Dual supply of a backup power supply

A power backup system comprising:

• a multi phased energy storage, and
• a backup supply comprising an AC voltage input and a DC voltage input,
• the multi phased energy storage comprises at least two battery strings each comprising a plurality of battery modules wherein the connectivity of the individual battery module of the plurality of battery modules to its respective battery string is controllable by a switching arrangement associated with the individual battery module,
• wherein the switching arrangements comprises four semiconductor switches in an H-bridge configuration,
• wherein the switching arrangements of a first battery string are controllable so as to establish a DC output voltage of the first battery string,
• wherein the switching arrangements of a second battery string are controllable so as to establish an AC output voltage of the second battery string,
• wherein during normal operation, the backup supply is configured for:
• being supplied with an AC voltage from a utility grid via the AC voltage input, and
• being connectable to the first battery string via the DC voltage input, and
• wherein, during abnormal operation, the backup supply is configured for:
• during a first period of time, being supplied, via the DC voltage input, from the DC output voltage established by the first battery string, and
• after expiration of the first period of time being supplied, via the AC voltage input, from the AC output voltage established by the second battery string.