For years I've recorded solar panel data by hand. Filled two notebooks with columns of figures. My new charge controller, an EPsolar Tracer-BN, finally let me automate it.

morning activity; by 8 am the sun is still behind the hill but, 16 watts are being produced, and by 11:30 am, the battery bank is full

You can explore my home power data here:
(click and drag to zoom)

The web interface loads the RRD files into a web browser using javascriptRRD. I wrote a haskell program that drives the epsolar-tracer python library to poll for data, and stores it in RRD files. Could have used collectd or something, but the interface to the charge controller is currently a bit flakey and I have to be careful about retries and polling frequencies. Also I wanted full control over how much data is stored in the RRD files.

Full source code