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, Administrivia’s technicians 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.

Takeaway: Turning down the brightness on your laptop’s screen is an easy way to extend the length of time your laptop can run between charges.

4. ‘You’ll save energy if you disconnect all your chargers and peripherals when you’re not using them.’

Some of your peripherals may continue to suck power when they are turned off but still plugged in; but the Macs we tested did not. When plugged in—charged and idle—at full screen brightness, the MacBook Pro averaged 13.3W. When turned off but plugged in, it had a draw of 0.0W, according to our meter. Our 2011 21.5-inch iMac averaged 82.7W when on but idle, but that figure dropped to an average of 0.1W when the iMac was turned off but still plugged in. We attached a 27-inch Apple LED Cinema Display to the iMac, plugged both of them into a power strip, and plugged the power strip into the Watts Up meter. This setup drew an average of 136W when idle but still switched on. The power strip alone drew power at a rate of 0.0W with nothing plugged into it. The display drew 0.2W when plugged directly into the power meter but not connected to the iMac. With both the display and iMac plugged into the power strip, but turned off, the power meter reported a 0.5W power draw. When we turned the surge protector’s power switch to off, the draw dropped to zero.

Takeaway: Turning off devices is sufficient. The power draw of our charged MacBook Pro plummeted to zero when we turned it off but left it connected to the power meter. Turning off the power strip also brought the power draw down to a zero reading.

5. ‘If you use your Mac’s default energy settings, you don’t need to turn your computer off at the end of the day.’

We found that both the MacBook Pro and the iMac drew power at a rate of around 1W once they were set to sleep.

Takeaway: Though the wattage level wasn’t high, both Macs did continue to draw a small amount of power when asleep.

6. ‘Your computer sucks in so much power when it starts up that you’ll save energy by simply leaving it on.’

A significant spike in energy draw occurs when you boot your Mac, but it settles down very quickly. Our MacBook Pro gobbled up to 40W during the first minute of bootup, but the system’s consumption dropped back to its 12W-to-13W average by the 90-second mark.

Takeaway: The Mac doesn’t use much power when sleeping, about 16 watt-hours from 5 p.m. to 9 a.m., but that’s more than it uses during the first minute of starting up.

7. ‘Solid-state drives use less energy than regular hard drives.’

At full brightness, with Wi-Fi and Bluetooth on and a regular hard drive installed, our MacBook Pro drew about 14W when idle. At the same settings, but with an SSD in place of the regular hard drive, the MacBook Pro drew just shy of 15W. When copying a 10GB file from the machine’s desktop to a different folder on the same drive, the MacBook Pro drew 18.8W with the regular hard drive in place versus 20.1W with the SSD. When idle with Wi-Fi and Bluetooth on, the iMac averaged 82.7W using the hard drive and 77.6W with the SSD. In copying the file, however, the power savings disappeared, with the hard drive drawing approximately 85.3W while the SSD drew about 85.9W.

Takeaway: In our tests, the SSD appears to have used less power than the 3.5-inch, 7200-rpm hard drive in the iMac, but a bit more power than the 2.5-inch, 5400-rpm hard drive found in the stock MacBook Pro.

8. ‘Bus-powered FireWire drives use more power than USB hard drives.’

We connected a Western Digital My Passport Studio drive equipped with both USB 2.0 and FireWire 800 ports to our 15-inch 2010 MacBook Pro. With the drive mounted (but idle) via USB 2.0, the MacBook Pro’s display set to full brightness, and the laptop fully charged and with Bluetooth and Wi-Fi turned off, the Watts Up Pro meter reported a draw of about 16.1W. With the drive attached via FireWire (but idle), and all other conditions the same, the meter registered 18.4W. While we performed a Time Machine backup, the MacBook Pro with USB hard drive used 25.1W versus 28.2W over FireWire 800.

Takeaway: Our drive drew less power, whether idle or in use, when connected over USB than when connected via FireWire.