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Comment by greenyoda | Hacker News Reader

But if you're using both the light and the heat generated by the light bulb, aren't you making use of almost 100% of the energy that's being consumed?

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A heat pump can produce 2-5 times more heat than an electrical resistance heater (like an incandescent bulb). It pulls heat from the colder outdoor air with a pump, it sounds illogical until you study heat cycles... http://www.dbh.govt.nz/UserFiles/Image/Publications/building...

Heat pumps actually move existing heat indoors, from outside, using electricity to do the mechanical work, so it's much more efficient than just using electricity to create heat directly.

More specifically, heat pumps often have a COP of around 2, even in very cold weather (-20℃). That means that for every two joules that a light bulb turns into heat, you will save only one joule (and less in most conditions) on your overall energy consumption.

In very, very cold areas, the COP of the heat pump will reduce to 1, and in those cases the incandescent bulb isn't wasting any energy.

Thanks for putting specific numbers to it. It's really surprisingly efficient, especially at warmer temps (0℃) which are more common in most of the US.

That number is also assuming that the pump pulls from the air rather than the ground. Ground based pumps are apparently more efficient (well, energy effective; efficient is not quite the right word in this case) because the ground warms up in the sun more than the air.

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