Question Can you specify feed and back feed amp?

Hello, looking at maybe setting up some solar on my new property. My current electrical setup is a main panel on a pole (200 amp) which then feeds a sub panel for the house (200 AMP). On the main panel on the pole, I also have a 30 amp (10/3) running to a shed. I initially would like to setup solar just for the shed (with batteries). I was looking at an 18K (this way when I build my house later, I can use this there and expand). Can I specify to only pull up to 30 amps from the grid and not back feed? When i do want to back feed, can you specify back feed amount too?

Thank you for the info, 

Mike