A method of managing the overall power demanded from one or more power sources forming part of a machine, the method comprising: i) setting time-varying power demand constraints for the one or more power sources, wherein the time-varying power demand constraints 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 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 ; iii) inputting the time-varying power demand constraints and the operating characteristic to a power control processor arranged for autonomously regulating the overall power (P) demanded from the one or more power sources , the inputted power demand constraints representing a remit of delegated autonomy for the power control processor ; iv) for the or each power source , assessing one or more power source operating parameters and/or one or more fault conditions associated with the or each power source ; and v) using the power control processor to autonomously control the power (p) demanded from the or each power source by: comparing the power demand constraints , operating characteristic and power source operating parameters and/or one or more fault conditions with data pre-stored in a storage device; and generating a power demand control signal (P) 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 for the one or more power sources, wherein the time-varying power demand constraints 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 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 ; iii) inputting the time-varying power demand constraints and the operating characteristic to a power control processor arranged for autonomously regulating the overall power (P) demanded from the one or more power sources , the inputted power demand constraints representing a remit of delegated autonomy for the power control processor ; iv) for the or each power source , assessing one or more power source operating parameters and/or one or more fault conditions associated with the or each power source ; and v) using the power control processor to autonomously control the power (p) demanded from the or each power source by: comparing the power demand constraints , operating characteristic and power source operating parameters and/or one or more fault conditions with data pre-stored in a storage device; and generating a power demand control signal (P) 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 , operating characteristic and power source operating parameters and/or one or more fault conditions with data pre-stored in a storage device; and generating a power demand control signal (P) 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 . <EOS>