ATHENS (Reuters) - Greece's Helleniq Energy stuck to its plan to install more renewable capacity in the coming months but said on Thursday it was concerned about restrictions related to power grids and a lack of energy storage.

Helleniq, Greece's biggest oil refiner, operates solar and wind parks with a total capacity of 381 megawatts with plans to increase that to 1 gigawatt within the next 18 months, Chief Executive Officer Andreas Shiamishis said when commenting on first quarter results.

"But (we) remain cautious, as challenges related to grid constraints and storage technologies remain unresolved," he added.

Tapping its abundant solar and wind potential, Greece has been investing heavily in green energy projects in recent years but progress has outpaced much-needed upgrades of its power grid and the building of facilities to store renewable energy.

As many European grids are decades old and need adapting from generation by large, fossil fuel power plants to wind and solar power generation, Europe will need to invest 584 billion euros ($634.63 billion) in upgrades this decade, according to European Union estimates.

In Greece, power transmission system operator IPTO has said it is connecting new renewable energy stations at the fastest possible pace and the electricity grid's capacity will reach 29 gigawatt by 2030 from 18 gigawatt now through its decade-long investment plan of 5 billion euros.

($1 = 0.9202 euros)

(Reporting by Angeliki Koutantou; Editing by Kirsten Donovan)