SCHEMA:
CREATE TABLE table_24620684_2 (vehicle VARCHAR, clean_electric_grid_california__san_francisco_ VARCHAR)
USER:
When 100 g/mi (62 g/km) is the clean electric grid california (san francisco) how many vehicles are there?

QUERY:
SELECT COUNT(vehicle) FROM table_24620684_2 WHERE clean_electric_grid_california__san_francisco_ = "100 g/mi (62 g/km)"