The "proof of concept" prototype works, but the inverter (unloaded) draws more energy than the 15 watt solar panel can provide. Add a 6 watt load and the capacitor voltage drops to 5 volts within 20 minutes or so.

FUTURE

I would like to attempt powering a small freezer or refrigerator. By adding two more 3000 farad capacitors (5 total), I can eliminate the dc to dc up converter--let the inverter do all the work. The over/under voltage relays will need to be modified. Two hundred watts of solar panels should provide the power for the refrigerator (80 watts or so) for at least five hours on a sunny day.

A freezer (unopened and loaded) will easily keep items frozen for 19 unpowered hours. A refrigerator is supposedly only good for about 4 hours. Can a small refrigerator/freezer (with a chunk of ice filling the freezer area) maintain a decent temperature for 19 hours? I don't know--will have to experiment and measure to see. 

The concept here is to provide reliable ac power for five hours per day and depend on thermal storage to keep the items inside cool. To continue operation for multiple cloudy days, more solar panels (600 to 800 watts total) would probably be needed (solar panels do produce on cloudy, dreary days--just a reduced amount).