A study by the University of Oxford and the Regulatory Assistance Program found that heat pumps are several times more efficient than oil, gas or electric heating. The study, published in the journal Joule, found that heat pumps operate two to three times more efficiently than combustion heating or resistive heating technologies. Heat pumps harness nature's energy and take advantage of temperature differences, with the coefficient of performance (COP) being the main indicator of efficiency. The study also found that even air source heat pumps are suitable for sub-freezing temperatures, making them twice as efficient as electric heating systems.