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Doing some homework for a customer who wants to install electric baseboard heat. As I am trying to determine how many feet/watts I need. I ran across some guidelines that make sense, except they use the "X" number of watts to square foot ratios.

Why wouldn't it be "X" number of watts to Cubic foot of space? Wouldn't that be more accurate??

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    The simple answer is that nearly all homes have nominal 8 foot ceilings.
    – isherwood
    Commented Nov 24, 2016 at 0:32

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Floor area is used for two reasons:

  1. The temperature of a room results from a balance between the rate at which heat enters and the rate at which it's lost through the walls. The rate of loss, in turn, is proportional to the surface area of the walls, floor, and ceiling, which, for a normal-shaped building, is roughly in proportion to the floor area.
  2. Floor area is easier to measure.
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Doing this properly involves far more than square feet or cubic feet: you need to do a heat loss calculation. Inputs include:

  • Outdoor design temperature (how cold it might get)
  • Surface area of the building envelope, divided into types (wall, door, window, attic, foundation)
  • The U-factor of each type of surface
  • The amount of air leakage through the building envelope

The result will be the amount of heating power needed to keep the inside of the house warm enough at the design temperature. Then, you can throw in the efficiencies of the heating system, and end up with your heating requirements.

This is not to say that you must do a heat loss calculation; back-of-the-envelope techniques can give you a ballpark estimate. Do note, though, that it's incredibly easy to over-estimate your heating needs, which can lead to a more expensive and (for oil or gas) less efficient system.

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    All true, but it doesn't address the specific question posed.
    – isherwood
    Commented Nov 24, 2016 at 0:33
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The guidelines are probably set up by the manufacturer. Since there are so many variables related to heating they are probably just trying to keep it simple.

In fact some types of heating are probably more efficient when used all the time vs. barely used. And really the thermostat can adjust for issues if heat loss/needed is off by 20-30%. Your baseboard heaters could be on an hour a day or 10 hours.

Usually electric heaters are highly inefficient if they are being turned on and off all the time. I would try to install the minimum. Really the biggest concern with setting up heating elements like this are, how much is the air circulating? Meaning if I heat this room, will it bleed over to a surrounding area. Much different having one open space vs. 8 small rooms.

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All the heating needs come from the transfer of heat through the outside surfaces, and not the interior of the room. Hence, as Mark and Dan mentioned in their answers, you need to only find the amount of heat that escapes through the exposed surfaces and add an infiltration factor and "shazam" there is the heat requirement. You will still need to add a pick-up factor to get the correct wattage needed in each area. Go buy a book at your local book store on heat loss calculation.

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  • One thing: you can use any factor such as square feet, cubic feet,etc. and add correction factors for say higher ceilings, slab on grade, heated or unheated basements or whatever as long as you use the information needed to determine the heat loss or the size of the heating unit. Different authors use whatever calculations they are schooled in to determine an outcome such as the size of the heating unit.
    – d.george
    Commented Feb 5, 2017 at 12:50

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