Among the many things that Supergirl's time-travel episodes, "Prom Night!" and "Prom, Again!" change is the way the Girl of Steel recharges after exposure to Kryptonite. In a short, almost blink and you'll miss it moment in "Prom, Again!", the Arrowverse reveals what could be yet another Post-Crisis reset or the revelation of making the moon a new way for Supergirl to recover after her power is drained.
The sun has been featured as a source of Supergirl's powers. Alura tells the young girl in the pilot that Earth’s yellow sun will give her great powers. Early episodes show Supergirl draining her energy either by overexertion or exposure to Kryptonite and the use of special DEO sun lamps to recharge her. Sun lamps were used when she was knocked unconscious and into the ocean while chasing a missile and after her first case of solar flare following her encounter with Red Tornado. In later seasons, time and again, Supergirl turns to the sun like on Slaver's Moon with a red sun, when she regains powers when her sister tosses a sun grenade her way. And despite being saved by a Kryptonite suit to limit her exposure in seasons 4 and 5, she could still be found using the sun to recharge her energy, even being under sun lamps after her brief exposure to Kryptonite initiated by Red Daughter.
A new twist on the source of Supergirl's power emerged in "Prom Night!" After Alex removes Kryptonite shards from Kara's hand following her first encounter with the deadly green rock, the young Supergirl is still weak. However, she suddenly regains her strength and power after opening a door and stepping into the rays of the full moon shining outside. This is either a newly discovered second source of Supergirl's power or once again the Arrowverse's post-Crisis reset has chosen to rewrite her history. Either way, that short moment leaves fans with many questions and possibilities.
If this is indeed what is quickly becoming the new Arrowverse trope of blaming any retcon on the post-Crisis reset, it creates many problems going forward. The moon or moonlight becoming the only way for Supergirl to recharge after overexertion or Kryptonite exposure is fraught with complications. If this is a rewrite, then the aftermath of her Red Tornado encounter has been irrevocably altered to change the concept of the solar flare. The DEO will need to spend time creating moon lamps. Supergirl’s body clock may need to be switched from day to night so she gets more time in the moonlight. And of course, there are certainly issues with the strength of the moonlight due to the moon’s many phases.
Another avenue to explore is if this isn't a reset, why hasn't the moon been used as a restorative power source before? One theory for why using the moon as a sort of lunar Supergirl battery hasn't been used is that Alex, one of the world's foremost authorities on Kryptonian physiology, simply forgot that moment after her sister’s first exposure to Kryptonite. There is also the possibility that after Supergirl's first Kryptonite exposure, subsequent exposures have Kryptonite lingering in her cells blunting the moon’s effect. Or it could be something as simple as the show remembering basic science in that moonlight is just another form of reflected sunlight.
It would make sense for Supergirl to change the source of her power to be changed to the moon, after all the moon is often regarded as a feminine symbol of power. Regardless of the reason, this retcon of ways to combat Supergirl's exposure to Kryptonite just adds weight to the interest of fans for the much-desired Danvers sisters' Midvale prequel series. Watching young Kara discover other superpowers and learning just what the young superhero can do would provide a multitude of story ideas for such a spin-off.