Thanks for reply and advise kind regards  

Hi going to show a stationary engine next year powering a 240V AC  2.2KVA  generator and I am going to power various lamps . Is it ok to power a 135W SOX lamp with a 90W SOX ballast I have seen the 135W SOX lamp runs at 164V at 0.95 A & the 90W SOX lamp runs at 112V at 0.94 A . So the current draw is similar but the voltage is a bit lower but I read up that it is the current draw is important so would this combination work ? kind regards

It will work certainly if the gear uses an electronic starter and I've seen 90W lamps in 135W fittings in real life. It may not work on Leak transformer gear. It will probably shorten lamp life though.