I completed a solar energy lab, where we looked at power flows, which were represented through different parts of a circuit measured with 3 separate ammeters. We used four different modes and varied the irradiance and load. only charging = energy from solar panel to circuit without battery switched on charging + consumption = " " + battery switched on discharging + consumption = some lights from solar panel switched off + battery switched on only discharging = battery + solar panel switched off I've had to calculate the overall efficiency of each setup too and have done so by: [ (dis)charging + consumption ] / [ PV Power + (dis) charging ] = overall efficiency I've attached the results to this thread, but I don't understand much about them and haven't been given any information about the theory behind it. If anyone could either point me in the direction of some literature, or help me answer the questions I have below I would very much appreciate it. Then I could interpret the data a bit better. :) 1) Why are only 2/4 (dis)charging results negative? The negative was a result of the current being negative, but I don't understand why 'only charging' and 'charging+consumption' had negative current, and 'discharging+consumption' and 'only discharging' had a positive current. 2) Is the overall efficiency different for each setup because of the battery using power generated by the solar panel? And because we changed the irradiance of the cell by switching some cells off?