Power supply

A power supply comprising:

• input terminals for connection to an AC source;
• output terminals for connection to a load;
• an AC-to-DC stage;
• and
• a battery,
• wherein the AC-to-DC stage and the battery are connected in parallel between the input terminals and the output terminals, the power supply operates in either a first mode or a second mode, the load draws current from only the battery when operating in the first mode, the load draws current from both the battery and the AC-to-DC stage when operating in the second mode, and when operating in the second mode:
• the AC-to-DC stage draws an input current from the AC source and outputs an output current having a waveform that is periodic with a frequency twice that of the input current and a ripple of at least 50%;
• the current drawn by the load is greater than the output current during one or more first periods;
• the current drawn by the load is less than the output current during one or more second periods;
• the load draws current from the battery and the AC-to-DC stage during the first periods such that the battery discharges;
• and
• the load and the battery each draw current from the AC-to-DC stage during the second periods such that the battery charges.