As the device will be used outdoors and the solenoid only needs to be operated perhaps once every three to four days on average, almost any solar cell will work as the amount of energy needed is very small. I understand that since it's a short pulse that is being generated, it's unlikely to damage the coil on the solenoid even if the voltage got up to 30 to 40 volts. My concern lies more in the potential to damage the capacitor. I note that Nigel Goodman stated that if it's charging a capacitor, it would not be in a no load state. Once the capacitor is charged, I think the solar panels would be in a no load state or am I mistaken?