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A generator's advertised rating is often an intermittent rating, not continuous. Continuous output is less.

Generators care more about VA then power consumption

  • VA = power consumption divided by power factor
  • The VA figure is the same as power consumption for resistive loads
  • VA is noticeably higher than power for inductive loads such as motors.
  • Where VA rating isn't known, a fair estimate for a motor is about 20-25% above its power rating.

Decide what you want to run on it and add it all up. Often that power requirement works out too expensive, and you can then decide where to cut back.


There are other options than a generator, depending on what you want to run. Eg a UPS, a lead acid battery & charger, gas lighting, a fire or oven for heat etc. Probably all of those are more reliable than generators, and less noisy and easier to use. See Backup power for more info.

Life expectancy

Life expectancy varies widely. Some machines are designed with short lives of under 100 hours run time. High reliability machines like Listeroids can run daily for decades. Ensure your choice can do what you need.


Generators with 240v, 110v and 12v charging outputs are all available. Consider future use as well as immediate use.

  • 12v output can charge car batteries and run some 12v appliances
  • Running a 240v tool on a 110v output can enable use of a tool with greater power consumption than the generator can provide, though the performance penalty is heavy


Listeroids are a very different breed of machine to the others on the market. These are copies of a 1929 Lister design, still made as per the original.

  • extremely long service life
  • large, heavy, and definitely not portable
  • low rpm engines
  • very good fuel efficiency
  • low cost
  • normally supplied as an assembled and painted engine, but with no further work done
    • no generator fitted
    • no final testing
    • sometimes incomplete cleanout of moulding sand from the engine
    • no cooling radiator
    • no oil etc
  • standard of paintwork can be inadequate
  • Listeroid introduction
  • Typically shipped in from 3rd world countries, where they're popular

To make one usable,

  • any grit in the engine is cleared out
  • a generator is added, usually belt drive
  • a cooling system is added.
    • An oil drum is a popular simple radiator choice.
    • Where antifreeze will be needed, a car radiator & fan is a better option
  • paintwork is patched or repainted
  • engine oil is added

Noise levels

Noise is a big issue for many users.

  • Most open frame generators are 96dBA or more
  • "Silent" rated generators can produce 70dBA

Some generators are available with built in soundproof shrouding at extra cost. Its also possible to put generators in a vented enclosure to reduce noise, and this may be a good idea if you're going to use one a lot. Air inlet and outlet ports can use a serpentine path to cut noise. Needless to say lightweight construction isn't what's wanted.


Pullcords aka recoil starters are most popular for portable generators. These are fairly straighforward to use.

  • Some have a pulley with a notch. The rope knot sits in the notch, and the rope wraps round the pulley. Simple & effective.
  • Some have an automatic retracting pull mechanism. These are easily converted to the above type if necessary.

Starting handles used to be used where more physical force is required than suits a rope pull, eg on 7kVA units. The main issue with a starting handle is that a single backfire can break thumbs or ribs and injure wrists. Correct starting posture is important:

  • Always check your thumbs are on the same side of the handle as your fingers, never let a thumb wrap round the handle.
  • It's possible to break a rib if a backfire forces your arms into your chest with great force, or damage a wrist.
  • Stand with enough clearance to avoid chest injury if it backfires
  • Place your hands inline with your forearms, avoid letting your wrists drop
  • take proper care every time, don't underestimate the potential for injury

Electric starting makes life easier, at a price.

Remote & automatic starts are generally found on large ex-military and ex-corporate machines.

2 stroke engines are relatively cheap, but famous for problems with starting. Consider how failure to start would affect things in your application.

Diesels that are too big/hard to pull start generally have a large flywheel and a compression release. On such units, release compression, spin the engine upto speed then close compression and it should fire. A cloud of thick smoke is normal when starting a diesel. It contains plenty of toxic carbon monoxide, breathing it is inadvisable.


Portable generators usually have no oil warning system. You need to check the oil from time to time, a neglected oil system can kill the machine.


Portable generators use IT earthing, meaning no effective earth system.

When powering a gas boiler, its often necessary to earth the output wire used as the neutral for the boiler to work. Usually neither 240v wire is connected to the gen frame, but sometimes one can be; check before picking your neutral or you may get a live gen chassis. With dual voltage output, use the conductor common to both outputs as the neutral. Earthing may be achieved with an earth spike into the ground, connected to the genny's earth output or frame, and to your neutral wire. You now have TT earthing.

Connection to a house

Connecting an extension lead to a portable gen is simple and reasonably safe. Connecting a generator to a house installation introduces complications requiring solving, at least one of which can result in death if overlooked (backfeeding power). See this article on installing a transfer switch.

Starting spray

Starting sprays (eg Easystart in UK, and in Australia 'Start ya bastard') are often used with generators. These are aerosol cans of a volatile fuel designed to start under a much wider range of conditions than regular petrol or diesel. Spray some in the carb then start. Useful to get them going when stubborn, as often happens.

It's rumoured that regular use can end up damaging the engine, but a uk.d-i-y discussion could find no evidence or reason to support this.

Power quality issues

Power quality issues are usual with portable generators.

Under heavy load generators can produce large voltage sag, resulting in appliances running slow or not working. This carries a small risk of frying refrigeration compressors.

Generators are inductive sources, and can in some situations produce damaging voltage transients if a load switches off. This isn't usually a problem. Google 'load dump.' This is the one situation where surge absorbers can help protect fragile/valuable electronic equipment.

Some generators produce a less than steady output, with visible flicker on lighting. This seems to be due to a design issue with the voltage regulation system. (The flicker is too slow for it to be related to waveform shape.)

Capacitive loads on traditional type machines can cause overvoltage.

Invertor generators use electronics to produce a clean steady output. The result is a much cleaner more stable output, but also a less electrically robust machine. These have much less peak current output ability than traditional nonelectronic machines; this can be an issue for fridges & freezers.


Petrol deteriorates when stored over winter in plastic. The volatiles evaporate and gum forms. Petrol that won't start can be mixed with new to gradually use it up.

Diesel generators can be run on red diesel, much cheaper than road diesel.

Occasionally people run generators off domestic heating oil. This is 28 second oil, whereas diesel is 35 second oil. Heating oil lacks the lubrication of diesel, some engines don't seem to mind it, some don't last long. It's not a recommended practice. A little lubricating oil should be added to it.

Diesel isn't volatile and can be stored from one year to the next.


Proper servicing can make a big difference to engine life.

Stretching the power rating

Its possible within limits to run appliances on a gen of lesser VA rating.

  • Power tools etc can be used in series with a dropper, or 230v tools used on a 110v output. Performance suffers badly of course.

Maximising return

Diesel generators can be run on red diesel, cheaper than road diesel.

When a generator is used long term, it can be set up to produce hot water from the heat of either

  • the cooling system, for big machines that use water cooling
  • or the exhaust

Cooling system heating: The gen's cooling system is connected to a heat exchanger in the HW cylinder. Retrofit exchangers are available, or a coil of microbore can be used.

Exhaust heating:

  • Exhaust Heat output is greater than cooling circuit output
  • A heat exchanger captures exhaust heat. The circuit requires antifreeze in winter.
  • Temporary outdoor setups not in winter can direct exhaust over the bottom of a water container, or around the cylinder.