Available space matters when picking a radiator. A double panel radiator gives off more heat than a single panel radiator of the same size. Switch to a double panel option when you need a stronger radiator. For example, a single panel radiator 20 inches (50.8 cm) (about 500 mm) long and 20 inches (50.8 cm) wide puts out 1333 BTUs of heat. A double panel radiator the same size puts out 2597 BTUs of heat. Room greater than 18 feet (5.5 m) (six meters) long on any side are tough to heat. Larger rooms benefit when you make space for two or more radiators. Adding a second radiator splits the heat output needed from one radiator in half. This means you can keep smaller radiators that will maintain an even temperature throughout the room. The radiators should be placed on opposite sides of the room. This way, they heat the room evenly. Iron radiators aren’t in wide production, but some companies still offer them. In addition to their old-school appeal, iron radiators heat and cool slowly. Modern panel radiators tend to be all hot or all cold, which can be uncomfortable. Iron radiators are often found in older homes. Hot water iron radiators give off 170 BTUs per hour per square feet. Steam iron radiators give off 240 BTUs per hour per square feet.

Summary:
Pick a panel style. Get multiple radiators for larger rooms. Get an iron radiator for gradual heating.