Mike I started imaging off the back of a landrover using a 2.2KW Chinese generator, it gave great service and is still going today but when I eventually built an Observatory it was barely adequate - I purchased a 6.5 KW Chinese Generator and it broke down on the first night, motor run a bearing. Retailer replaced it the next day and it's successor has run for 3 years without any problems. The only thing is that the Chinese seem to be a little over optimistic with their ratings - my 6.5 KWH would reliably deliver more like 4.5KWH. The biggest cost of running it was petrol, so much so that a cost comparrison showed that Solar units would be much cheaper (The cost of Solar has dropped considerably over the last 18 mths to 2 years) - the proviso being that you can get your hands on some cheap batteries. The recent advances in computers also mean that the power needed to run the observatory is now not as great as it used to be.
http://www.pbase.com/loc46south/image/154996463
If you need further info PM me and I will help where I can
Cheers