The first rule of this is that price IS an object; it's entirely possible to make a wish list for your solar energy system that causes it to cost more than the home it's attached to. So, set up your budget (including tax credits and assessment offsets for this, based on your locale) before you start. In general, the more efficient a solar panel is at converting sunlight to electricity, the more expensive it is, and the differences are nonlinear; going from a typical 17% efficient conversion to a 30% efficient conversion can cause the cost of the panels to go up by a factor of 100. (This is why NASA's solar panels are more efficient than the ones you can put on your roof; for NASA the cost of getting those solar panels into orbit dwarfs the cost of the panels themselves, so they might as well pay the extra for the greater efficiency). Other factors to think about are the cost to replace them as they wear out; long exposure to sunlight will eventually degrade solar panel performance. The expected life span of a solar panel array is about 20 to 30 years, when all is said and done.