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.
But Dyson had this great spherical concept, so maybe there is a way to get a lot of that energy while still, like, living on the planet.
Or is it saying that you would get 173,000 terrawatt-hours of electrical energy? It refers to energy then gives a number in units of power.