A Solar water heater (SWH) is composed of a heating unit "collector" and of a storage unit "tank". How does it work:

Imagine a garden hose lying in the sun - the water inside will be heated by sunshine. If you paint it black you will minimize the reflection losses and the water will be hotter. You want to heat the water for a large swimming pool? Just arrange a large array of black painted hoses (see image). In western countries the swimming pool will need hot water during summer in the day time - perfect, your hot water delivery is right in time with the demand and you dont have to store it!

Now you change this hose to a more effective row of glass tubes and cover them with a large glassbox - the water is hotter again because you are reducing the convection losses. Thats understandable?
Now change the glass box for an outer glass tube and you evacuate the air between the outer and the inner glass tubes (like in a thermos jug): Because of the tubes shape you get energy even when the sun shines from left or right and because of the vacuum you will have probably no more energy losses.
If you lean this tubes against a frame you will change the water temperature inside: Because of the larger proportion of hot water than cold water the cold water will flow downwards.
This is your collector!
You want to store this hot water and use it later? Connect the upper end of the tubes to a well insulated tank - the water will be hot for several days.
If your tank is higher than the collector you have to limit the water amount by a float valve - like you have in the flush box of your toilet.
This whole system is called a "Thermosyphon" - you can see it in the pic above.

If your tank is lower than the collector you need a closed pressure tank or you must let the hot water flow through the tank in a closed circuit - the so called "heat exchanger" like in a immersion heater.
The less components you have to install the better the system. A good SWH is maintenance free, can last 10-15 years and will pay back your investment after 5-8 years: That's the famous "return of investement" or ROI - don't believe it if somebody promises you a ROI after 1-2 years. Thats "talking for selling".

But how can we ensure to install the right size of SWH for you?

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