Do PowerMacs dream of electric bills?

People often wonder if they should turn their computer off, or leave it on but put it into “sleep mode”. I decided to do some analysis a while back, here are the results.

If you look up the specs, a Sawtooth Power Mac G4 in deep sleep uses about 4 watts of electricity. In MA you pay $0.04823 per kWh, so it costs 4 / 1000 kW * 24 hours * 365 days * $0.04823 = $1.68 per year to leave a Mac sleeping instead of turning it off. So, it’s not going to break the bank.

Heat-wise, conservation of energy tells us that no more than 4W of heat is being emitted by the machine. Compare that to the average human body, which radiates 50-100W (estimates vary, Google if you feel the need to check). So, the G4 isn’t going to heat up the room significantly when it’s asleep either.

What about the environment? Well, let’s assume the worst possible case, that your electricity is all coal-generated. That means emissions are around 0.43kg per kWh of electricity used by the consumer. So in a year of sleep, your Mac would cause the emission of 15kg of CO2. Sounds pretty bad, doesn’t it? Well, according to space research you produce 1kg of CO2 per day by breathing, or a massive 365kg per year. So the Mac is 20x better for the environment when asleep than you are.

What about the hardware? Well, in deep sleep almost all the hardware is powered down exactly as if the machine was turned off. The hard drive, which is the piece most likely to wear out, is powered down. The CPU and RAM are incredibly unlikely to wear out even if you run them 24×7 for the next ten years, at which point the machine will be so obsolete it’s worthless. So turning the Mac off won’t make it last any longer or protect your ‘investment’.

So, I conclude that there is absolutely no economic or environmental need to turn your Mac off. If you have a UPS, you may as well leave the Mac asleep when you’re not using it.

Of course, if your computer is a PC which doesn’t have an Energy Star “deep sleep” mode, the above calculation may be off by a factor of 10 or more. However, most PCs are now Energy Star.

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