Due to a large number of posts, which I received by email or at fairs, I decided to reply to you through this article on our blog. I will try to answer as soon as possible. I am waiting for your questions by commenting on this blog post. 
1. solar panels for domestic hot water and / or 2. solar panels for heating and DHW (domestic hot water)?
If we want a system of solar panels only for domestic hot water we have to think if we need it only for the period 1a spring - autumn or 1b. the whole time of year? This clarified aspect we will look for a system depending on the performance or depending on the price (the price is not always a decision factor when we choose a water pump or a solar panel for our home). Un solar panel cheap will have the advantage of relatively low starting cost and over time (years or months) an increasingly lower yield. A high-performance solar system is more expensive initially (not by much, as this product will allow) but comfortable in terms of performance / service over the years, not as if you were buying a solar panel or a cheap water pump. The choice in this case is relatively easy because solar panel manufacturers have already predefined models based on water storage capacities precisely to facilitate consumers' choice of the right system according to the daily needs of hot water. If I still came up with these explanations, what would you choose one solar panel cheaper with a shorter lifespan, or a solar panel /water pump something more expensive (how much can you afford or in installments), with a longer lifespan? I am waiting for your answer regarding the water pump / solar panels. 
2. In general if we want a system of solar panels or the water pump to help us heat and produce hot water or DHW heat at the same time, we must know that it will be from the start only throughout the year, not less than a year. Here things are already complicated and variables such as: what area does the house have? with what and how do we heat it at present? how can we know concretely what solar input we have in winter? These elements will be professionally evaluated by computer simulation. It is extremely important that in addition to "words" there is a concrete anticipated assessment of the energy saved during the winter.