A quick web search reveals this:

Still, no matter how well a collector performs or how perfectly it's placed, there's no getting around the fact that it relies on an energy source that only works part-time. “You can never meet the demand exactly, because you can't turn on the sun when you need it,” says Robert Waters of Viessmann, a manufacturer of solar thermal systems. In the some parts of the United States, solar panels might provide as much as 95 percent of a household's hot water in summer, but as little as 20 percent in winter. And unlike solar-generated electricity, which can be stored in batteries or sold back to the power company, hot water is a fleeting asset; even a well-insulated storage tank turns cold after a few overcast days. That's why virtually all solar hot-water systems are supplements to, rather than replacements for, conventional water heaters.