Optional solar roof on the new Prius Prime adds about 3–4 miles of range/day

It’s a 185 watt panel with a list price of $610.

It seems the best use case is if you commute to work, park outside at work, and don’t have access to charging. In that case it would save about 1/10 gallon (or 1/3 liter) of gas/day. If your gas is $4/gallon it pays for itself in roughly 4-5 years. If you pay $2/L that time comes down to 2-3 years.