Hi Antony. Lots to unpack here.
First the panel you bought. If P=VI, then the panel you bought is 240 watts. That's a big panel! That's the type of panel you would see on a house roof. (but as we will see below, your sticker tells another story).
Next the controller. A 20A/24V will definitely handle your panel.
The panel is not fixed at 12V. The VMPP rating is the Voltage and Maximum Power Point which means that your panel, in full, vertical sunlight, should deliver 17.3 V. Note that panels will often deliver even slightly higher peaks. Hence the need for a controller.
Now let's talk about the controller (and the panel's sticker). The sticker says 20W; that's actually a small panel. The IMPP and VMPP confirm this. If you multiply the two, so 1.20A X 17.3V, you get 20.76W. So in actual fact, the 20A, 12V rating you got when you googled it, does not match the sticker. As for your 21V measurement, like I said, the VMPP of 17.3V can sometimes be exceeded.
In short, the controller you will need is not that big. It needs to handle ±2A and 18V; those are fairly cheap and standard. What you will have to decide is whether you want a PWM or MPPT controller.
Now my advice. Get a bigger controller because you may end up getting more panels. Given the choice, get an MPPT controller but check out their RFI. Many solar controllers cause RFI, bad sometimes...
What follows is a more detailed technical description of what you've asked (sorry you asked?? 😎)
NOMINAL POWER: This is also known as the power rating of the solar panel. In your case, it's 20W, meaning the panel should generate 20 watts of power under standard testing conditions.
POWER OUTPUT TOLERANCE: This shows the variance in power that the panel could produce. In your case, it's -0/+3%, which means the panel could produce up to 3% more power than its nominal rating but will not produce less.
LSC: This stands for Short Circuit Current. It's the maximum current that can flow through the solar panel when there is a short circuit. Your panel's LSC is 1.30A.
VOC: This stands for Open Circuit Voltage. It's the maximum possible voltage across the solar panel when no current is flowing through it. Your panel's VOC is 21.7 volts.
IMPP: This stands for Current at Maximum Power Point. It's the current at which the solar panel outputs the most power. Your panel's IMPP is 1.20A.
VMPP: This stands for Voltage at Maximum Power Point. It's the voltage at which the solar panel outputs the most power. Your panel's VMPP is 17.3 volts.
MAXIMUM SYSTEM VOLTAGE: This indicates the maximum voltage that can be safely handled by the solar panel's internal components. Your panel's maximum system voltage is 600 volts.
MAXIMUM SERIES FUSE RATING: This is the maximum current that can pass through a fuse in the system without blowing. It's a safety feature to protect the system from excessive current. Your panel's maximum series fuse rating is 15 amps.
NORMAL OPERATING CELL TEMP: This is the average temperature that the solar cells will operate at under normal conditions. Your panel's normal operating cell temperature is 45 degrees Celsius, plus or minus a certain amount that isn't specified here.
Please note that all these values are typically measured under standard test conditions (STC) which include a solar irradiance of 1000 W/m², cell temperature of 25 °C, and air mass 1.5 spectrum. Real-world conditions can cause the performance of a solar panel to differ from its rated specifications.
Now let's look at the differences between PWC and MPPT.
Solar controllers, also known as solar charge controllers, are essential components in most solar power systems. They regulate the voltage and current coming from the solar panels going to the battery, preventing overcharging and ensuring the longevity and efficiency of the system. There are two main types of solar charge controllers: Pulse Width Modulation (PWM) controllers and Maximum Power Point Tracking (MPPT) controllers.
Pulse Width Modulation (PWM) Controllers: PWM controllers are relatively simple and cost-effective. They match the solar panel voltage to the battery voltage by slowly reducing the amount of power applied to the batteries as they approach full charge. This is accomplished by "pulsing" the full charge on and off very rapidly. The downside of PWM controllers is that they do not optimize the power output from the solar panels as conditions change.
Maximum Power Point Tracking (MPPT) Controllers: MPPT controllers are more advanced and efficient. They continually adjust the input voltage from the solar panels to find the maximum power point and convert the excess voltage into more current. This optimizes the performance of the solar array and can significantly improve the efficiency of the system, especially in colder temperatures. MPPT controllers are generally more expensive than PWM controllers but can provide up to 30% more power from the same solar panels under certain conditions.
It's worth noting that the best type of solar charge controller for a particular system depends on the specifics of the system, including the power of the solar array, the type of battery, and the climate and environment where the system is installed. But like I said Anthony, check the RFI!!!!
My final, final recommendation: I would suggest using an MPPT type controller for amateur radio applications due to their improved efficiency and advanced technology. However, I'll note that product availability and models may have changed since I bought my last controller, so I can't provide a specific product that's currently available.
That said, one brand known for their high-quality solar charge controllers is Victron Energy. They manufacture MPPT charge controllers which are reputed to have low radio frequency interference (RFI). You might look at a model like the Victron SmartSolar MPPT 75/15 which can handle up to 75 volts and 15 amps, that way you can grow with it. This model has Bluetooth built in for monitoring and configuration, and Victron has a strong reputation for quality. I'm not sure of the price difference, but you could also save a little and get a 75/10. Here is the link to both: https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-75-10-75-15-100-15-100-20
Sorry for the lengthy reply to such a simple question. I was in the mood.... 😁
Last point. Considering you bought the panel at a super cheap price and the controller is overkill, maybe look on Ebay or get something much smaller just to get your feet wet!!