Investors are probably wondering whether all this growth is sustainable. My own guess is we're probably looking at one more leg down before costs are driven decisively below those of grid energy. Once solar power becomes cheap power there is no stopping the solar boom, any more than the minicomputers of the 1970s could stop the PC. My own estimate is we're still two-to-three years away from that point in most of the US, which doesn't yet account for the cost of its carbon waste. Until grid parity is general here, the best investments will be in companies that combine a strong export market with high efficiency, like SunPower, and companies that specialize in finding buyers, like SolarCity. At the time of publication, the author had no investments in companies mentioned in this article. Follow @DanaBlankenhornThis article is commentary by an independent contributor, separate from TheStreet's regular news coverage.