My company has been installing rooftop solar equipment since the 1970s, when the words “solar panel” meant solar hot water, and photovoltaic modules — now commonplace in both residential and commercial settings — were found mostly on satellites, being far too expensive for most earthly projects.But PV costs have come down to earth. The grid-connected systems we install today typically cost around $4.30 a watt, or about what the PV modules alone cost six years ago. For a 9-kw system like the one shown in this article, that brings the gross cost to $39,000 or so, and municipal, state, and...

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