When the credit on your meter is less than one pound the meter "beeps" warning you to either purchase more credit or, press the emergency credit button (this will normally give you two pounds). Pre-payment meters are normally configured not to go off between 23:00 and 07:00 hrs because of off-peak heating. However, if you use up all the remaining credit, including the emergency during those hours, your supply will go off at seven am and you will owe not only the emergency pay back but any credit you had left on the meter plus the cost of any additional units you consumed during the night.