Today's Portable Heaters come in various configurations (tower, tabletop, torpedo, wheeled, hand-carried) and in various powering options (electricity, natural gas, propane, diesel, kerosene). Previously rated only by BTUs of output, they're now also rated via the square footage that they can warm. Some portables suited to farmstead shop and machinery-maintenance areas include (1) the forced-air type that uses one of the liquid fuels to fire the a high-output burner and run a high-velocity fan to distribute heat from; (2) the forced-air radiant type, which consists of a radiant heating element along with a fan that circulates warmed air; and (3) the electric forced-air type. which may require a 240-volt circuit to both heat the element and run the fan to distribute the warmed air.