Lightbulb Energy Savings, part II

Posted: September 8, 2011 in analysis

In part I of this article, we discussed whether or not high efficiency lightbulbs would cause sufficiently higher heating bills to offset their energy savings, and determined that, for one case of a St. Paul, MN suburban homeowner, they would not.

There are some more areas to explore about this, however.

LED Lighting

But what if my friend is concerned about warmup time, light quality, or the dangerous substances in CFLs, and decided to light his house with LEDs instead?

The electrical bill drops even more dramatically than before, of course, though the numbers are getting so small that the additional 50% efficiency doesn’t translate into a large absolute change.  Lighting costs $11.34 in electricity.  If he buys decent quality bulbs, each will be about $42, but they last so long that he’ll need to replace just over half a bulb per year, on average, costing $23.50 per year.

The heat given off by LEDs is negligible for home heating purposes, so all 867.2 therms of heating must come from the furnace at $0.65 per therm, raising the heating gas bill to $563.68, for an average total of $598.52.  The energy efficiency of LEDs does not yet offset their higher cost, and I have been hand waving the start up cost (replacing every fixture at once) by giving an extended average over many years.  For the first two examples, this was not significant, but at $42 a bulb, it could take 20+ years to actually see the lighting savings (and in practice, it would be that long before any individual bulb would wear out).

But, the problem with LEDs is the cost of the bulbs, which is still quite high, and not the missing heating.  Cheaper LED bulbs cost less than half as much, but have poor reviews for light quality and sometimes can hum or buzz, according to reviews.

Thermodynamics and Other Places

Now for my logical thermodynamic argument that efficient lighting must provide financial savings.  Incandescent light bulbs are essentially electric heaters that give off waste light.  If it was actually cheaper to heat a house with electric heaters in the form of light bulbs than with a gas furnace, then it would also be cheaper to heat a house with electric heaters in the form of actual electric heaters than a gas furnace, and anyone in that situation should turn off their gas and go all electric.  Those people also save money from CFLs, since all their heat costs the same and the CFLs benefit them strictly with cheaper replacement costs.

This has been a very long and complicated bit of math which basically boils down to two numbers: $0.09 per Kwh for electricity and $0.65 per therm natural gas.  Converting to the same units, natural gas is 4 times as efficient in energy delivery by cost than electricity in St. Paul.  Is this a national anomaly?  I won’t redo all the calculations for other areas, but let’s do a quick survey of the efficiency ratio in other major metro areas to makes sure that St. Paul isn’t an aberration in energy prices.  In Chicago, gas is 5 times more price efficient.  In San Francisco, it is 5.2 times better.  In NYC, it is 4.3 times better.

But what about rural areas?  This gets a bit more complicated, since when there aren’t gas mains you need to get propane trucked in, and the fixed costs of this operation vary widely depending on your arrangement with the service (who owns the tank, whether you are charged per trip, and so forth).  But, we’re discussing changing the lighting of someone who has that all arranged already, so all that matters is the marginal cost of a gallon of propane, converted into heating energy.  In rural North Dakota (near Crosby), the ratio is only 1.2, meaning that burning extra propane to make up for the loss of waste electric heating will be almost (but not quite) as expensive as the electrical savings.  However, in rural Maine, the ratio is 5.9.  I cannot find anywhere in the US where the ratio is below 1, and electricity is actually cheaper than gas.  Such a place might exist, but I haven’t seen it.


In my analysis thus far, I have held to the theory “a therm is a therm is a therm”.  That is to say that it doesn’t matter where or how the heat is being input into a system, it affects the system in the same way.  This is not strictly the case, as I am aware and have been reminded by another friend of mine, who actually configures energy-efficient heating and lighting systems professionally.  The issues here get very complicated very quickly.  Central heating is affected by ducting placement and insulation, and waste heating from lighting is affected by lighting height.  The sense I get from this friend, however, is that this factor weighs in favor of central heating, as lighting heat efficiency is most often hurt negatively by the lights being near the ceiling.

I have also taken a few liberties with the startup and replacement costs for the lighting equipment.  As I mentioned in the LED section, I am treating an annual replacement cost based on usage, but to change light bulb types in a home, every bulb would be swapped simultaneously regardless of usage.  Over time, the cost would average out to be equivalent, but that might take a while.  I am also only factoring in usage during the heating season itself, and light bulbs are of course used and replaced during the summer months as well.  I think, however, it is best to limit the calculations to wear on lighting caused during the time frame under consideration.


There are some valid concerns about the widespread adoption of CFLs in the US, but higher home heating bills is not one of them.  In every scenario, in every region of the country, in every circumstance, overall energy bills come out cheaper when replacing incandescents with CFLs.  In nearly every case this difference dramatically pays for the cost of the CFLs themselves.  Recall, also, that there is a significant saving on cooling energy in the summer (which, per degree cooled, is far less efficient than a degree heated) which CFLs help with and none of these calculations consider.  CFLs also save time in less frequent light bulb changes  (My friend theoretically spends 2 hours a winter changing incandescents, vs. just a few minutes for CFLs and no time at all for LEDs) are less of a fire hazard, and have other intangible benefits that help to offset some of the issues with the technology.  These tradeoffs can and should be discussed, but the energy argument is almost certainly invalid.


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