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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

Table 3, for a 100-W incandescent bulb or its compact fluorescent equivalent. 

Stated another way, for a kilowatt-hour of electricity consumed by incandescent lights as compared with fluorescent equivalents, the net heating fuel savings is 5.2 cents for fuel oil, 4.0 cents for natural gas, and 6.2 cents for propane. These heating savings figures may be more impressive when compared with the current national average retail electricity rate of 7.95 cents/kWh. 8

Because the DOE cost-justification analysis considered a 10,000-hour cumulative comparison, its results would be modified as shown in Table 4, if all bulb usage occurred at times when heating was needed. 

Here, the compact fluorescent bulb is still more economical for oil and gas heat, while the two bulb types have equivalent total costs compared with propane heating. Present-value discounting (at 10 percent) modifies these figures slightly more, giving light bulbs a slight advantage over propane heat (see Table 5)

Thus, a seasonal analysis yields considerably different answers for cold-region customers who heat with propane or fuel oil. For them, light-bulb heat can serve as a potentially efficient partial substitute. Note that this analysis does not merely trade off energy use versus the higher initial cost of an energy-saving device. It also involves potentially significant savings in fossil fuel used for home heating. 

Additional Scenarios and the Social Perspective

We can move to other scenarios (beyond the above numbers) to see whether light-bulb heat could ever be more than a curiosity. To that end, we consider a consumer who might change out types of bulbs seasonally, as is done with snow tires, or perhaps turn on different lamps (with different bulbs) in winter versus summer. 9 We also replace the DOE-assumed 10-cent electricity rate with the residential retail average of 9.3 cents. Assuming an illustrative six month heating season produces the results in Table 6. 10 

Once again, while compact fluorescents come out slightly ahead when compared with two of the three fuels, it may be a reasonable choice for an oil or propane heat consumer to use incandescent bulbs when heating is needed. 

Other modifications also are possible. Reducing the assumed purchase price of compact fluorescent bulbs (from the $14 in the DOE analysis) does improve their economics. 11 For example, at a $5 bulb purchase price, the present value of using compact fluorescents all year improves by $5.59 for each fuel, making bulb-switching uneconomic in each case. 

Finally, the retail price of electricity typically contains costs that are not avoided if electricity use is cut (such as for distribution poles and wires). For a societal analysis, the better comparison is to marginal generation and transmission costs of the electricity that is consumed (or saved) by consumer choices about which lights to use. 

Of course, the marginal cost of electricity generation will vary depending on the seasonal generation mix in a region, the availability of transmission, current system demand, and other factors. For example, wholesale prices in the Pacific Northwest traditionally have been lower when water flows permit substantial hydroelectric generation. If low prices occur when heating demand