When Terrence Dwyer received a knock on his door and a flyer for a solar panel system small enough to fit on his deck, he was quickly sold. Solar systems that plug into regular wall outlets have been popular in Europe for years and are gaining traction in the U.S. for their affordability and simple installation.“We thought absolutely, let’s do this right away,” said Dwyer, who lives in Oakland, California. These small-scale solar systems could become attractive to more homeowners now that Presid...