When 100 g/mi (62 g/km) is the clean electric grid california (san francisco) how many vehicles are there?
CREATE TABLE "table1_24620684_2" (
  "vehicle" text,
  "operating_mode" text,
  "epa_rated_all_electric_range" text,
  "epa_rated_combined_fuel_economy" text,
  "clean_electric_grid_california_san_francisco" text,
  "u_s_national_average_electric_mix" text,
  "dirty_electric_grid_rocky_mountains_denver" text
);
SELECT COUNT("vehicle") FROM "table1_24620684_2" WHERE "clean_electric_grid_california_san_francisco"='100 g/mi (62 g/km)';