Many times you have to decide to invest for your home stuff, and I was thinking a lot about how to heat the water. In Spain the typical ways are (butane, electrical o sunny heaters).
As you know the last, always makes you a doubt point. To summarize, where I live there is about 8ºC to 15ºC in Winter, and these days the water input was 15ºC and the output rises to at least 50ºC (my thermometer doesn’t measures higher). The system is a 22 heat pipe tubes with a 220 liter tank.
I strongly recommend you.