I improved @Facade Kitsune's improvement of @Factorio Blueprint's Power indicator alarm. Instead of the alarm going off all the time while in low power mode(<20%), it only sounds once every 2 seconds. Also a secondary alarm that goes off indicating <50% that only sounds once.
Accumalator>>Combinators(Bottom)>>Power meter>>Combinators(Middle)[20%]>>Timer Combinators(Top)>>20% Low Power Alarm(Top) Accumalator>>Combinators(Bottom)>>Power meter>>Combinators(Middle)[50%]>>50% Alarm(Bottom)
There's also a fancy looking 100 that will turn on when the power is at max. And a silent alarm next to it that will only show a "100% Power!" alert, which you can remove if you wish for it to not clog up your alerts.