So why have a solar tonneau cover? Well if you have a pick-up truck and want power then you need to bring along a generator. Your generator needs gas so you need some cans of gas. Unless you use your generator all the time then you need to keep it in running order. That means running it every few weeks to keep the gas in the carburetor from evaporating and leaving it unusable. That cost you money for the gas that you burn while not actually using the power. The gas that you have to keep on hand has to be continually cycled with fresh gas to keep it from going bad.
So now solar. It is maintenance free no moving parts. When you use the power from the battery or batteries the solar panel(s) along with the charge controller will recharge your battery to replace the power that you have used, then stop charging the battery. All you need is sunshine and even on a cloudy day you will be producing some power to recharge with.
If you run a generator to charge (for example) a cordless drill battery, then you are using a 5 horsepower motor burning perhaps $5 worth of gas an hour to charge a small battery. That is not efficient.
If you use solar then you only produce the power you need, no waste.
If you use an inverter in your truck it's even worse .Then you are running a 300hp motor to charge a small battery. You need to keep the truck motor running to avoid draining your truck battery and leaving you needing a boost.