The present disclosure relates to a method for determining an electric energy storage system state-of-power value (SOPESS; SOPESS(k)), the electric energy storage system state-of-power value being indicative of the total maximum amount of electric load that an electric energy storage system (18) can deliver or receive at a constant load level during a predetermined future time range without violating a state-of-power, defined by a state-of-power value (SOPi; SOPi(k)), for any one of said battery units (24, 26), the electric energy storage system comprising at least two battery units (24, 26) electrically connected in parallel to each other, the method comprising: - for each battery unit (24, 26) in the electric energy storage system (18), determining the battery unit state-of-power value (SOPi; SOPi(k)), the battery unit state-of-power value (SOPi; SOPi(k)) being indicative of the maximum amount of electric load that the battery unit (24, 26) can deliver or receive at a constant load level during the predetermined future time range without violating electro-thermal limits of the battery unit (24, 26), - for each battery unit in the electric energy storage system, obtaining a battery unit measured load value (Li; Li(k)) indicative of the electric load actually imparted on the battery unit (24, 26) at a certain time instant, - on the basis of the battery unit measured load value for each battery unit in the electric energy storage system, determining a load distribution amongst the battery units of the electric energy storage system, and - determining the electric energy storage system state-of-power value on the basis of the battery unit state-of-power values (SOPi; SOPi(k)) and on the load distribution.