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Squeezing BTUs From Light Bulbs

Incandescent light bulbs create a cogeneration benefit by warming the indoor spaces they illuminate.

Fortnightly Magazine - August 2006

Consider the old-fashioned incandescent light bulb. Many public agencies promote replacement by compact fluorescents that consume less electricity. However, incandescent light bulbs create a cogeneration benefit by warming the indoor spaces they also illuminate; they can be viewed as little resistance heaters attached to a source of light. A consumer willing to use them selectively in winter can save fossil fuels otherwise used for heating, while avoiding the corresponding increase to summer air conditioning demands that year-round use would cause. 1

Compact fluorescents also give off heat, but less, because their wattage is lower for the same light output. As well, some consumers may prefer an incandescent for other reasons, such as the color (or temperature) of a light, how quickly it turns on, whether it can be used with a dimmer switch, whether it will fit an existing lamp or fixture, or whether it has a pleasing appearance. 2

When cogeneration heat is factored in, old-fashioned light bulbs can sometimes be the best choice for cool-season use. The best scenarios for this involve locations where electricity generating costs are low, and where consumers heat with home heating oil or propane. Even where compact fluorescents are more economic, a homeowner in a cold climate may be doing the right thing for himself or the environment by keeping incandescent bulbs due to habit, or for the quality of light. 

It is perhaps odd to speak of one of the oldest available electrical devices in terms of modern energy efficiency. However, it serves as a quirky, if useful, example of how consumers can be better empowered to find efficient adaptations the more they face energy prices based on marginal costs—particularly including time-of-day or seasonal rates for electricity. 

Turning to the Numbers

Table 1 shows a common analysis used to promote bulb changeout. 3

These calculations compare a single 10,000-hour compact fluorescent bulb with the 10, 1,000-hour incandescent bulbs it would replace. At the assumed electricity rate of 10 cents/kWh, the cost advantage of the compact fluorescent bulb is apparent. 4

This analysis can be extended to include the heat output of the bulbs, using the standard relationship as follows: 5

One Watt = 3.414 Btus/hour

Thus, a single 100-W incandescent bulb will produce 100 x 3.414 = 341.4 Btus/hr. By contrast, a single 27-W fluorescent bulb will produce 27 x 3.414 = 92.18 Btus/hr.

Table 2 shows the heat content of three home heating fuels, their typical efficiency in home furnaces or heaters, and their approximate prices as of October 2005. 6,7

This information leads to these costs per 1,000 BTU of home heating: 

Fuel Oil:
$2.65/gallon / 128 MBTU/gallon = 2.07 cents/MBTU. 

Natural Gas: $13.50/MMBTU/ 850 MBTU/MMBTU = 1.59 cents/MBTU. 

$1.94/gallon / 77.6 MBTU/gallon = 2.50 cents/MBTU 

 A consumer’s out-of-pocket heating-cost savings is shown in