A power indicator with an alarm. Each row lights up with 10% more power. An audio alarm is triggered once when the power dips below 20%. The alert threshold can be configured in the alarm. The colors can be configured in the constant combinators. It's possible to use fewer constant combinators at the cost of configurability and fewer lights at the cost of brightness (but you'll save on power).
Originally from https://factorioprints.com/view/-Kl-a5-7hM9UdJQEYzjU, upgraded to be easier and simpler. Removes unnecessary parts (such as combinators) and makes it easier to make (no longer requires a substation).
Combines the audio alarm from https://factorioprints.com/view/-KjcT8FkYRMzsnu9Qguq with the light indicator from https://factorioprints.com/view/-KkvU0W4_ayHgU8XV1cf and simplifies the circuit network stuff.