That's rather meaningless number intending maybe to shock some folks who don't understand math/physics etc for many reasons, just randomly: atmosphere takes a bit out; we will never be 100% effective (but 50% is not unreasonable); we can't cover even 1% of earth surface with solar panels even when ignoring most are oceans, that would be absolutely massive use producing staggering waste and due to curvature of Earth only a fraction of them would be useful at a given time.

So what you are basically saying we have to support solar with other sources, always, by principle. Still good supplementary stuff unless you are too north/south from cca equator, there it will never be a major thing.

They speak in term of raw energy, carried by the photons the sun sends us. If we had 100% efficient solar panels and covered the entire earth with them we'd get 173,000 terrawatts of electrical power.

What's ??? about it. That actually sounds much less than expected. It's not saying we can or should capture it all, just the possibilities if we can harness even 0.1% of it.

Why every day, not every some other time unit? The unit "terawatts" is a rate, not a quantity of energy.