Hi,

Really surprising to see a microcontroller (ADC to be more specific) that supports so high voltages

(signal ranges 100-300V, 5-20/5-30/10-40A)

Anyway, I dont think you can do that power calculation directly...

power=(Vpeak*Ipeak*Cos(angle))/2

One alternate way is to build a list of all COS values for the the probable angles in a LOOK-UP Table and then use it as and when necessary. Another issue involved here is the Floating point numbers (for which each person has a different perception, atleast in all the forum questions and answers I've seen), so you should adpot the best possible approach to your problem.
the second consideration is the multiplications. You should also design methods for performing multi-byte multiplications (ofcourse that should not be as difficult as tackling the first problem).

Regards,
Harsha.