Electric cars could cost the same as gas-burning vehicles by 2024, a year earlier than previously anticipated, because of falling battery prices, according to a new Bloomberg NEF survey.

Prices for electric-vehicle battery packs could drop below $100 per kilowatt hour by then, said James Frith, a BNEF analyst who analyzed data from across the lithium-ion battery value chain. At that price, many electric cars are expected roughly to equal gasoline-powered models—though it won’t be true for every electric car...