A few random thoughts...

The number of solar energy watts theoretically hitting your solar panel is not the number of watts in 12 volt DC going to your battery storage. Expect system losses of more than 10%.

In the informal experiment I ran a few years back, the closest I got was ~70% of the number in a panel’s name. One of the 21 watt panels actually measured out at 14.3 watts — that’s at roughly solar noon on a nice sunny SOCAL day. Add some clouds and move the sun left or right of center and I would expect the numbers to drop off.

My only recommendation is to buy a bigger panel than you expect to need. Then test it before you need it to make sure you have all the wires/cables to make the system work.

I have a 100 watt panel and Renogy Wanderer Charge Controller. No affiliation. The smaller, portable units you might throw in your backpack to take hiking/camping came with built in controllers and USB output.