[URSpider94]

My take: Further increases in efficiency could mean that you could install more capacity on your roof, or the same capacity in fewer panels. Assuming that you can fit enough panels to cover 95% of your demand on your roof, installing more panels doesn't make sense (over-generation is not really profitable today). At the same time, a lot of the installation costs are independent of number of panels (permitting, estimates, wiring to the junction box), so installing fewer panels won't bring the cost down tremendously.

At the same time, tax subsidies are scheduled to start phasing out, and utilities are making noises about discontinuing some of the more favorable rate plans that allow 100% net metering (generated power is credited at the full retail rate, not at the wholesale rate). Either of those changes would negatively impact ROI.

It's hard to know what will happen in the future, but with the current environment, many people can lock in a favorable 20-year ROI for a system today.