The idling of heavy-duty trucks results in additional emissions, fuel consumption, and cost. Small fuel cell auxiliary power units (APUs) (now in development) are promising alternatives to idling the main engine. A solid oxide fuel cell (SOFC) APU is particularly attractive, because in conjunction with a reformer, it could operate on widely available diesel fuel. Because fuel cell APUs may not only reduce environmental impacts, but also reduce operating costs, this application has been cited as an attractive early market niche for fuel cells. Our objective is to determine whether SOFC APUs are likely to soon be economically feasible for those trucks that idle most and what energy and environmental benefits are probable. We estimate the APU market size as a function of APU costs by applying Monte Carlo sampling and net present value (NPV) economic analysis to our ADVISOR-based vehicle fuel consumption model. Emissions and fuel economy benefits estimates are then presented as a function of varying market penetration levels, assuming installation on only new vehicles and removal at the end of the 4-year SOFC lifecycle. With modest economic incentives from government and continuing fuel cell technology improvements, we find that SOFC APUs might be economically feasible for up to 15% of the long-haul truck population (60,000 trucks) in the United States in the 2015 time frame, resulting in a 30% reduction of heavy-duty truck fuel use during overnight idling and a 40% reduction in oxides of nitrogen (NO,) produced during idling.