* Network agency says grid firms must prepare
* Some 1,000 MW of reserve power should be on standby
* Worries about possible supply gap later this decade
By Tom Käckenhoff
BONN, Germany, May 4 (Reuters) - Germany's power grid faces a difficult job to prevent blackouts next winter, when demand is high and closure of nuclear plants still constrains supply, the country's new energy network regulator said on Friday.
"The situation remains tight ... I do not envisage that the coming winter will be much different from the previous one in this regard," Jochen Homann, head of the regulatory authority called Bundesnetzagentur, told reporters in Bonn at the release of the agency's annual report.
The country switched off 40 percent of its nuclear capacity a year ago in the wake of Japan's Fukushima disaster, risking strains to the system's performance in providing round-the-clock power.
The grid, as a result, had to cope with three tricky situations last winter, when spot power supply fell short of levels needed to match consumption.
The Bundesnetzagentur oversees flows on energy grids, but transmission grid firms are responsible for ensuring them.
Homann, a former economy deputy minister who took the helm of the watchdog in March, said some 1,000 megawatts (MW) of reserve capacity needed to be accessible in the 2012/13 winter to ensure that any supply glitches could be covered.
Looking beyond next winter, when the effect of the nuclear shutdowns should be most acute, policymakers and energy firms are discussing ways of meeting German demand in the remainder of the decade as more nuclear plants go offline.
Bundesnetzagentur estimates that the power capacity situation will be balanced up to 2014, when 12 gigawatts (GW) of closures should be matched by newly constructed plants.
Further ahead, between 15 and 16 GW of additional capacity needs to be built, its data shows.
A high-level meeting at the Berlin Chancellery on Wednesday highlighted concerns about a looming capacity gap.
Homann said his agency recommended no further phase-outs on top of those planned.
The stalling of a new hard-coal-fired power station at E.ON's Datteln site should provide a reason to keep old capacity alive there, rather than stick to old plans to shut it down, Homann said.
He was also concerned about sluggish construction of transmission lines, which means that power from new renewable power facilities might not be transported quickly enough to consumption centres.
Out of 1,834 kilometres of transmission lines deemed necessary, only 214 km have materialised and a mere 100 km are operating, he said.
Additional expansion plans that take into account the nuclear withdrawal schedules are due to be presented in early June. (Reporting by Tom Kaeckenhoff, writing by Vera Eckert, editing by Jane Baird)