Light/Dark side powers?

[hala]

What does it mean? If I'm good I can use DS powers, or will they be less affective?

[Grishnakh]

Just as effective. Simply more expensive (more FP to cast).

LS Mastery makes LS powers cost 50% and DS powers cost 150%.
DS Mastery makes DS powers cost 50% and LS powers cost 150%.

In both cases, a very high CHA modifier will reduce the penalty for casting "opposite". The EFFECT of each powers is determined by Level, WIS, and sometimes CHA. Not DS/LS Mastery. A Jedi Master with LS Mastery will have an equal Force Storm to a Sith Lord with DS Mastery (assuming equal level, equal stats).