Wednesday, February 15, 2012

Light bulbs and implied discount rates

What does your choice in light bulbs say about your attitude toward the future? In terms of discount rates, quite a lot.

An incandescent 60W bulb costs around 50 cents to buy, while a 15W CFL costs about $9. The government estimates that an incandescent bulb will cost $4.80 in electricity annually, while the CFL will cost $1.20 if used an equal amount.

Assuming a five year time span - roughly how long a CFL bulb is expected to last, and during which a new incandescent bulb will have to be purchased every year - if you just add up the costs, the incandescent will cost $12 more. Why would anyone buy an incandescent bulb? The answer is time preference.

Plugging the numbers above into Excel and using "Goal Seek" finds an implied discount rate of 39%. That is, someone would have to value one dollar a year from now 39 cents less than a dollar today in order to be indifferent between an incandescent and CFL light bulb.

People discount the future when making decisions, and the discount rate is not always consistent between all activities. Few people would want to pay a 39% rate on a credit card, but some are willing to do the equivalent when the cost is on the electric bill instead of the credit report.

There are a number of other potential explanations: maybe some renters don't expect to stay a full five years or have electricity included with the rent; some consumers might be cash-constrained and can't afford the pricey bulbs; or there could be some cognitive bias or plain lack of information about electricity costs. But, given the plenitude of "green" or energy conservation campaigns and general worry about global warming, behavioral factors might also push in the other direction.

If varying discount rates are distributed throughout the population, there may be enough people in the "tail" - with extremely high discount rates - to keep incandescent bulbs on the shelves for some time to come.

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