U.S. homeowners installed more solar power systems in the first three months of this year than in any other previous quarter,according to data from the Solar Energy Industries Association (SEIA), a trade group for the U.S. solar sector. The first quarter of any given year typically sees the fewest solar installations because of winter weather, the group notes, but the period from January through March of this year saw a solid increase over last quarter — 11 percent — and a 76 percent increase over the same period last year. The average cost for a residential solar system is now $3.48 per watt, or 10 percent lower than this time last year, the SEIA report says. It also notes that, cumulatively through the first quarter of 2015, nearly one-fourth of all residential solar installations have now come on-line without any state incentives.