T12s are 10W per foot. 4' = 40W, 3' = 30W, 2' = 20W...
T8s are 8W per foot. 4' = 32W, 3' = 24W, 2' = 16W...
T5s are ~7W per foot (since most T5s are not made in even feet this number is rough estimate). 46" = 28W, 34" = 21W, 22" = 14W
These are the normal ratings, there are variations between manufacturers though. You can see each smaller diameter bulb gets progressively more efficient, thats mainly why they were made. For roughly the same amount of light the smaller bulbs would save electricity. There are some oddballs, like the T6's I mentioned earlier as well as T10's both of which are rating at 10W per foot like T12's and are compatible with most T8 or T12 fixtures. I dont really get into the short bulbs, there ratings since they are fractions of a foot are more loosely rated and you do see a couple watt discrepancy. Then for increased output they made the T5's burn at the same watt per foot as the original T12s (and higher) with the T5HO which is rated ~10-16W per foot. Again the T5's arent even feet so this is ballpark... 60" = 80W, 46" = 54W, 36" = 39W, 24" = 24W
These arent set in stone there is a little variation between manufacturers. Again I stress the rating on the bulb is just what the bulb is intended to run at, not the actual power it will - that is a function of the ballast (with a little influence from the characteristics of the bulb too). Like DarkSoul said you can overdrive bulbs by hooking them up to higher power ballasts or by paralleling ballast outputs together. I run 32W T8s at about 50-55W on my 75g planted tank. The stamp on the bulb really means little. I'm also running 54W T5HO bulbs at 80-85W on the setup I'm building right now by using them on higher powered VHO ballasts.
