Solar panels are now becoming popular with energy savings and environmental aspects. Most of the time the solar panel installation experts are available from manufacturing companies and are familiar with most of the structural problems and they already have attachment details on how to connect solar panels with roof.
Solar panels are not too heavy in weight, at the most they may be around 35 lbs in weight maximum which can hardly add a load of 3 psf on roofs in addition to snow and roof dead loads. Most of the roofs are designed with heavier ground snow loads such as 30 psf or 35 psf in northern regions where the there are more blizzards and lots of snow. Comparing this with weight of solar panels the gravity load is not big of a difference, and most of the roof trusses and rafters are capable to rest this load. However, the issue may become critical from design point of view when we think about uplift.
The uplift from wind may become more critical rather than gravity load itself on solar panel. Depending on size of solar panels it may induce larger uplift loads on rafters/roof trusses depending on which region its being used. In higher wind region it may cause greatest uplifts on roof rafters/trusses which may require to review connection detail/typical design for connection provided by solar panel manufacturer. Experts at solar panel providers have tested worst case scenarios but it also advisable to coordinate with them on any project specific issues that may cause review of typical connection detail. Sometimes due to high winds it may become necessary to deign bolted connections to attach solar panels to roof rafters/trusses.
From my experience in dealing with solar panels so far is, the attachment was with (3) 12d nails to 2×6 blocking to top chord of trusses wither toe nailed or face nailed and then the bracket to solar panel is attached to 2×6 flat members. Also, interesting aspect is, solar panels are not possible with slopes less than 2:12 or greater than 6:12. I would like to see comments if someone can confirm or argue this. This is from my experience with specific type of solar panels.
(the above article is based on various readings and experience that is faced with recent design and shall not be treated as professional advise. Architect/Engineer or solar panel experts are responsible for their design and the final decision)