According to the Apple spec page, it's 1.8A @ 12Vdc. That'd be 21.6W, plus some overhead for the power supply itself.
It should be printed on the supply brick, though.
edit: I looked on the brick, and it says 0.5A 100-120VAC. That puts it at more like 50W max, but I suspect the actual to be quite a bit less, depending on any USB bus powered devices connected. The ratings are maximums.