UPDATE 1-EDF to invest 8 billion euros in power storage

* EDF says storage key to balance intermittent renewables

Investment to focus on grids, retail solar, Africa offgrid (Adds CEO quotes)

By Geert De Clercq and Benjamin Mallet

PARIS, March 27 (Reuters) - French state-owned utility EDF plans to invest 8 billion euros ($9.94 billion) between 2018 and 2035 to become a European market leader in electricity storage, the company said on Tuesday.

EDF said in a statement it will develop 10 gigawatts of additional storage capacity worldwide by 2035, in addition to the 5 GW it already operates.

EDF, which already operates pumped storage hydropower plants and industrial-size power storage batteries, added that it aimed to become a French and European market leader, with an offer of storage batteries for clients in the retail power sector.

It also aims to develop a portfolio of about 1.2 million off-grid clients in Africa by 2035 through local partnerships.

"With storage we can smooth out the intermittence of renewable energy and guarantee the performance and balance of the grids," EDF chief executive Jean-Bernard Levy told reporters.

EDF's investment in power storage will be focused on boosting the resilience of the power system, on individual storage for retail clients with solar panels, and on off-grid solar plus storage systems in Africa.

Working with local partners, EDF aims to develop a portfolio of 1.2 million off-grid clients in Africa by 2035.

Asked for a detailed breakdown on how and when EDF will spread the planned eight billion euro investment, Levy said the company did not yet have a detailed plan.

Levy said the company will double its research and development into power storage to 70 million euros for the 2018-2020 period.

In the coming two years, the group's 'EDF New Business' unit also plans to invest some 15 million euros - about a third of its investments - in projects and startups linked to power storage and flexibility.

EDF's storage plan is the second major investment project in the energy transition towards more renewables.

In December, EDF said it would invest about 25 billion euros to build 30 gigawatt (GW) of solar capacity from 2020 to 2035, mainly funded by equity partners, project financing and loans. ($1 = 0.8046 euros) (Reporting by Geert De Clercq; Editing by Sudip Kar-Gupta)