All power use is limited by the Unicorn's Magic Capacity, which you cannot see. If you use them too much, you will run out and the Unicorn will need to recharge. This takes several hours, but they are always recharging. You can tell when your Unicorn is at half power by the appearance of the Power Shortage moodlet, which gives no mood impact. The Power Failure moodlet will come later, if you keep using powers. This gives a small -10 mood hit. If you use the powers once every few hours you won't run out. Powers like Teleportation and create fire are really cheap, but Blessing Sims and Flora can only be used three times with max energy. Extinguishing fires takes no energy.
Whatever you do, don't remove the Power Failure moodlet if you're cheating. Your Unicorn might lose the ability to generate magic. I suspect power fills after the moodlet is gone. I had a Unicorn go three days without any magic during playtesting. The problem was not resolved.
This is from the unicorn info on here. But I also think somewhere I've seen that not all unicorns have powers, some do some don't. Hope this helps
