∆E = 0
The law of conservation of energy. It’s the first law of thermodynamics, one of the most important physical laws of the universe. And just because there’s magic involved doesn’t mean we have to break it.
In a nutshell, the first law means that you have to end up with the same amount of energy you started out with—no more, no less. If you burn a tank of gas, for example, the chemical energy in the liquid fuel is converted to kinetic energy (the motion of your car) and heat. The energy contained in your sandwich might get turned into you running a marathon. (Or, in my case, just sitting on the couch, but for longer.)
I’m not going to argue for total scientific realism in fantasy fiction (where’s the fun in that?), but I do think we can stick to the spirit of the law. Supernatural powers ought to come at a price. For example, in the first book of Stacia Kane’s Downside series, the price of summoning and controlling a particularly powerful ghost is a human soul. Doesn’t get much creepier than that. Even in the teen drama The Vampire Diaries, vampires who don’t drink human blood are weaker than their murderous counterparts.
The price doesn’t have to be a human sacrifice: it can be something as simple as time. In Harry Potter, just because the kids have magical abilities doesn’t mean they can snap their fingers and get whatever they want. It takes years of study and practice before they can accomplish spells. And there are limits. Wizards can’t create food they don’t already have.
Supernatural powers get a little boring if they come too easily. It's one reason I'm not a big fan of day-walking vampires. When the bloodsuckers are confined to the dark and can't enter your home without invitation, it makes their creepiness more visceral, and their gift of immortality more limited. A paranormal gift feels more precious when it comes at a cost, whether it’s a character’s personal energy, the time it takes them to study and perfect their art, or some sort of sacrifice. The alternative is too God-like to be interesting.
What's your favorite price for supernatural power?
(P.S. You can check out Part One of this series here.)