Supergirl continues to find ways to tell compelling stories about people in costumes with other-worldly powers, without needing to turn dark, “gritty” and misanthropic. This flies in the face of everything we know about superheroes in the 21st century! Clearly, someone has not gotten the memo.
“Human for a Day” was the traditional “hero loses their powers” storyline, which every superhero series has to do at some point. It’s pretty much required. In this case, the mechanism of Supergirl losing her powers was lifted right from the recent Superman comics—Superman (and Supergirl) have the power to unleash a “Solar Flare,” depleting their cells of all their stored solar radiation to create a massive blast. This leaves them powerless for 24 hours (in the comics) or even 48 hours (according to the TV show.)