I have a mirror light with two T8 flourescant tubes. It is a hit and miss affair as to whether the lights come on when switched on. Sometimes they come on and other times I can turn the switch on and off for about 1 minute and they stay off. Sometimes removing a bulb and turning it upside down will get the lights on but they don't light again after turning it off.I know that 240v is getting to the ballast but don't know how to measure what is coming from the ballast to the bulbs. They are two 15w bulbs. Can I use a new ballast that is rated at 2 x 18w as I can't find 15w ballasts on the internet? Thanks Rich.

Forgot to add to above that I have replaced with bulbs with new ones.
Rich.

Why do you have 240volts going to the mirror light? Is the ballast rated for 240volt power?

How do you know you have 240 Volts to the Ballast, Where do you live?

Sorry. This is in the UK. 240v mains supply. Ballast is rated for 240v incoming.

Is the fixture earthed well? Fluorescent ballasts need a good earth conductor to operate properly.You may notice the fixture works well on damp days, and not on dry days.

Is the fixture earthed well? Fluorescent ballasts need a good earth conductor to operate properly.You may notice the fixture works well on damp days, and not on dry days.
Its earthed well. I think the cheap Chinese ballast is faulty. Do you know if a 2 x 18w one will be an acceptable replacement for the 2 x 15w one I have ay the moment... as long as all other specs are within parameters?

If its Earthed well, and voltage is good, I always start with bulbs, as you have that, that leaves the sockets or the Ballast, The newer balast seem to work better that the ones in the past.
Sometimes the Ballast is more Expensive than the whole fixture,

Have seen ballasts for around the £20 price but need to know if twin 18w ones will be okay.