JOHANNESBURG, Jan 15 (Reuters) - South Africa"s state power provider Eskom will from Monday carry out a staggered shutdown of both units of its Koeberg nuclear power station for scheduled refuelling and maintenance, putting an already overburdened power system under additional strain. In a statement, Eskom said it would take Unit 2 offline from Monday for five months, then do the same with Unit 1 for a similar duration afterwards. Each unit generates 920 megawatts (MW). Eskom"s ailing fleet of power stations, which are mostly run on coal, struggles to meet national demand, and Africa"s most industrialised nation has suffered power blackouts for years, a situation which the maintenance work could exacerbate. "The extended unavailability of the units due to the planned long outages does mean the electricity supply system may be under additional strain during the coming year," the statement said. At the beginning of last year, Eskom had to implement scheduled power cuts because of a maintenance issue at Koeberg, prompted by a rising leak rate in one of its steam generators. read more