The operation of a solar heater and the efficiency of the solar water heater will always and exclusively depend on the materials used in the solar heater and its insulation. It will also depend on the insulation of the water tank. With a good installation we can even surpass the performance of some cheap commercial heaters. And what happens if it is cold and there is no sun? The operation of a solar heater will be optimal when your solar heater is well built. In this way it will take advantage of direct sunlight, and it will also take advantage of even diffuse radiation. On a cloudy day we probably won't be able to heat much water, and we may not raise it to the desired temperature, but even raising our water tank a few degrees can lead to savings. Or the extension of its conservation.
How does the tank keep the water hot on a cloudy day?
If the tank is properly insulated, the water will not cool down much.
If we lose a few degrees over the course of a few days, but we recover them thanks to the good functioning of the solar heater, the energy will be conserved.
Imagine that the tank is at 46ºC, and every day we lose 8ºC due to the lack of sun.
If 3 days go by, in theory we lose 24ºC.
If the collector can increase our temperature by 11ºC daily, thanks to the diffuse light, and the little sun that comes out between clouds, well, that's it, conserving.
So that we lose 8ºC every day and the collector increases us 11ºC.
So we will have gained in that term of 3 days, not only will we have conserved, but we will have gained a total of, 9ºC.
So from 46ºC we will have 55ºC.
All this if we do not use that water of course, it is all theory.
But I have done this many times, if one day it is weak, I consume little. And the next day, between conservation and gain, I finally got the water really hot.
We can also heat the water with an electrical support resistor.
You know, the old-fashioned electric tanks.
Or we can also heat the water by passing it through a gas heater. Or through the inside of a chimney.
Operation diagram of a solar heater
If we finally use an auxiliary support, the savings are directly the difference between heating warm water (our accumulator) or cold water from the street.
In other words and with numbers.
Heating from 24ºC (tank) to 36ºC, or heating 14ºC (street) to 36ºC, there is an abyss of energy expenditure.
How does a collector heat our accumulator?
It all starts at dawn. The solar heater will be cold.
Let's assume the tank is very cold because we used up all the water the day before.
So, cold solar heater and cold tank. OK?
The sun comes out and the inside of the solar heater starts to heat up.
It heats up because it is painted black, the black color attracts the heat from the sun.
Yes? We can see this by standing in the sun with a black sweater on.
The glass or plastic screen (in my case polycarbonate) lets in solar radiation (which is what heats everything).
It quickly heats up everything «black».
And the screen doesn't let the heat that is produced inside the box escape. (This is called the greenhouse effect)
What's under the paint? A metal body and copper tubes that will trap that temperature.
What's inside the copper tubes? Cold water that will be
This effect of allowing sunlight in and not letting the heat produced escape is called the greenhouse effect, like the greenhouses of rural agriculture, like the damned effect that is changing our planet.
When the solar heater exceeds the temperature of the tank, the device that controls both things (normally it is a differential thermostat with temperature probes in our collector and tank), will start the water pump.
The circulating water «steals» the heat from the collector, and spills it into the tank. Literally.
And so on. until the collector cools down.
And so on many times until the day is over.
What the hell?
A bottle as a solar heater
This is an example to understand the design of the operation of a solar heater.
Take a 1.5 liter bottle.
Paint it black.
Fill it with water.
Put it inside a transparent bag
Finally, leave the bottle on the street, with all this assembly. Or on a patio where the sun shines on it.
That's your solar heater, and your tank.
No kidding, it works.
After 1 hour, the water in your bottle is incredibly hot.
What will you do with it? Shower?
No, it's very little water, it's absurd.
But you can pour it into an insulated bucket to keep it at its temperature.
Then you store that hot water in the bucket, cover it.
You will repeat the process again.
It's no joke, you could fill a 50 or 60 liter bucket in one day doing this, and have a pleasant shower at night.
If it takes 1/2 hour to heat up a 1.5 liter bottle (you can use reflectors), imagine how much water you would get in 5 hours.
A not inconsiderable 150 liters.
Who would do this in a modern civilization? Nobody.
And if we put a 150 liter bottle in the sun, hahahaha, fantastic!!!!!
Where are there bottles of that size? And who brings us the "little bottle"? to the bathroom?
So, the trick that is used in solar heaters is this very thing.
Each cycle of circulation between the collector and the tank is like the back and forth of the little bottle.
And the accumulator is our bucket where we were filling little by little with each bottle.
Isn't it simple?
Graphic example of the operation of a real solar heater during the day.
enthusiast with two decades of experience using this operating system, which has enriched my approach to development and technical problem solving. 
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