How many watts does a fluorescent ballast draw to figure the total load of a fixture that has 2 lamps.

Find the datasheet for the make and model number of the ballast and lamps you want to use, it will be listed there.

Each ballast and lamp combination is different.

Do you already have the fixtures? If so what is the ballast part #? At a minimum what type of lamps, T8, T5, and how long are they?