Thank you for your help - would this process be the same for other devices that I wanted to "hard wire" ie smart TV's etc?
Can I use the airport express as an extender? and will it provide internet access point in the garage?
You can use the AirPort Express to extend the wireless signal > http://support.apple.com/kb/HT4259
Which port plug would I put the solar systems RJ45 cable in?
You have to connect it to an Ethernet port, but not the DSL port (that's for connect the AirPort with the modem)