I just put in a 12kw system. It was $32k before federal rebate, and it was the cheapest bid by 5 different contractors. Not sure where your getting $15k
FWIW. The panels are $0.86/wh each with a micro-inverter that runs ~$160 per panel and 32 panels. So, ~$5000 for inverters, ~$11k for panels, which brings you to ~$16k and your not even tied into the power of the house, or adding the mandatory shutoff, or the new meters, or the Emphase system that tracks by the minute power consumption and usage of each panel, etc etc. So lets call the rest $4k
Total of parts ~$20k. $12k for construction in the Seattle area seems fair for a complex electrical install that includes a 10 year install and 25 year system warranty.
I think like many things, the last mile of work is way more expensive then people realize. Solar is more than the panels themselves
Note: I could have used cheaper panels, but they take up more roof. I could have also saved money by skipping the per-panel micro-inverters, but then the system is more brittle and less efficient
$7000 + $15,000
Then you apply the federal credit after that.
This is for a grid-tie system, I don't think Tesla is pricing theirs with a powerwall here.
Edit: apparently they're pricing it with 2 powerwalls, still cheaper by a sizeable margin however.