LONDON (Reuters) - Many wind turbines mounted on homes in British cities are contributing to global warming, not fighting it, according to a new study.

And although many environmentally-friendly homeowners also hope to cut their bills by generating their own power, most micro-turbines will never save as much money as the equipment costs, according to the study by the Building Research Establishment Trust.

“In large urban areas such as Manchester, even with very favorable assumptions about efficiency, lifetime and maintenance, micro-wind turbines may never pay back their carbon emissions,” the report says.

“Even in the most favorable location considered in the study, there is no financial payback within the expected life of the systems, with the current system and electricity costs.”

The study analyzed the likely performance of three of the most common household wind turbines in Manchester and Portsmouth in England and Wick in Scotland.

In many cases -- and across most of Manchester -- more climate-warming carbon dioxide is produced in the manufacture, installation and maintenance of the turbines than they save by generating “green” power over their expected lifetime.

“These studies have shown a large variation in the expected CO2 payback periods from a few months in good locations to situations where they never pay back, in poor locations,” the report says.

Only those climate-conscious homeowners in the best locations in the two smaller cities studied can expect to save more carbon dioxide than their turbines are responsible for producing.