System losses (inverter) are greater than I anticipated--probably the inverter's cooling fan.

The very low cost inverter I selected (see components) consumes more energy than my 15 watt panel provides. This means that any attached AC load will drain the capacitors down regardless of the amount of solar energy received. 

I would suggest using a larger solar panel (I just happened to have the panel and inverter used for this assembly) or an inverter that uses less energy in standby mode.