The city owns a total of roughly 11,000 sedans and sport utility vehicles, about half of which are used primarily for emergency purposes by the Fire Department and the Police Department. The electric vehicles — a majority of which are expected to operate entirely without fuel — would make up about half of the city’s nonemergency fleet, officials said. The program is expected to require a capital investment of between $50 million and $80 million over 10 years, to support the creation of additional charging capacity for the vehicles. Amy Spitalnick, a spokeswoman for Mr. de Blasio, said fuel savings would help offset additional costs, such as the higher purchase price for electric cars.