A recent comment on popular personal finance blog, Bad Money Advice, got me thinking about the many seemingly sensible things that are nonetheless false. This particular comment advises using incandescent bulbs in winter to save on your heating bill - by increasing your electricity bill.

Incandescent bulbs are not efficient at lighting, but they are very very efficient heaters. And since they heat surfaces instead of the air, they are actually more efficient than many other kinds of heaters.

Every been skiing when its 20 degrees, but you feel warm because the sun in shining brightly? Its a bit like that…

CFLs are fine in the summer… but in the winter replace them with incandescent bulbs to reduce overall electricity consumption.

The premise - that using compact fluorescent lights that efficiently turn electricity into light will increase your heating bill during cold weather - is probably on some level right. We are reducing the total heat generated through non-furnace means in the house. But the question here is whether the increased costs in heating outweigh the decreased costs in electricity.

Efficiency

Bex contends that incandescent lights are very efficient heaters. I suppose from an engineering perspective, they are...if viewed as a heater, an incandescent light bulb is about 90-95% efficient. That's pretty good, about the same as a high efficiency furnace. What they're piss-poor at is distributing that heat. This is hardly shocking - the main driver of furnace inefficiency is the heat exchanger. Turning gas or oil into heat is extremely efficient even in crummy old furnaces, so long as their air supply isn't clogged...the difficulty has always been getting that heat out of the exhaust and into clean, breathable air you want to circulate in your house.

With lightbulbs, well, touch a light bulb and you'll get burned, but a foot away you are unlikely to notice much effect. Most lighbulbs are near the the ceiling, and since the hotter air surrounding the bulb will rise, the benefits will not be felt at standing or sitting levels.

Controlled Operation

Even in cold months, we don't always need to be adding more heat to our houses all the time. A furnace, controlled by a thermostat, turns on and off as we need more heat. Lights are turned on and off as we need more light. Not being the same, this gives us an excess of heat during the early evening - when we are generally active, cooking dinner, cleaning, generally running around (and using many lights), while the furnace will still need to work just as hard in the later evening when we are on the couch or in bed, using only a few lights.

Thermodynamics

You pay for energy. In most places, people heat their homes using either heating oil or natural gas. We do this because it's more cost effective to directly burn fuel for heat than the change it into electricity first. The reason for this is waste. Every time energy changes for, some is wasted. Electric heating has been converted several times...the process looks a bit like this: fuel->heat->electricity->heat...there's also some line loss in its electricity stage. The lost energy still has to be paid for, so the retail price of electricity is higher than the retail cost of fossil fuels.

Two Sets of Bulbs?

Bex suggests switching your light bulbs twice a year, to take advantage of the heat benefits in winter without wasting electricity in summer. There's some marginal time cost to that, plus the hassle of finding storage space for all those extra bulbs. Not deal breakers if there were real savings to be had, but since there's not...

"you feel warm because the sun in shining brightly? Its a bit like that…"

WTF. This doesn't even make sense. The sun radiates a lot of heat. So much so, that it warms up things that it shines on. Incandescent bulbs radiate very little heat, mostly they heat objects in their immediate vicinity, so you don't feel warmer standing under them. Perhaps bex is thinking of heat lamps which provide more of their light in the infrared spectrum - radiant heat.

End Result

Your furnace is intended to heat your house. It's designed for it, it's good at it. Light bulbs are intended to provide lighting, and so they are not particularly efficient as heaters. Switching to CFLs, so that you're lights do a better job at lighting and a not-noticeably-worse job of heating will reduce your energy consumption. Not just move it from one bill to another. Less total energy consumed means less money spent.

If light bulbs were the best way to heat your house, basic economic principles tell us that houses would be designed with banks of lightbulbs instead of a furnace. They're not. People have done the math and the furnace is a better heater. Use it.

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