I have been working on ways to reduce the power that the solar charge controller draws without charging the battery or powering loads. The SSR brings the coil current down to 1.7mA with a 2.2K resistor on the Arduino output. That's about as low as I think you can get it. On the relay output side, the system can power a Banana Pi (or Raspberry Pi) through the relay without problems. The relay is a Panasonic AQV252G.