No I goofed a bit too, by assuming 2kWh used every day but $800 in installation cost (which is for 1kWh). Fixing that error increases the payback to 8.76 years/kWh of installed battery. But only if you assume rates don't change for all of those ~9 years.

But rates will likely increase. So will the delta between off-peak and on-peak rates. I think the true payback period is between 5 and 8 years.