We're all interested in saving energy. Maybe you conserve to be a better global citizen or to save money on your utility bills. Maybe you use your laptop on the go and want to squeeze every possible minute of battery life out of it. But what if the daily computing practices you follow to save energy end up wasting it instead? What if your assumptions about Mac power usage are wrong? To investigate this possibility, Macworld's lab compiled a list of eight widely held opinions about energy conservation, grabbed our trusty power meters, and started logging power usage.
We used two systems: a 2011 21-inch iMac and a 2011 15-inch MacBook Pro. We connected them to a Watt's Up Pro power meter equipped with a USB connection that allowed us to capture energy usage logs while we ran various tests. Here's what we found out.
1. 'Laptops use less energy than desktops.'
The iMac we tested averaged around 83W with the screen set to full brightness, and with Bluetooth and WiFi enabled. That's six times more than the 13.4W that the MacBook Pro drew at similar settings when fully charged. When the MacBook Pro's battery was at a 50 percent charge and plugged in, however, our 15-inch laptop drew 80W, just about the same as the iMac.
Takeaway: Over the course of a day, laptops do use less energy than desktops.
2. 'Turning off Bluetooth preserves your MacBook's battery life.'
We detected little, if any, difference in our laptop's energy draw with Bluetooth enabled versus with Bluetooth disabled. In fact, our MacBook Pro with SSD drew an average of 13.9W with Wi-Fi and Bluetooth off and 13.8W with Wi-Fi off but Bluetooth on.
Next, we paired a Bluetooth mouse and keyboard with our MacBook Pro with SSD, and ran the tests again. This time the power draw increased a touch to 14.3W. Looking at the OS X's Energy Saver preferences, we noticed that the estimated battery life of the charged MacBook Pro dropped from 7 hours, 24 minutes with Bluetooth turned off to 7 hours, 5 minutes with Bluetooth turned on and the wireless keyboard and mouse paired with the system.
Takeaway: Though the meter didn't show much change in the amount of power our laptop drew with Bluetooth off versus on, as long as nothing was paired with it, the power draw increased--and the estimated battery life decreased--when we paired a wireless keyboard and mouse with the laptop.
3. 'Dimming the screen on your MacBook prolongs battery life.'
The validity of this hypothesis is easy to demonstrate. At full brightness, with a fully charged battery and with Wi-Fi and Bluetooth turned off, the MacBook Pro drew about 13.4W of power. When we set the screen to half brightness, the power draw dropped sharply to 9.4W. And with the screen set to the lowest level that allowed us to make out what was on the screen, the power draw fell further, to 8.2W.
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