FACTBOX - German electricity transmission grids

July 25 (Reuters) - Vattenfall Europe [VATN.UL] is the latest big German power company to propose selling its high voltage grid to help unify the country's 1.7 million kilometre transmission networks amid tight regulation.

Policymakers have put pressure on dominant utilities to give new players access to former monopoly markets to help scale back soaring energy prices and increase pan-European integration.

Splitting grids for power transport from generation plants in Europe's biggest and most centrally-located country with a 60 billion euro ($94.21 billion) power market is central to these plans to create more equality.

- Vattenfall's German high voltage grid of 380/220 kilovolt is one of four such geographically defined grids. The others are owned by RWE (RWEG.DE), E.ON and EnBW. Other countries have normally just one grid company, often state-owned.

- There are hundreds more German energy companies operating medium and low voltage grids to reach end consumers, in which the big players have many shareholdings.

- Costs to overhaul the grids are rising to absorb an increasing amount of volatile wind power supply but earnings have been squeezed as the energy regulator has enforced cuts to charges made by the operators to customers.

- But the regulator earlier in July revealed decisions allowing power utilities higher returns than previously planned from next year in recognition of rising costs and to keep the infrastructure attractive for alternative investors:

Power providers may earn a return of 7.56 percent before taxes on their existing networks, up from 6.5 percent this year.

But owners of existing gas grids will see their allowed charges drop to 7.56 percent from 7.8 percent.

For new grids, power and gas operators from next year will be allowed to earn a return of 9.29 percent on invested capital.

- An average private household pays 4 percent out of its power bill to account for charges from high voltage grids.

- European power grids are meant to be unified under Brussels plans for a single energy market. This will require big future investments to transport power over long distances from production sites to centres of high consumption.

(reporting by Vera Eckert)