Two grocery stores caught my eye yesterday. I was in the freezer aisle in Fry’s Food and Drug, and noticed the lights in the freezers were turning on as I approached. At first, I figured this was a malfunctioning freezer light. But then I noticed all the freezer lights were doing the same thing – turning on as I approached, and turning off as I walked away. The freezers had a motion sensor mounted above the door that only switched on when a person was in front of the door.
A few hours later, I was in Fresh & Easy, and noticed the sign on their freezer doors proclaimed that they use LEDs in their freezers to save electricity. I didn’t notice motion sensors on their freezers, so apparently the LED lights are on all the time.
Here are two simple methods of saving electricity. Compared to the cost of the electricity to run the freezers, it may be negligible, but over time it can add up. The biggest savings probably comes in the freezers not having to remove the heat generated by the light bulbs. The stores must think the savings is worthwhile, otherwise they could have installed cheaper freezers with fluorescent lights that are on all the time.
I had a professor who was obsessed with “back of the envelope” calculations, and his students teased him on it. Sometimes, those calculations are useful for rough estimations and simple comparisons. If I assume standard grocery store freezers are lit with two 40-watt fluorescent bulbs each, and that there are 20 freezer unites in the freezer aisle, that is 1600 watts, or 1.6 kilowatts. If the lights are on from 6am to midnight, that is 28.8 kilowatt-hours per day. In 30 days, that is 864 kilowatt-hours. Assuming that a motion sensor cuts the time the lights are on in half, about 430 kWh is saved. Greater savings occur because the light bulbs are generating less heat that has to be removed by the freezer.
Last month, I used 480 kilowatt-hours at my house. My “back of the envelope” estimation is that the supermarket saves electricity at least equal to that used by a small household. In comparison to the store’s overall electricity bill, that may not be much. In comparison to how much a typical family uses, the saving is significant.
By the way, has anyone else noticed the older-style freezers with the open tops have almost disappeared in favor of those with glass doors?