Earlier this year, I blogged about a new website set up to ease the chore of shopping for solar panels, EnergySage, and since then the company's own blog has described two financial benefits of solar which I hadn't thought about before. First, solar panels can help you save for retirement; in fact, it's hard to imagine an investment that anyone who's not a Russian oligarch could make for such a high return and such a low risk. Second, photovoltaic power can cut the cost of driving an electric car—which, mile for mile, is already a third or fourth of a gasoline car—in half.

My own panels have earned me about $2,000 a year in electricity savings as well as the tradable certificates I receive for each megawatt-hour of energy they generate. Considering how much I had to pay up front, that amounts to a return of just under 20%. That outdoes the stockmarket, not to mention the lower-yielding investments that many retirees rely on. The investment isn't completely risk-free; you're subject to the vagaries of weather, equipment failure, fluctuations in the certificate market price, and family members who evidently take the solar array as an excuse to leave all the lights on. At one point, we had a run of cloudy days and our array fell behind its expected monthly output. We later caught up, though. (I'm still working on the family members.)

I talked to Diane Hammond, a Massachusetts homeowner featured on the EnergySage blog, and she said she bought her solar array last year when one of her certificates of deposit matured and all the new ones were paying just 0.5%. "There just isn't any place to put your money to get a return on it," she told me. She, too, estimated her solar array has had an effective return of about 20%. "I've been so excited I'm not paying the electric company all that money," she said. "It puts a little bit extra in my pocket."

EnergySage's CEO, Vikram Aggarwal, who has a background in financial planning, told me: "It's one of the best financial products you can buy. All financial advisors should be promoting solar." He also argued that the panels increase the resale value of a house, which I'm more skeptical about. I watched a few episodes of House Hunters on HGTV recently and got a sobering lesson in the irrationality of homebuyers—they'd routinely reject an entire house for want of a $100 bathroom fixture. So I doubt that a lifetime's worth of free electricity will even register with them. But, realtors who are reading this, I hope you can prove me wrong.

Regarding the electric car, the EnergySage blog cited Neeraj Aggarwal (no relation to Vikram) and I contacted him to follow up. He told me he leased a BMW i3 at the same time he was preparing to install a solar array and decided to increase the array size from 8 kW to 10 kW. Working through the cost of charging the battery and the savings afforded by his array, he reckoned the electric car costs 2 to 3 cents per mile to operate. A equivalent gasoline-powered car might cost 20 cents per mile. Given the distance he typically drives, Aggarwal estimated he saves $250 a month, or half the cost of the lease.

Whenever the subject of electric cars comes up, people always ask about range. Aggarwal said he gets 80 to 100 miles on a full charge, which is just enough for his daily commute. He uses special mapping apps to get driving directions that take charging stations in account, so he can top off his battery over dinner. "You have to be a little more prepared or analytical in terms of driving the car," he said. This minor inconvenience is offset by lessened routine maintenance—no oil changes, for example.

To take full advantage of the economics of solar, you do need to buy the array rather than lease it. Although leases save the upfront cost, they give you only a fairly small reduction in your electric bill. Fortunately, buying an array is easier than it used to be, because banks are now providing loans specifically for solar arrays. In my case, I was able to take advantage of a loan program run by my utility, PSE&G, which recently announced a renewal of the program.

It's not often you can do good and do well at the same time. Take advantage of it while you can.

Photos courtesy of Neeraj Aggarwal and Diane Hammond