The system uses a photoelectric converter on the bottom of the drone to capture energy from the laser. These kinds of systems do a pretty decent job of wireless power transmission, although the efficiency isn't great. At the transmission end, power-to-laser efficiency can be between 50-85%, depending on how fancy your laser is, and at the receiving end you're likely losing around 50% of your energy again.

But electricity is cheap, and if you're plugged into the mains, these losses are acceptable if it means your surveillance drone never needs to come down.