MADRID, Dec 17 (Reuters) - Spain"s electricity grid operator REE (REE.MC) said it agreed to sell a 49% stake in its telecom infrastructure unit Reintel to U.S.-based buyout fund KKR (KKR.N) for 971 million euros ($1.10 billion). Reintel operates a network of 50,000 km of fibre optic laid along the electricity grid and railways, REE said late on Thursday. The unit generated 104 million euros of earnings before interest, taxes, depreciation and amortization in 2021, the company said. The sale is part of a strategy to raise funds to finance investments in the country"s electricity grid to connect renewable power plants under construction. ($1 = 0.8822 euros)