(Originally posted by me elsewhere, but I thought I’d repost it here for fellow fantasy writers and system wonks):

Let’s start with this all-important link: http://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)

You can reasonably safely skip down the chart until you get to about 10^-7 to 10^29 range, as this is more or less straining the limits of ordinary human capacity to visualize.

Specifically, let’s take a happy hop-and-skip to about the 10^-1 to 10^9 range, which is more or less our day-to-day energetic interactions.

As I read through this chart, what really struck me, over and over again, is how relative to human expectations, light and motion are cheap, while heat is expensive. Of course, these are all subjective perceptions; energetically, a Joule is a Joule. But it can be pretty weird to realize that your friendly neighbourhood chocolate bar (1.2×10^6) has about one quarter of the energy of a Kilogram of TNT (4.2×10^6), and about 5 times the energy of a WW2 “pineapple” style hand grenade.

The next energy equivalence that really struck me was solar energy vs bullets: Specifically, the solar constant at our distance from the sun is 1.4×10^3 Joules per second. The kinetic energy of an M16 rifle round is 1.8×10^3. Got a decent solar panel and a way to convert energy into motion? You could be firing off bullets all day on solar power.

Need to melt some ice? Each gram will cost you 3.3×10^2. Need to kill a man via X-rays? Now available for the low, low price of 3×10^2.

What’s that, you’re an old-fashioned wizard who lays waste to his foes via lightning bolts (1.1×10^9 J) or fireballs (ballpark it to 1KG of TNT 4.2×10^6 J) or a tornado (> 1×10^15 J)? Maybe you’re the classier sort of evil sorcerer who boils an opponent’s blood (5 litres, heating (4.184 J/ml/celsius) from 37 to 100 celsius, 5000 ml x 4.184 x 63 = 1.3×10^6. Why, it only proves your mettle to use hundreds of thousands of times more energy to strike down your foes than necessary, right?

But what if you’re one of those awful modern sorcerers who rather likes using that would-be energy of a lightning bolt, and firing off one hundred and fifty thousand .458 Winchester Magnum rounds (elephant gun, 7.3×10^3 J per round) instead?

And that doesn’t even touch on how terrifying conjuring is, energetically. Magically creating mass from nothing but energy? (9×10^13 J per gram!) Conjuring a dove (900-2100 g) pretty quickly starts demanding Tsar Bomba levels of energy.

So I guess what I’m trying to say is, forget Gandalf. The guy at your kids birthday party, pulling a rabbit (400-2000 grams) out of his hat? Back away slowly, and pay his invoice.