In the northern half of the U.S. — and even much of the South — installing a residential solar hot water system doesn’t make any sense. It’s time to rethink traditional advice about installing a solar hot water system, because it’s now cheaper to heat water with a photovoltaic (PV) array than solar thermal collectors.

In short, unless you’re building a laundromat or college dorm, solar thermal is dead.

In the early days of PV, when PV equipment was much more expensive than it is now, homeowners with PV systems (especially off-grid homeowners) were instructed not to use electricity for heating. After all, since electricity is precious and expensive, and since PV power usually costs even more than grid power, it made sense to save electricity for uses like refrigeration, lighting, and home entertainment.

For decades, we all assumed that the greenest way to heat domestic hot water was to use a solar thermal system. But then two things happened: PV equipment got cheaper, and heat-pump water heaters became widely available.

The logic of using a PV system to heat water was first explained to me in early 2006 by Charlie Stephens, a policy analyst for the Oregon Department of Energy. I reported the details of that conversation in an article, “Heating Water With PV,” published in the May 2006 issue of Energy Design Update.

“If you want to do solar water heating and solar space heating, solar thermal remains too expensive,” Stephens told me. “It’s not as cost-effective as using an air-source heat pump coupled to a PV array. In our climate, a properly sized solar thermal system can provide 100 percent of your hot water in the summertime, but it won’t do diddly in the wintertime. So you paid $4,000 for a system that provides 40 or…