The global energy transition -- the shift from fossil-based fuels such as oil and coal to renewable sources like wind and solar -- is estimated to become a USD 23-trillion market by 2030, US Energy Secretary Jennifer Granholm said on Tuesday as American companies seek closer ties with Indian firms to tap this opportunity. Speaking at the ministerial meeting of the US-India Strategic Clean Energy Partnership (SCEP) here, she said the US will have to add 2,000 gigawatts of clean energy to its electric grid by 2035 to meet the goal of using 100 per cent electricity from clean sources.