looking machine there is some great scientific knowledge being harnessed. So easy to take for granted.  Right, beam me up Scotty, I’ve done my work here. At least, back to the office to see if they’ve eaten all the chocolate biscuits. Again.

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