Keg/Kettle conversion query

[Mossy]

I've acquired a 50L keg that I'm going to use as a boil kettle but I just have a few queries that yous might be able to answer. First off, it takes long enough to get 25L of wort up to a rolling boil on the burner that I have so I was thinking of putting a heating element in the keg. Does anyone know of a good element to use? Would I need to get two for that amount of wort? Can you get an element with a power adjuster on it? And finally, how expensive on the electricity are they to use?
If anyone can give me some answers or guidance is really appreciate it.
Cheers

[Tom]

Re the cost, it depends on your elements. If you have a 2.2kw element then the electricity usage is 2.2 units per hour. Airtricity is something like 20c / kwh at the minute, so 20c x 2.2 = 44c per hour. Then it depends on how long it's on for (Strike and Sparge water, boil etc).

I think that's how it works.