Anything around $3/W if perfectly fine these days. That means you likely save money on a monthly basis, and to me that's the big thing early on.
We're aiming for $2.45/W in Philadelphia this spring/summer. I think you'll see prices take their next step downward in 2018 as markets mature and customer acquisition processes change dramatically. Tesla(of course) will be leading the way with some kind of online package you can buy just like ordering a Model 3.
Tucked away in the headlines today was a NYC Tesla store marketing cars/solar/storage all in one showroom. This is the start of their next phase.
Earlier this week it was announced that residential solar installs were down 16% vs. 2016, mostly due to slowdown with leases and mostly from SolarCity(now Tesla). What were seeing isn't a slowdown, it's a shakeout. All these big companies(Vivint, Sunrun, etc) have massive expensive sales teams just like SolarCity had, and their pricing is inflated to match. Tesla has pivoted away from this sales method and will be taking over the market anew fairly soon. You'll be able to buy a rooftop array online from Telsa for $3/W no problem, and likely lease one with a very short lock-in period.
Gonna be awesome.