When we published initial results from the testing of solar water heaters, we knew the analysis was a work in progress. For instance, when the October 2009 issue of Consumer Reports went to press, some data from the hottest months of the year were missing. And one of the key deciding factors in buying a solar water heater—how much money it can save you—tends to change like, well, the weather. Here's an update on our testing of the solar water heaters installed on the roof of our Yonkers, New York, headquarters:

Second panel boosts savings. When we added a second panel to the $6,000 Heliodyne 50180 system—a move that gave this model Energy Star qualification—we saw a definite jump in the percent savings from the 32 percent we reported, even during a period with unseasonably cool weather. Still, the $1,500 extra you might pay for this two-panel system could result in a longer payback period even considering whatever state tax credits you can get on top of the 30 percent federal credit. Read "Energy Tax Credits at a Glance" for more information.

A newcomer shows promise. An Eagle Sun model, $6,000 to $9,000 installed, is the newest system on the roof. While the model is rated by the Solar Rating and Certification Corporation to supply 70 percent of the energy needed to heat the water for a two- to four-person household, we won't have enough data to report on until sometime next year. Still, the expected savings from the Eagle Sun are much higher than those of the first solar-thermal systems we installed. (The photo shows Senior Lab Technician Eric Hado (left) and Senior Project Leader David Trezza installing the Eagle Sun system on our roof.)

Higher savings are achieved. Installing the Hot₂O Solar Hot Water System by Fafco, a do-it-yourself system (we recommend pro installation), cost us $2,200. But the company has updated its installation instructions. After we modified the setup, which included insulating the hot-water tank, the average savings climbed from about 29 percent to roughly 42 percent.

Performance stays consistent. We took the Rheem RS47-21BP, $5,000 to $6,200 installed, out of service to make room for the newer systems, but until the day we removed it in mid-September, its average savings remained at the roughly 32 percent we first reported. And the SunMaxx 3570 (PDF), $5,500 to $7,000, is maintaining the same 23 percent savings we saw in the system's first weeks in place; its single array of tubes is roughly equivalent to a single flat panel in terms of performance.

As we move into fall and then winter, we'll see whether evacuated-tube systems like the SunMaxx really perform better and save more money than flat-panel designs in cold weather.—Ed Perratore | e-mail | Twitter | Forums | Facebook

Essential information: If you're considering a solar water heater, read our recent report on alternative energy (specifics on solar water heaters are available to subscribers). The American Solar Energy Society kicked off its 14th annual National Solar Tour on October 3. To find a related event in your state, visit this page. If a solar water heater isn't for you, check out our coverage of tankless water heaters.