I have bought 2 of these panels, they charge a 100AH battery easily.
The specifications listed on any website may not be accurate, but they are just as good as the actual specifications.

Actual specifications:
Pmax: 20 Watt
Vmp: 17.5 V
Imp: 1.14 A
Voc: 21 V
Isc: 1.25 A
Dimensions: 55cm (length) 35cm (width)
(I have measured with a multimeter, voltage, current, and they are approximately as stated)

It comes with a junction box at the back of the panel.
Inside the junction box, there is a PN diode (also known as a Bypass Diode).
The Bypass diode is connected in parallel between the - and + of the panel.
(In photovoltaics, a PN diode is used as a Blocking Diode or Bypass Diode)
The anode of the diode is connected to the - of the panel.
The cathode of the diode is connected to the + of the panel.
Then, two cables with alligator clips come out of the junction box.
Wire colors: black and red.
Black is the -.
Red is the +.

As a beginner, I recommend it to anyone. You can create your own solar system with less money and it will definitely be more efficient than a ready-made solar system.
On the other hand, for 18 euros, it is very good.
//Greece is the sunniest corner of Europe.

For the fun fact:
A friend of mine once bought a standalone solar system (70 euros) with a controller that turns on the lights by pressing a button. After a week, when you pressed a button, the controller chip gave the wrong command and turned on other lights or when you pressed a button to increase the lumens of a lamp, it gave the command to decrease the lumens. In short, it's not worth it.

A simple solar system consists of:
Solar panel.
Solar Charge Controller.
Battery.
Consumer (LED lamp).

With these 4 components, you can create your own solar system.
Before buying these components, it is important to see and study:
1. Is my panel properly positioned? (Take a look at the neighborhood, how are the panels of the neighbors positioned? Usually, there is a 40-degree difference between the panel and the flat surface.)
2. Can my panel charge my battery?
3. How many hours does my panel need to charge my battery?
4. How many hours can my battery provide power to my lamp?
5. How many hours do I want my lamp to be on?
6. What is the power consumption of my lamp?

When you see "free" light, you will feel great relief.

/*Important tip, be careful if you leave the panel in the sun and then go to connect it with cables after some time. Remember that it holds current and you may close the circuit and get shocked by the current it has at that moment.*/

It is not something difficult, but it is not something easy either.
Personally, as a 16-year-old student in a vocational school, I found it easy.

Thank me later.

I have one of these panels but for a year now... it is of very poor quality... Firstly, measuring with a multimeter, I saw that its characteristics have nothing to do with reality. Secondly, half of the panel doesn't work... I realized this by covering half of it with cardboard and not seeing any change in the multimeter reading. I suspect it because the half looks like a different material!! Do not buy it!!!

I bought two of these about a week ago for the price of €26.99 each! I relied on the image I saw and the characteristics that the stores selling it have published. The image and the characteristics have nothing to do with reality.

The stores that sell them present it as a quality product and attribute the following characteristics to it:

"The high-performance and high-accuracy Solar BAO plus photovoltaic panel is a new generation modern monocrystalline technology photovoltaic generator that contains cells with anti-reflective coating and glass for producing more energy (more kWh per kWp) from your installation.
All photovoltaic panels imported to Greece have a positive tolerance of up to +5% of their nominal power in watts, giving them the ability to achieve yields of up to 16.9% compared to incident solar energy.
The photovoltaic panels are of very high quality and have European certifications CE, ISO 2000-2001
Rating Power (Pm) 20W-25W
Tolerance 0 +5%
(Vn) 12v
Rated Voltage (Vm) 17.8V
Rated Current (Im) 1.25A
Open Circuit Voltage (Voc) 21.70V
Short Circuit Voltage (Isc) 1.27
Dimensions 550x340x20
Weight 3.0 KG
ISO9001, ISO14001"

The product has the following characteristics written on the back:

Maximum Power (Pmp) 20W
Open Circuit Voltage (Voc) 21V
Short Circuit Voltage (Isc) 1.68A
Voltage at Maximum Power (Vmp) 18V
Current at Maximum Power (Imp) 1.12A
Maximum System Voltage DC1000V
Nominal Operating Cell Temp. (NOCT) -45 to 80°C
Maximum Series Fuse 3A
Power Measured in standard conditions (STC):
Irradiation 1000W/m^2, AM1.5, Cell temperature 25°C, CE
Weight: 1.75KG
Dimension: 460x350x17mm

If someone compares them, they will understand that it is a different product... even though I informed the store about this mistake, the response was that "these are the details given to us by the supplier."
Seeing the actual characteristics of the photovoltaic panel, one would think that it has better prices and delivers the desired watts with a smaller size and less weight, so it is cost-effective.... Not at all.... Their quality is very poor... stay away...