Using carob as a substitute for cocoa
While you can certainly substitute powdered carob for cocoa, the flavors are distinct enough that you will likely need to adjust your ratios and sugars to make carob blend into a recipe better. Use less sugar than you would for powdered chocolate.

I found this description http://www.natural-health-restored.com/what-is-carob.html of Carob, and this comparison http://www.natural-health-restored.com/carob-vs-chocolate.html of carob vs. chocolate.

A note about the source http://www.natural-health-restored.com, I have never used this site personally, and can't vouch for its accuracy. It also seems to be centered around Vegan eating, so read into the site's commentary what you will.

Additionally, with powdered chocolate, you can never get quite the same reconstituted flavor with other fats/oils as you can with dark solid chocolate (the fats/oils are different from the solid chocolate's original fats). I suspect the same to be true for Carob, however I have no proof.

So (without exact ratios):


use less sugar than for a chocolate recipe
be aware of the flavor difference (don't think that it's spoiled just because it doesn't taste like chocolate)
oil or butter ratios may need adjustment


Personally, I used carob at a 1:1 ratio and found my carob muffins to be drier (more oil?), stronger in flavor (less carob powder?), and a touch on the sweet side (less sugar?). 

My suggestion would be to find a recipe made for carob instead of just substituting in one for the other.

Good luck!