You left the (appliance) on.
Is there a single word for "on" in this case that would apply regardless of the appliance? For example, "you left the iron on." or "you left the lights on". I'm guessing that "en" isn't used in this case.
2 Answers
Prendido o encendido podría funcionar bien aquí.
Encendido/prendido, "Dejaste la plancha encendida/prendida", because of the gender.