Our robots use 12 volts. The maximum voltage for the Jaguar is 24V, but it works fine at 12v. And since we can only use the (12v) motors that come with the kit, you couldn't use a 24v motor anyways.
It wouldn't hurt to spend an hour or so reading the rules, too.
http://usfirst.org/roboticsprograms/...nt.aspx?id=452