After reaching a high point of over three million barrels per day (bbl/d) in the late 1970s, demand for residual fuel oil in the United States has steadily declined (product supplied as seen in the chart above is a proxy for demand). Residual fuel is used as fuel for large ships and for electricity generation, industrial process and space heating, and other industrial purposes. Between 2000 and 2010, average annual residual fuel use fell from approximately 900,000 bbl/d to 500,000 bbl/d. It averaged nearly three times that in the 1940s and 1950s. As its name implies, residual fuel oil is the remaining fraction resulting from the crude oil refining process. Because residual fuel is a heavy product, it has limited uses and relatively high emissions.