Dow Chemical Co. and residential homebuilder D.R. Horton tomorrow are expected to announce a national rollout of the Powerhouse, a new kind of roof shingle that’s also a solar power system, the Denver Business Journal reported. The Michigan-based company will begin selling the Dow Powerhouse Solar Shingle in Colorado, followed by a dozen more states in the next year. D.R. Horton, based in Fort Worth, Texas, is one of the largest builders in the Denver area. Dow didn’t say how much the new shingles will cost or how much power they can generate. The company did say it intends for the shingles to be cost-competitive and for the total cost of the solar roof to be less than the combined cost of a conventional roof and supplies of conventional power.