In times long ago (a few years ago), solar electric (photovoltaic) modules were very expensive. Eking out every watt of power from each module was critical. There was a huge incentive to mount your solar modules with “optimal tilt and orientation.”
Unfortunately, most roofs are pretty darn fixed when it comes to slope and compass direction. We have to live with what we’ve got. But what happens when you want to mount solar modules on a flat roof. The conventional wisdom is to tilt modules toward the south. Generally speaking, approximately 25 degrees tilt toward due south is theoretically optimal for us in Southwest Florida. All else being equal, you should shoot for this orientation and tilt if you want to optimize the output of your solar modules.
However, this does not take into account one very important factor – cost! The cost of building a rack system on a flat roof that tilts modules can be quite expensive. The aluminum rack structures can be a costly part of the total system cost. The design wind loads that are we are required to meet generally rule out ballasted systems that use extra weight to hold tilted modules down onto a roof. We have to secure the system into the building structure. Tilting modules makes securing modules very hard, and expensive!
With module prices plummeting to the lowest prices ever, the relative cost of installing a tilt mount system is cost prohibitive these days. If you have enough roof space, you are usually better served by installing more solar modules rather than striving for optimal tilt.
There are other advantages of tilting modules, like better natural cleaning through rainwater and better heat dissipation through convection, but again the new paradigm of solar energy economics makes these factors less important.