I think I need to start ignoring your comments. DOE says solar, heat pump, and tankless gas water heaters are more efficient than conventional water heaters. I agree, its fairly obvious. What I was saying is 1) payback is better for grid tied solar PV than solar water heaters, and the solar PV adds power at peak demand often to the grid making that power more valuable. 2) solar PV is scalable, which means you can build a bigger system. 3) Solar PV is replacing grid electricity normally, while solar water heaters often replace abundant natural gas with electricity. This is often not worth the extra money from a ghg or pollution point of view. 4) If you want to heat water from PV you can, and you can heat all of it, not just a portion. IF you use a heat pump water heater, this often uses less roof space. See DOE and I don't disagree at all. When you look at today's prices and tomorrows energy needs, solar hot water heaters do not make sense in the United states for just about every household, solar PV is a better use of roof top. The two main cases for solar water heaters are swimming pools and those off the grid. A very high percentage of existing solar hot water heaters are for those with swimming pools, and my friends with pools and no solar hw, just don't heat them in the winter - big energy savings right there. If you don't need hot water all the time and live where it doesn't freeze, some of the cheap solar water heaters might make sense especially when self installed, but this is a very small portion of the population. Run the numbers yourself. Post them here so we can all look at them. The prices of solar PV have dropped and water heaters have become more efficient. And simply because this thread is about coal, solar hot water heaters replacing gas water heaters do not reduce coal use at all, but solar PV has the potential to do that.