Are power wall battery systems affected by cold in northeastern areas or are they insulated in some way, since they will be on the outside of the structure and exposed to cold?
Since batteries store energy chemically and chemical process slow down when it gets cold, there will be some loss in capacity.
Here is what Elon Musk says:
"Musk said that the batteries will have thermal management systems to allow them to power houses in hot and cold climates, too—the batteries have an operating temperature range of -20C (-4F) to 43C (110F).
The bigger problem Tesla had developing the batteries was keeping them cool with a special cooling system when there is a lot of current coming out with the pedal to the metal. Of course you are not going to be stepping on the pedal of your house (unless you are driving a Tesla RV), but I would not be overly concerned about the cold on your powerwall. If you did live in a place where it did get below -20C, then you might want to put it inside or ask Tesla about a solution to heat it up.