Researchers at the government-funded Environmental Energies Technology Division at Berkeley have released a study (PDF) that shows a significant delcine in the actual cost of solar installations over the past decade. The cost of solar seems to have declined by about $3/watt between 1998 and 2007, though this average is gleaned from systems both small and large, which have very different needs and costs. The San Francisco Chronicle highlights the study’s findings that the cost of solar has yet to equalize across states. The per-watt price for solar still differs by as much as $2/watt or more depending on the state in which you live.