Apartment Solar - What is the '?'?

I live in an apartment (insane Bay Area home prices are for a different blog post). As a result, I can't buy solar panels and put them up on my roof like I want to. I do get alot of sun on my patio though. I really want to go buy a large solar panel somewhere and use it to power some part of my house (see technical diagram). I know where to get all of the pieces in the diagram except for the '?'.
See, the '?' is a the piece that takes the green power from my solar panel on my balcony (DC, variable current) and supplements it with the brown power from my wall outlet (AC, constant current) and outputs constant AC current to my refrigerator (or computer/washing machine/whatever), always using as much power from the solar as possible and as little as needed from the wall to finish the job. I don't want to buy a battery or sell to the grid - if I have too much solar power, I am happy to discard it. I don't want to rip out my wall or otherwise alter my apartment, and I don't want anyone to "install" this. I want to do it all myself. The internet knows everything, so I am asking you internet: What is the '?' I need to make my apartment solar?

Update: I wrote a response to this in a later blog post (Apartment Solar Update)with a few updated ideas. Still not a perfect solution though, if you have ideas, leave them in the comments.