I am trying to figure out how to operate a solenoid on loss of sunlight so that I can drop an insulating panel into place over a Trombe wall after roughly 3PM every evening in winter. I guess I could use a low voltage DC circuit to operate a relay which then operates the higher voltage DC solenoid circuit. How do I de-energize the DC solenoid circuit so that I don't run the battery down? I have zero electronics experience. Any help would be welcome. Thank you.