space heater wattage & btu

The capacity of small space heaters (and furnaces, air conditioners and other heating and cooling systems) is measured in BTU or in watts (electric heaters).

Some general references

Typical central furnaces have an input BTU around 60.000 - 100,000 BTU's.

The capacity of kerosene and gas space heaters - 10,000 Btu to 40,000 Btu per hour, in most cases - is also rated in BTU's.

Electric space heaters have a much lower heat output: most of them have a capacity ranging from 1kW to 2kW (1,000 watts to 2,000 watts), that is, in BTU per hour: 3,410 BTU – 6,820 BTU. Many of these electric space heaters have now three or four power heat settings (say, 400 watts, 800 watts and 1,200 watts).

Insulation, air sealing and the size of the heater

A tightly insulated home, properly air sealed, will need space heaters with a smaller wattage and less BTU's. Whenever possible, you should insulate and air seal your home and rooms before buying any heater…

Do not select an oversized space heater. Most space heaters come with a sizing table that can help you to choose the best size.

Watts and BTU
The energy consumption of an electric heater is usually measured in Watts (Watts = Volts x Amperes); 1 Watt= 3.412 BTU's.
BTU stands for “British Thermal Unit”; a BTU is the amount of heat you need to heat one pound of water by 1º F. You should consider the size of your room or area to be heated, and its insulation, to estimate the right size of the heater in BTU's.

Gas heaters size (vented heaters)

In the case of gas vented heaters, there is some broad rules to find out how many BTU's you need for a room or space. Here is one proposed by Rinai (a major manufacturer):

1. First, calculate the volume of the room (by multiplying its square footage by its height);
2. Second, multiply the volume figure by... a) 4, if the insulation of your home or room is poor; b) 3, if it’s average; c) 2, in case of a good insulation.

The resulting value is just a broad calculation and a rough rule of thumb, but it can help to figure out how many BTU's your gas heater should have.

Sizing an Electric Room Heater

If your goal is just heating you while in a stationary position in your living room, then a compact radiant heater with a two heat settings of 400 watts and 800 watts can be a good solution.

But you may have other goals: heating the entire room in a comfortable way, even if it takes longer (in this case, a 1,500 watts oil-filled space convection heater can be a good option) or getting a quick heating of a room with a wall-mounted gas heater (involving, say, 40,000 BTU's per hour or more), etc.

For a good electric heating electricity cost calculator, see: Sust-It

Kerosene heaters BTU's

A kerosene heater that is too large for a space will cause you to switch it off and on repeatedly, with resulting kerosene odor and other "minor" problems.

Here is a common rule of thumb to estimate the size of a kerosene heater: Square footage of the room x 22 (or a factor near this value).

That will give you the number of BTU/hour you need. But that’s just a very broad reference.