A method of managing the overall power demanded from one or more power sources (6, 7) forming part of a machine, the method comprising: i) setting time-varying power demand constraints (10, 15) for the one or more power sources, wherein the time-varying power demand constraints (10, 15) are set as a function of time and vary with time and are based on an operational objective of the machine, and wherein the power demand constraints (10, 15) include a maximum power demand limit (PMAX) and a minimum power demand limit (PMIN); ii) setting a desired operating characteristic representing a mode in which it is desired to operate the machine and/or the or each power source (6, 7); iii) inputting the time-varying power demand constraints (10, 15) and the operating characteristic to a power control processor (4, 16) arranged for autonomously regulating the overall power demanded from the one or more power sources (6, 7), the inputted power demand constraints representing a remit of delegated autonomy for the power control processor (4, 16); iv) for the or each power source (6, 7), assessing one or more power source operating parameters and/or one or more fault conditions (20, 21) associated with the or each power source (6, 7); and v) using the power control processor (4, 16) to autonomously control the power (p) demanded from the or each power source (6, 7) by: comparing the power demand constraints (10, 15), operating characteristic and power source operating parameters and/or one or more fault conditions (20, 21) with data pre-stored in a storage device; and generating a power demand control signal for the or each power source; such that the overall power demanded from the one or more power sources is within the delegated remit of autonomy and the operation of the machine and/or the or each power source is optimised for the input operating characteristic. i) setting time-varying power demand constraints (10, 15) for the one or more power sources, wherein the time-varying power demand constraints (10, 15) are set as a function of time and vary with time and are based on an operational objective of the machine, and wherein the power demand constraints (10, 15) include a maximum power demand limit (PMAX) and a minimum power demand limit (PMIN); ii) setting a desired operating characteristic representing a mode in which it is desired to operate the machine and/or the or each power source (6, 7); iii) inputting the time-varying power demand constraints (10, 15) and the operating characteristic to a power control processor (4, 16) arranged for autonomously regulating the overall power demanded from the one or more power sources (6, 7), the inputted power demand constraints representing a remit of delegated autonomy for the power control processor (4, 16); iv) for the or each power source (6, 7), assessing one or more power source operating parameters and/or one or more fault conditions (20, 21) associated with the or each power source (6, 7); and v) using the power control processor (4, 16) to autonomously control the power (p) demanded from the or each power source (6, 7) by: comparing the power demand constraints (10, 15), operating characteristic and power source operating parameters and/or one or more fault conditions (20, 21) with data pre-stored in a storage device; and generating a power demand control signal for the or each power source; such that the overall power demanded from the one or more power sources is within the delegated remit of autonomy and the operation of the machine and/or the or each power source is optimised for the input operating characteristic. comparing the power demand constraints (10, 15), operating characteristic and power source operating parameters and/or one or more fault conditions (20, 21) with data pre-stored in a storage device; and generating a power demand control signal for the or each power source; such that the overall power demanded from the one or more power sources is within the delegated remit of autonomy and the operation of the machine and/or the or each power source is optimised for the input operating characteristic.