Here is an article which discusses the power dissipation in a typical RS-485 network. This may be helpful for estimating power supplies and thermal considerations. Note that the newest RS-485 devices have significantly lower power dissipation than older designs. There is also a discussion about using non-terminated networks for lower power.
http://electronicdesign.com/article/design-solutions/Reduce-Power-In-Your-RS-485-Network.aspx
