I just jumped in with no objective. A 100W panel and a few charge controllers for different applications.
I've been thinking of small systems as this, such as to take my fish/reptile tanks off-grid. The reptile tanks are ideal because they really only need to run when the sun is out (to recreate the sun with artificial lights and heat
), the fish tanks need some heat and circulation during the night, however, and the battery makes it not worth it in the long run, unless just for experimentation.
The one application that has paid back the most is the MPPT controller set up to charge my e-bike. I cannot quantify that, but the payback is not just an obscure financial calculation, but the objective enjoyment that all that power and acceleration I'm experiencing is basically free - after paying for the system, of course.
Let me run a number... that system cost $130. @ $0.20/kWh, it will be 650kWh to break even. The e-bike uses about 160Wh to go 5 miles, with some assist, so about 4000 charges or 20,000 miles to break even. I'm going to need more tires.