"Backyard" photovoltaic solar setups increasingly abound, but they usually run "blind" with charging performance often unknown. Simple LED indicators,although better than nothing (see perhaps http://www.instructables.com/id/Single-LED-ammeter-FLED-based/ ), may not be revealing enough for keen investigations. Full current flow monitoring (both INTO & OUT OF a solar setup), perhaps via a cheap (US$5-$10) pocket style DMM (digital multimeter), can be near essential in many setups.
An ammeter can give valuable system insights!
Under charge it can reveal slight changes in panel performance due to air quality, cloud cover, aging, placement, inclination or seasons and also drop offs arising from such tedious issues as leaf/dirt/bird droppings or corroded & suspect connections. Naturally panel damage (or even theft - it happens!) will be quickly spotted too. The relative merits of assorted batteries and panels that may become available can be investigated as well.
Discharge performance checks of new devices (or alerts to excessive or un-authorized loads) can be quickly noted, while longer term battery degradation can also be monitored.
Step 1: RC Electronics WU100-BK "Watt's Up" Watt Meter
However this is an Instructable after all, so follow on for DIY approaches !