Turn It Off?

Home Home and Auto Appliances Energy Use Turn It Off?

Dear Dollar Stretcher,
I've got a question concerning energy use/consumption/conservation: Is it worth turning off a light for only a few minutes? Will this save money (a few cents) or is the energy required to start the bulb more than it would use in this time? What about a computer?? Is it better to leave a computer on all the time or turn it off between uses? If it is better shutting off appliances between uses, how much time must it be off in order to save more than the start-up cost?
Andre D. in Hearst, Ont.

Andre asks a good question. Is it always wise to turn off an appliance when it's not in use? Are there times when it makes sense (and cents!) to just leave something on? Let's see if we can't come up with some answers.

Take lighting for instance. How many of us have turned off unattended lights while mumbling about our spouse or children? Lights are so easy to spot. And so clearly wasteful. But, how much does it really cost us? According to the Lincoln Electric System (Lincoln, NE) a 100 watt incandescent bulb costs 0.6 cents per hour. Not six cents, but six tenths of a cent!

Now if your house is like mine, they didn't leave the 100 watt lamp on, it was the 250 watt reading lamp! So now we're up to 1.5 cents per hour. And yes, if I look closely I can probably find another couple of 60 watters on, too. Those cost .3 cents per hour. So an average evening of four hours times the 1.5 cents plus the 60's totals 8.4 cents. Sure, I'm going to say something, but I probably don't want to raise my voice (and blood pressure!) over something that comes to $3 a month.

What about Andre's computer? Well, most home computers have a power supply rated at about 180 watts. That's good for about 1 cent per hour. Add the color monitor and printer at 1.2 cents per hour. So you're spending less than 2.5 cents per hour for your pc. Really a pretty good bargain if you think about it.

OK, so we've looked at some things that are fairly inexpensive to operate. What happens when you get into the big electric users like air-conditioners? Let's start by finding out how much it costs to operate them. Again using Lincoln Electric System figures, we'll consider a typical 3 ton central unit. We'll take one with a SEER of 10.0. That should cost a shade less than 25 cents per hour to operate. So if we can turn it off we'll save a quarter each hour for our effort.

But suppose we were going to be out for two hours. Would we actually save 50 cents by turning it off? Doesn't it cost more to re-cool your home than to just keep it cool the whole time?

There are a lot of variables here that make it difficult to generalize the cost of leaving it on versus turning it off. But, if you'll read through an example with me we'll see that it is cheaper to turn it off and how you can prove it to yourself in your own home.

In our example we'll assume that we're home for an hour, gone for two, and back again for one more hour. Our first option is to just leave the a/c on all four hours. Using our 25 cents per hour that would cost $1.

Now suppose we turn it off while we're gone. We'll save 50 cents in the middle two hours, right? Well, not exactly. When we get back we'll spend more than 25 cents in that hour to get back to our cooler temp. But will we spend less than the 75 cents that it would have cost to keep it on the whole time? Yes, we will save some money.

One way to think of it is what is the average temperature that's being maintained in the room. If the unit's on continually you keep the temperature at 78 degrees the whole time. Turning it off allows the temperature to creep up to say 82 degrees while you're away. That brings the average for the four hours up to 80 degrees (78 + 82 + 82 + 78 / 4 = 80). The air-conditioner will not cycle on as often to maintain the higher temperature.

Want to prove it for yourself? Try this. One day when your air-conditioner has been on for a few hours measure how many minutes in an hour that the compressor is actually on. You'll be able to tell by your inside fan circulating the cool air. Suppose it runs for 15 minutes. Four hours would be a total of 60 minutes of actual electric usage.

Once you know that, turn the system off for a couple of hours. Then turn it back on and calculate how many minutes the compressor runs in the next hour. Let's say it ran 35 minutes. Add that to the 15 minutes from the first hour and your total for the four hours is 50 minutes of electric usage. You've saved 10 minutes of electric usage.

Now for the final part of Andre's question. What about the power used to 'start' an appliance? First, very few appliances take more power to start. Some, like florescent lights, have a requirement, but when they cost tenths of a cent per hour to operate, the start up is minuscule.

Larger motors, like air-conditioning compressors, do take more juice to start. But, and this is important, that happens every time the unit cycles on. So if you turn the unit off while you're gone, you avoid all those cycles. Had you left the A/C on you would have paid for those start ups even though you were gone!

What's the bottom line? Common sense. If you're leaving a room for ten minutes and coming back it's not a big deal if a small lamp is left on. But if you're going to be gone for more than a few minutes large electric users like air-conditioners should be turned off.

Thanks to Andre for a thought provoking question.

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