If plug-in hybrid vehicles proliferate as expected, utilities will be able to handle the added power demand without building new plants or straining transmission power grids as long as owners plug in overnight, the New York grid manager said in a report issued on Monday.

If New York motorists start plugging in significant numbers of plug-in hybrid vehicles (PHEVs), we will see new demands on the grid, said Stephen Whitley, president of the New York Independent System Operator.

Fully electric vehicles and PHEVs are expected to increase power demand in New York state, the area covered by NYISO, some 2 percent by 2030, the NYISO report showed.

However, if deployed with technology and incentives to encourage favorable charging patterns, PHEVs can offer valuable new ways to store electricity produced in off-peak periods, Whitley said of his staff's report. That energy storage potential could enhance the grid's use of wind power.

PHEVs are expected to be rolled out to consumers in significant numbers in the next few years. President Barack Obama has called for a million plug-in hybrids on U.S. roads by 2015.

The report assumes 1.5 million plug-in hybrid sales by 2016 and 50 million by 2030, including 2.5 million in New York.

Plug-in hybrids will account for 25 percent of all U.S. automobile sales by 2020, according to two studies, one by the electric industry group Electric Power Research Institute and the environmental group National Resources Defense Council, and the other by the U.S. Energy Department's Oak Ridge National Laboratory

The NYISO report supported the long-held notion of wind power advocates, who say the renewable, no-emissions power source works well with the coming plug-in hybrids.

Winds in most areas are higher at night.

If vehicle batteries are charged during high-demand daytime hours, particularly in the summer, it could strain the grid and cause the need for costly new power plants, the report showed.

Rate design to encourage off-peak charging, coupled with time-of-use rates, and smart grid/advanced metering initiatives, would facilitate favorable charging behavior, it said.

Automakers and utilities say it will be the second generation of plug-in cars that will take advantage of the smart use of power -- two-way communication to stem power use when it is costliest and most stressful for the grid.

A plug-in hybrid will not emit carbon dioxide when running on its electric motor, but there is concern that they will indirectly increase emissions from power plants that burn fossil fuels like coal and natural gas.

The NYISO paper says that if the PHEVs are recharged overnight, they can run on wind power, lessening the need for increased generation from fossil fuel power plants.

Plug-in hybrids connect to conventional electric outlets to charge batteries, which power an electric motor, but the autos also rely on a gasoline internal combustion engine.

The cost of electricity is about one-third to one-fourth the cost of gasoline to drive the same car the same distance.