!. Try to work out approximately what power you actually need..Do this by adding up the current rating of all the equipment,lights heaters fidges ,freezers etc etc etc Plus of course all those machines that tou have lurking in your workshop. Note-It is most unlikely that you will want to run everything at the same time so cut yourself some slack! It may be possible to shed some load before switching on a big electrical load or starting a big electric motor. Always keep in mind electric motors may need up to 6 times the current(amps) to start as to run.!!.A 4 Kw electric motor might need 15-20Kw of generator power to get up and running .Only for a couple of seconds then the current(Amps) drop right off again..But if the power is not there it won't get away.
2/Work out how much time you are going to use it!.12 hours a day or just when the power goes out.! For standby work or usage a bigger old high hours purchase is just fine if you make sure that all the control gear is up to date. A relatively cheap course of action.On a 50 /500Kva range genset you can upgrade the controller for as little as $500 This will give you a state of the art digital unit that is an easy install and is usually bulletproof.
3/Good used gensets can be bought for about 30% of the current new price.. Many used units ,ex Gov or Telstra standby sets may have done as little as 200 hours and be 20 years old . This is quite common..
4/ For a prime power application ,say 50+ hours/week it,s best to pay some moreand chase a later model LOW HOURS!!!!! unit.The newer units are a little more fuel efficent . A good general rule of tyhumb is you will burn 4+ litres of diesel for every 20 HP in real power you use per run hour.
5/ Seek a set that the vendor can supply a current load test from preferably an independent person. Most reputable dealers will assist with this and if possible ask to be present when this is being carried out..Nothing like seeing for yourself..
6/ The imported Chinese produced lookalikes are not as good as the originals as yet,but are improving.. Specially in the range 10 /50 Kva. Makes such as New Holland and Kipor have been rolling out of China now for 20+years and are really quite good. Do make sure that there are spares available and service items if you choose to go this route.Filters ,belts etc.Many so called Perkins,Deutz Or so called Cummins sets have never been closeto Europe or USA than Beijing.,. Some of these units are manafactured under licence(copies) but many are not!Materials and quality control in these examples are either non existent or sadly lacking..
The vast range of small petrol sets .1-5 Kva is cheap and plentiful and are NEW.Cheap and with warranty..Good luck. Best to buy a diesel for any serious application . The fuel burn is minimal compared with petrol units. BE VERY CAREFUL AND HAVE A PROPER ELCTRICIAN GO OVER THE SET BEFORE YOU EVEN THINK ABOUT STARTING IT. ELECTRICITY KILLS. BE BLOODY CAREFUL!!!