Why does my bill vary so much from month to month with the Tarifa Transparente?

With the Tarifa Transparente, the energy price changes every hour, so your monthly cost depends on when you consumed as much as how much you consumed. If in a given month you run more appliances during peak hours (10:00–14:00 and 18:00–22:00 on weekdays), your average price will be higher. On top of that, billing cycles don't always cover the same number of days, as they depend on when your distributor reads your meter.