False flat rate tariffs
As flat rate tariff sounds attractive, it has been used for tariffs that are not really flat rates. It might seem like they are, because they assure you that what you pay each month never changes... unless you exceed your consumption limit.
Sometimes this is an annual limit. In other words, you pay the same every month, but your consumption is checked at the end of the year and you have to pay more in compensation if you've exceeded your limit. This process is called regularisation. With Endesa Única there is no regularisation.
There is nothing wrong with these options. The only problem is that they are being marketed by calling them something they are not. These options are a world away from Endesa Única.
If we think about the gym, it would be like them telling you at the end of the year that you have done too many hours of exercise and that you have to pay another 50 euros at Christmas.
There are many examples of false flat rates with electricity and gas tariffs.
It looks like you always pay the same, but at the end of the year it's time to do the sums and see if you have to pay more or get a refund. The threat of a surcharge for excess consumption is always hanging over you. These are not true flat rate tariffs. This is not Endesa Única.