Fuel cells are a promising technology for combined heat and power (CHP); however their efficiency and carbon benefits are often overlooked because of their reliance on fossil fuels. This paper sets out the numerical methods and evidence of real-world performance needed to compare fuel cells with other low-carbon technologies. It is demonstrated that firstly, the efficiency of present-day fuel cells is high enough to outperform the best electric heat pumps, even when these are powered exclusively by the best modern power stations. The equivalent COP (coefficient of performance) of today's fuel cells ranges from 5 to 00, compared to 3-4 for the best ground source heat pumps. Secondly, this high efficiency means that even when fuelled with natural gas, the heat from a fuel cell is zero- or even negative-carbon when electricity from central power stations is displaced. (C) 2015 Elsevier Ltd. All rights reserved.