Plug-in hybrid electric vehicles (PHEVs) can run in all-electric mode with grid-supplied electricity or in hybrid electric mode with liquid fuels. We use 1999 California electricity market data to show that millions of PHEVs could charge economically during both peak and off-peak hours with real- time electricity pricing and modest gasoline prices. However, the present value of fuel cost savings at current prices is probably smaller than the magnitude of potential marginal vehicle costs. We simulate the effects of large PHEV fleets on the system load curve under three charging scenarios and find that 1 million compact car PHEVs would not significantly affect the system peak. Larger fleets could require the expansion of system capacity if not charged during the hours of lowest demand. Our forecasts of possible PHEV adoption suggest that only in the most aggressive transition scenarios would there be several million PHEVs in California within a decade.