All wall heaters are sized according to wattage, and when choosing one you want to match the heat output of the heater to the type of room the heater will be used in. To properly size a wall heater for efficient operation, you need to consider both the size of the room and the amount of heat loss that you will be fighting in order to keep that room comfortably warm.

First, determine the square footage of the room. Simply measure the length and width of the room, and multiply the two numbers. For example, if your room measures 10 feet by 14 feet, it would be 140 square feet.

Next, look at the general insulation levels of the room. Insulation levels in newer houses are typically quite good – R-19 in the walls, R-38 in the ceiling, and R-25 in the floor; windows have double pane glass; and doors and windows have good weather stripping and caulking. Houses built between the mid-1960s and the mid-1980s that have not been weatherized will typically have lower amounts of insulation – R-11 in the walls and R-19 in the attic, with no floor insulation – and windows that are not as energy efficient. And for older houses that were built prior to the mid-1960s, insulation might be very light or even non-existent, with single-pane windows and little in the way of protection against air infiltration.

Now, armed with the size of the room and the relative level of energy efficiency, you can make some choices regarding heater sizes. A typical rule of thumb is to allow 10 watts of heat per square foot if the room has high levels of insulation and generally good energy efficiency; 12 watts per square foot for rooms with more moderate efficiency; and 15 watts per square foot for those older, colder rooms.

So, for a 140-square-foot room with moderate energy efficiency, you would want a heater that's capable of putting out approximately 1,680 watts (140 x 12). Chances are you won't find a heater with exactly that wattage, so always choose the next highest one – in this case, it would probably be either 1,750 or 2,000 watts.

These examples assume a room with a standard 8-foot ceiling and an average amount of glass in it. Of course, not all rooms are created equal, so there are a few other sizing adjustments you may wish to make as well: