Should you leave electric devices on?
Or will you save energy by turning them off when not in use?
Is it better to leave electric devices on? I've heard many people say that it takes more energy to turn something off and then back on than just to leave it turned on. This doesn't make sense to me - what's the scoop?
Answer from Green Energy Efficient Homes
There are three arguments given for why you should leave things turned on instead of turning them off between uses, and none of them stands up to scrutiny.
The first argument is that there is a surge in power draw when certain devices are turned on, and this power draw makes up for any savings that might have been obtained by turning the device off.
The second argument is that, for devices that heat or cool - electric heaters or air conditioners - it takes more energy to bring an indoor space back to the climate controlled temperature when the heater or AC is turned off and later back on, than it takes to maintain that temperature constantly by leaving the device turned on.
The third argument is that cycling a device on and off increases wear and tear on the device, so when you leave electric devices on it saves money in the long run because there are fewer breakdowns caused by this cycling.
The power surge argument is the one used to justify leaving lights turned on. It is true that there is a spike in power usage when you turn on a light. But the spike is for such a very short time that it has virtually no impact on the overall power consumption of the light. As I explain in Turn off lights, the main problem with this argument is that the amount of power surge described is so astronomical that if it were true, every time you turned on a light it would blow the circuit breaker.
The steady state argument - that keeping a room at a steady temperature uses less energy than trying to bring it back to comfort temperature after the heater or air conditioner has been turned off for a while - doesn't hold up to basic physics. Heat is constantly flowing from a colder area to a warmer one, and the bigger the temperature difference between the two areas, the greater the heat flow. If you are heating a room and it's cold outside, heat will migrate from the room to the outside. If you're air conditioning a room and it's hot out, heat will migrate from outside into the room. Of course, it takes more energy to bring a room from its cold winter state, or hot summer state, to a comfortable temperature, than to keep it at a comfortable temperature it has already reached. But people tend to forget that, while the room was moving towards the outdoor temperature because of the heat flow through the walls, the heater or air conditioner was using no energy at all, while when you leave electric devices on like heaters or air conditioners, the room that was kept at steady state required a constant input of energy to maintain that steady state.
The steady state argument might also be applied to devices where powering down, and then back up, involves more energy consumption per unit of time, than the device uses while operational. For example, leaving a laptop running doesn't use that much power, especially if the disk drive isn't spinning. Hibernating the laptop, and later turning it back on, requires the disk to spin, which in theory increases electricity usage. But in fact, when I measured the difference between hibernating and leaving my laptop on, I found that the instant I started hibernating, power use dropped by 30%. I think that's because, while the laptop is on, the CPU may be operating at full capacity, but as soon as you start hibernating, the CPU drops to a low power consumption mode (it doesn't take much CPU capacity to write the memory out to the hibernate file), so the act of hibernation itself is actually a lower power consumption activity than leaving the laptop on.
The wear and tear argument - that the surge of power when turning a device off and back on damages the device - doesn't hold water from a cost perspective when applied to a realistic scenario. It's true that you will add wear to a device each time you turn it off and back on. But it's also true that there is wear and tear on most devices while they are left on, and if you leave electric devices on you are using power the whole time, whereas when you turn them off you might pay a small wear and tear cost from the cycling, but you use no energy at all from the device while it's turned off.
The last point to consider is that when you leave electric devices on because you assume it's more efficient, for the short period you plan not to use them, than turning them off, you're not accounting for those times when you might not use the device for longer than planned. For example, if you leave your room air conditioner running because you are planning to go back into your bedroom in a few minutes to read, but you wind up taking all morning to do garden chores outside, your room might be nice and cool when you get back at lunch time but there was no real value to keeping it cool while you were out gardening.
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