Tyler Nienhouse/CC BY-SA 2.0

Incandescent light bulbs may be on the way out around the world, but every now and then we hear from commenters bemoaning their demise and arguing for their superiority. While the "well, they give nicer light" argument has always left me a little cold, another defense of these inefficient illuminators has given me pause for thought:

Isn't the heat they produce useful?

The argument stems from one of the main criticisms of incandescents—that as much as 90% of the electricity they use goes to producing heat, not light. But, say the defenders of the bulb, if the heat from the bulb warms the house and displaces the energy used by actual heating systems, then it's not really wasted, is it?

On the face of it, it makes some sense. In fact we've seen some interesting designs for capturing and reusing that waste heat. But once you pause for a minute, the argument starts to fall flat. Here's why:

1. They are not efficient heaters. Incandescent bulbs are essentially electric resistance heaters. And because of the inefficiencies of producing electricity and transmission losses, even dedicated electric resistance heaters are far less efficient than using natural gas, propane or an air-source heat pump.

2. They are not positioned where heaters should be. Heat travels upwards. And yet many, if not most, light bulbs are hanging from the ceiling. You wouldn't put a space heater up by your roof, so the idea that an incandescent bulb is providing an effective replacement to your heating is a little shaky too,

3. When You Need Light, You Don't Always Need Heat. This is probably the biggest argument against the case for "bulbs as heaters". Because in most parts of the country, for the majority of the year, we don't need heat at the same time as we need light. In fact, we often don't just not need it, but we actively pay to remove it from our homes. So in the summer time in the South, you are not just paying for your inefficient lighting and the heat it produces—but then you are also paying to power your HVAC to remove that heat. That can't be right.

As always, of course, there are some exceptions that may just prove the rule. Most notably, as Paul Wheaton demonstrated in his excellent video on heating the person, not the house, task lighting using an incandescent bulb and a shade/reflector can act as a useful heat lamp, providing heat exactly where it is needed and not warming up the surrounding air. In fact, it's something I'm considering deploying in my own efforts to heat my home office efficiently. And because folks who are utilizing this type of heating are likely to be at the hardcore end of the green/energy efficiency spectrum, the chances are they'll be using this at night or in the winter when light and heat are both in high demand.

There is a huge difference between waste and byproduct. And it is true to say that heat from a bulb "could" be used to offset other energy use. But "could" is not the same as "will". More often than not, waste heat is just that. A waste.

Sorry folks, incandescents still suck.