Yeah, generally A grade uses the most power, but you get the best performance. A and C Module weights are around the same though.
Looking at just power issues, and not worrying about weight, then go for B or C for your non critical modules, and if you can run your system mostly fine or are just barely over the limit, then you will be just fine when usiing power priorities.
Just remember to turn the FSD and fuel scoop to lower priority which knocks off quite a few percentage points of active power usage. Also look for anything non combat related like discovery scanners n such.
Set cargo scoop to lower priority as well (if not disabled completely) - the cargo scoop actually has a big power draw so that will save you some power.
Just be glad you arent working the vulture....that thing needs some seriously major tweakage to get the power running right...but it is so worth it for that agile lil guy lol