Your probably better off without them, invisibility is a waste of time in this game, unless you are using a high stealth character and your out of LOS the enemies will still see you and run straight at you to attack, they will even follow you when you realise how broken it is and make a run for it.
Invisibility alone is pretty weak, although you can usually walk up behind NPCs that aren't already alert to your presence and get a hit in. Don't pop one in combat, though, by itself, it's useless. You might make some archers look for you for 5-10 seconds if you can avoid getting hit by a melee combatant that long.
However, Invisibility plus high level stealth (with the master perk) is game breaking if you have constant access to reusable invisibility (basically high level illusion plus master stealth). The combination can be used to essentially perma-stealth undetectable (although you have to keep doing it after every attack)... the closest thing Skyrim has to 100% chameleon.