Judging from the backlog in orders of MPPT controllers, customers are waking up to the advantages of this technology. The old faithful Outback FM-60 and FM-80 and the newer Xantrex XW-MPPT 60 are now joined by the new Morningstar TS-MPPT 45 and 60. Plus the Morningstar SS-MPPT 15 controller becomes very useful for smaller systems. I predict that in five years we will hardly be using traditional controllers. But what is the correct way to size an off-grid system with a MPPT controller, since its impact will vary with the module temperature and with the state of charge of the battery? Start with a healthy de-rating of the module (probably beyond the PTC rating) and the controller (most will be better than 95% efficient) and then just use the de-rated watts of the module. It is so much easier than traditional sizing that it seems like cheating….