A method for determining an electric energy storage system state-of-power value (SOP ESS; SOP ESS (k)), said electric energy storage system comprising at least two battery units (24, 26) electrically connected in parallel to each other and said electric energy storage system state-of-power value being indicative of the total maximum amount of electric load that an electric energy storage system 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 (SOP i; SOP i (k)), for any one of said battery units (24, 26), the method comprising: - for each battery unit (24, 26) in said electric energy storage system, determining said battery unit state-of-power value (SOP i; SOP i (k)), said battery unit state-of-power value (SOP i; SOP i (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 said predetermined future time range without violating electro-thermal limits of said battery unit (24, 26), - for each battery unit in said electric energy storage system, obtaining a battery unit measured load value (L i; L i (k)) indicative of the electric load actually imparted on said battery unit (24, 26) at a certain time instant, - on the basis of the battery unit measured load value for each battery unit in said electric energy storage system, determining a load distribution amongst said battery units of said electric energy storage system, wherein the step of determining said load distribution comprises determining a load sharing factor (δ i; δ i (k)) for each one of said battery units (24, 26), said load sharing factor (δ i; δ i (k)) being indicative of the proportion of the total electric load of said electric energy storage system being imparted on said battery unit (24, 26), and - determining said electric energy storage system state-of-power value on the basis of said battery unit state-of-power values (SOP i; SOP i (k)) and on said load distribution, wherein the step of determining said electric energy storage system state-of-power value (SOP ESS; SOP ESS (k)) comprises, for each battery unit (24, 26) in said electric energy storage system, determining a battery unit product (BUP i; BUP i (k)) by multiplying said battery unit state-of-power value (SOP i; SOP i (k)) and said load sharing factor (δ i; δ i (k)), wherein the step of determining said electric energy storage system state-of-power value (SOP ESS; SOP ESS (k)) comprises using the battery unit product (BUP i; BUP i (k)), among said battery unit products, which is closest to zero for determining the electric energy storage system state-of-power value (SOP ESS; SOP ESS (k)). - for each battery unit (24, 26) in said electric energy storage system, determining said battery unit state-of-power value (SOP i; SOP i (k)), said battery unit state-of-power value (SOP i; SOP i (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 said predetermined future time range without violating electro-thermal limits of said battery unit (24, 26), - for each battery unit in said electric energy storage system, obtaining a battery unit measured load value (L i; L i (k)) indicative of the electric load actually imparted on said battery unit (24, 26) at a certain time instant, - on the basis of the battery unit measured load value for each battery unit in said electric energy storage system, determining a load distribution amongst said battery units of said electric energy storage system, wherein the step of determining said load distribution comprises determining a load sharing factor (δ i; δ i (k)) for each one of said battery units (24, 26), said load sharing factor (δ i; δ i (k)) being indicative of the proportion of the total electric load of said electric energy storage system being imparted on said battery unit (24, 26), and - determining said electric energy storage system state-of-power value on the basis of said battery unit state-of-power values (SOP i; SOP i (k)) and on said load distribution, wherein the step of determining said electric energy storage system state-of-power value (SOP ESS; SOP ESS (k)) comprises, for each battery unit (24, 26) in said electric energy storage system, determining a battery unit product (BUP i; BUP i (k)) by multiplying said battery unit state-of-power value (SOP i; SOP i (k)) and said load sharing factor (δ i; δ i (k)),