Can I build more houses than I have the capacity to power?
Your power plant capacity has no bearing on the size of your network. So you can build as many houses as you can afford. From the rules:


  A player may connect to any number of new cities in a round, as long
  as he can pay the building and connecting costs.


Of course, it's probably a bad idea to build a network substantially larger than you can power. Balancing power plants, resources and network size is the key to the game!