The author of today's post, Sheryl Canter, is an Online Writer and Editorial Manager at Environmental Defense.

One reason that old-fashioned incandescent light bulbs are such a particularly poor idea in summer is that they put out a lot of heat compared to compact fluorescent light bulbs (CFLs). If you're air conditioning your home, incandescent lights will require your air conditioner to work harder because you're basically heating and cooling the room at the same time.

Some people have translated this into the advice that incandescent light bulbs are good in winter because they supplement the heat to your home, making up for the additional energy they draw. I asked James Wang, Ph.D., a climate scientist at Environmental Defense, to calculate whether this was indeed true. His answer was no. Here's why.

A light bulb - whether incandescent or compact fluorescent - is essentially an electric space heater. This is true even though a portion of the bulb's output is visible light, since the light eventually dissipates to heat. CFLs produce less heat than incandescent bulbs because they use less power to produce a given amount of visible light.

Since a light bulb is essentially a space heater, its CO₂ emissions are comparable to that of an electric heating system. By comparing the CO₂ emissions from home heating systems that use electricity, natural gas, and heating oil, we can know whether supplementing heat with incandescent light bulbs is a good idea.

Electricity can come from many sources, some dirtier than others. The national average for electricity generation is 1.34 lbs. of CO₂/kWh (kilowatt hour). One kWh equals 3413 BTUs (British Thermal Units), so that's 393 lbs. of CO₂/million BTUs. About 7 percent of energy is lost during the transmission of electricity from power plants to homes, and for electric space heaters, the efficiency of conversion to heat is 97-99 percent. So total emissions are 424 to 433 lbs. of CO₂/million BTUs, given the usual mix of electricity sources in this country. If all electricity were produced from renewable energy, emissions would be close to zero, but that's not the case.

Natural gas emits 117 lbs. of CO₂/million BTUs. The efficiency of natural gas home heating systems is 78 to 97 percent, so total emissions are 121 to 150 lbs. of CO₂/million BTUs. (This omits the small amount of methane that leaks out of natural gas distribution systems.)

Heating oil emits 161 lbs. of CO₂/million BTUs. The efficiency of oil home heating systems is 80 to 89 percent, so total emissions are 181 to 201 lbs. of CO₂/million BTUs.

As you can see, electric heat (light bulbs) is the most CO₂- intensive heat source. The inefficiency stems mostly from the energy wasted in converting fossil fuel energy into electricity and then to heat, versus converting fossil fuel directly into heat.

So the next time someone tells you that incandescent light bulbs are okay for winter use, set him straight!