 However, in other regions, such as the Upper Midwest, charging during the recommended hours of midnight to 4 a.m. implies that PEVs generate more emissions per mile than the average car currently on the road.The results show a fundamental tension between electricity load management and environmental goals as the hours when electricity is the least expensive to produce tend to be the hours with the greatest emissions.This occurs because coal-fired units, which have higher emission rates, are most commonly used to meet base-level and
off-peak electricity demand; while natural gas units, which have relatively low emissions rates, are often brought online to meet peak demand.