I bought the Mark5 solar heater in September 2023, and for the past 5 months, the solar heater has been used purely as an electric water heater, since to take a shower, we have to turn on the resistance for 1 hour. Even on days with full sunshine, with minimal consumption during the day, by the evening around 7 or 8, the hot water has disappeared. I won't even mention the next morning when the water is cold. For the past 5 months, they have come twice from Calpak (plus another 4 times from external plumbers at my own initiative), and the excuses keep coming and going. Everything else is to blame except for the solar heater itself. The communication with their headquarters is so terrible that it should be taught as an example to avoid. Although the solar heater has a warranty, they refuse to come and replace it, and I have to pay 150 euros upfront for them to do a cap check and verify the obvious, that is, the hot water is missing, while I have already spent a total of 250 euros on visits. Meanwhile, if they take a look at the reviews, they will see for themselves that others have the same problem, but they don't want to admit it for obvious reasons. I truly regret the money I gave them. A tragic treatment from a company that wants to claim it makes top-notch solar systems.

It has been used for 3 years in Attica. Statistically, I mention that throughout the year I turn on the resistance 10-15 times at most, and this happens on rainy days. All other days, the water is hot. Of course, I am not referring to summer (which is obvious). In winter, with 2 hours of sunlight, it manages to heat the water, and with prudent use, it is enough for 4 people. Of course, the use must be consecutive (one after the other), because logically, when one person uses it, the water that is replenished in the water heater is cold, so it lowers the overall temperature of the water. Therefore, the longer you delay its consumption, the less hot it will be.
Because it has been mentioned in the comments, even if the water has been heated one day without significant consumption, it is usable the next day. My overall impression is very satisfactory and I would recommend it for purchase. I had no problems with its installation and operation, so I cannot comment on the company's customer service.

I installed it a month ago and I have not been satisfied at all.
The water has never heated up above 60 degrees, even on sunny days and when we are away for the weekend. With 3 people in the house, using it moderately and taking a 10-minute shower at most, we don't have enough hot water. My parents, who have a 20-year-old water heater without selective collectors, have much hotter water at 70-80 degrees. Considering its price and features, I expected much better performance. If I were to buy again, I would definitely choose a different one because it is much more expensive without offering anything more! Also, their after-sales service is not good at all. When you report the problem and ask for their opinion, they immediately tell you that they need to come for a measurement with a charge of 120€. Simply ridiculous...
Edit 08/11/2023: After 6 months of use, I confirm the above.
Its performance is good only in the summer when even with a simple kettle, I would have hot water. As soon as the temperatures dropped below 25 degrees, the solar water heater stopped working. In the winter, with low temperatures, I will resort to using the electric one daily, although I am sure of the result.
Edit 02/02/2024: The solar water heater heats the water with sunlight all day, but not enough, so with a 10-minute shower, the hot water runs out. The electric resistance needs to be on for 45 minutes for a 10-minute shower, and for two people to shower, it takes at least 1 hour and 15 minutes.
If by chance we go to sleep at night with the water being at 40 degrees, in the morning it has lost 20 degrees of temperature. This means there are significant losses during the night.
Unfortunately, I wasted my money...