Solar PV system cost, does it worth?

The cost of the solar system must (unless you are independently rich or someone else is paying for it for you) be looked-at as money borrowed. In other words: if you had to add the cost to your mortgage (as most of us would have to), you would be paying, say, about 5% per year, so, the electricity generated by a $25,000 solar PV system is not actually free: it costs you $1,250/year in interest (maybe more in lost opportunity costs if you are a good investor, but that is an entirely different story). In addition to that, there ARE operating costs: I am told that solar inverters are typically expected to last on the order of 10-15 years, after which you must replace them, and PV solar panels have some finite life. Also, when your roof (the one made of shingles) needs to be replaced or repaired, you will incur additional costs to remove and reinstall the PV panels.

The projection of the additional benefits ignores human nature. The individual that makes such investments will only reap the health and environmental benefits if everyone does it. Otherwise, your investment will only have marginal, near undetectable health and environmental benefits, and they will be harvested equally by the slob next door that burns used tires to power his generator.

Cost is only a measuring stick that we use to decide how to spend our efforts on an individual or corporate basis. You will get the right thing done when either a) solar costs come down to the point that the investment is a no-brainer for Joe Sixpack, or b) when the costs of extractive energy technologies (nuke, oil, gas, etc) are realistically allocated. Given the power of the energy lobbies, (b) is not happening really soon, so all we can do is work on making (a) a reality.