During the multiple hurricanes we had here a while back, we lost power for a week or so, and my wife decided she was tired of cold showers after a couple of days. I hooked two 120' black garden hoses together, plumbed them in series with the hot-water side of the water heater, and draped them over the roof. Viola! hot showers. VERY hot showers. Water smelled a bit like garden hose, but desperate people do desperate things.
The solar thing would work well then with a tempering valve. You could run the tank very hot, and then dial down the mix for use.