A computer-implemented method comprising: receiving, from a power bank device configured to supply electric charge to a battery of a rechargeable device external to the power bank device, a first fuel gauge signal indicative of a present fuel gauge of the power bank device, the present fuel gauge of the power bank device corresponding to a percentage of a capacity of a battery of the power bank device; obtaining, via the rechargeable device, a second fuel gauge signal indicative of a present fuel gauge of the rechargeable device, the present fuel gauge of the rechargeable device corresponding to a percentage of a capacity of a battery of the rechargeable device; characterised in the computer-implemented method further comprising: determining, via one or more processors, a first charging efficiency factor of the power bank battery, the first charging efficiency factor being determined based upon the first fuel gauge signal; determining, via the one or more processors, a second charging efficiency factor of the rechargeable device battery, the second charging efficiency factor being determined based upon the second fuel gauge signal; generating, via the one or more processors (158, 188), a number of potential rechargings of the rechargeable device from the power bank device, each of the potential rechargings corresponding to charging of the rechargeable device battery to a desired fuel gauge, the desired fuel gauge corresponding to a second percentage of the capacity of the rechargeable device battery, and the number of rechargings being determined based at least on the desired fuel gauge, the present fuel gauge of the rechargeable device, the present fuel gauge of the power bank device, and the first and second charging efficiency factors; and providing, to a user interface, an indication of the number of potential rechargings of the rechargeable device. receiving, from a power bank device configured to supply electric charge to a battery of a rechargeable device external to the power bank device, a first fuel gauge signal indicative of a present fuel gauge of the power bank device, the present fuel gauge of the power bank device corresponding to a percentage of a capacity of a battery of the power bank device; obtaining, via the rechargeable device, a second fuel gauge signal indicative of a present fuel gauge of the rechargeable device, the present fuel gauge of the rechargeable device corresponding to a percentage of a capacity of a battery of the rechargeable device; characterised in the computer-implemented method further comprising: determining, via one or more processors, a first charging efficiency factor of the power bank battery, the first charging efficiency factor being determined based upon the first fuel gauge signal; determining, via the one or more processors, a second charging efficiency factor of the rechargeable device battery, the second charging efficiency factor being determined based upon the second fuel gauge signal; generating, via the one or more processors (158, 188), a number of potential rechargings of the rechargeable device from the power bank device, each of the potential rechargings corresponding to charging of the rechargeable device battery to a desired fuel gauge, the desired fuel gauge corresponding to a second percentage of the capacity of the rechargeable device battery, and the number of rechargings being determined based at least on the desired fuel gauge, the present fuel gauge of the rechargeable device, the present fuel gauge of the power bank device, and the first and second charging efficiency factors; and providing, to a user interface, an indication of the number of potential rechargings of the rechargeable device. determining, via one or more processors, a first charging efficiency factor of the power bank battery, the first charging efficiency factor being determined based upon the first fuel gauge signal; determining, via the one or more processors, a second charging efficiency factor of the rechargeable device battery, the second charging efficiency factor being determined based upon the second fuel gauge signal; generating, via the one or more processors (158, 188), a number of potential rechargings of the rechargeable device from the power bank device, each of the potential rechargings corresponding to charging of the rechargeable device battery to a desired fuel gauge, the desired fuel gauge corresponding to a second percentage of the capacity of the rechargeable device battery, and the number of rechargings being determined based at least on the desired fuel gauge, the present fuel gauge of the rechargeable device, the present fuel gauge of the power bank device, and the first and second charging efficiency factors; and providing, to a user interface, an indication of the number of potential rechargings of the rechargeable device.