> Anyway, a very optimistic calculations means you need the flat area of at least 800 m^2 in a sunny climate, to charge a single electric truck to full power.

The math the person you're replying to was talking about was solar power vs ethanol. While your 800 m^2 sounds like a lot of land area for enough solar panels to fully charge a large electric truck, it's basically nothing compared to how much land area you'd need to grown enough corn to produce enough ethanol to power a similar truck. It's about 500 times as much land area, a fairly obvious result when you consider you get solar power from the land basically every day and corn once a year. Growing plants for ethanol is dumb.

Of course the benefit of a combustion engine is that you don't need to grow corn to make ethanol for it, at least as long as you can get fossil fuels out of the ground and the enemy doesn't disrupt your supply chain for drilling and refining fuel oil. But that's irrelevant since that wasn't the land-use comparison the parent poster was making.