Convection heaters come in two varieties: fan-powered and fan-less. The fan-powered heaters tend to warm up the space faster than their fan-less counterparts, but the fans make them noisier. Keep this in mind if you are going to be using the heater in a room designated for studying or sleeping. Regardless, the fan-powered variety is popular and quite powerful, sometimes with BTUs more than 16,000.
For quick heat in a small area, a radiant heater is the most powerful option. These heaters radiate heat rather than forcing heated air into a room, which makes them ideal when you need to provide warmth only to a corner of a room or to a specific object. Mr. Heater brand manufactures a model that puts out 42,000 BTU, although many smaller heaters will put out between 5,000 and 8,000 BTU.
While both convection and radiant heaters can be powerful, they require more electricity to run than oil-filled or propane-powered heaters. Either of these types of space heaters will raise the temperature without raising your electricity bill. Propane heaters can put out heat for up to six hours on only one gallon of propane, though the initial cost can be somewhat higher than more traditional fan-powered heaters, starting at $80 in 2010. The oil-filled heaters will take time to get fired up, but after they are warm, will provide heat without draining electricity and start at about $50 for 5,000 BTU in 2010.