What methods can be used to reduce the amount of power a Raspberry Pi uses?
Unplug it from mains power :)
But seriously: the Pi has a "base" power usage when run headless, without anything plugged in (no monitor, no usb devices, just the power supply). Use a Kill-A-Watt meter to measure the powerusage of this setup. Then do the following:
If you want to further reduce this power usage, you can look into disabling a few things.
According to this article, if you have an rPi2, you can reduce
the number of cores by adding the following lines into
# replace N with the number of cores you want to keep using
You could look into disabling power to the USB bus by issuing the following (from this post on this forum):
echo 0x0 > /sys/devices/platform/bcm2708_usb/buspower
NOTE OF WARNING: This might also disable your network port, so don't use this in a headless setup until you have made sure you can recover from it.
Use the Kill-A-Watt meter to check the impact these changes have to power usage.
If you need to use additional devices, such as an HDMI monitor, USB devices including WiFi dongles etc., keep the Kill-A-Watt meter hooked up to your Pi and look at the impact of adding each device in (shut down Pi, add 1 device, reboot Pi, make sure device is in use (E.g. WiFi dongle is connected/connecting to network) and observe the output of the Kill-A-Watt meter.
Other things you may want to search for include disabling the LEDs, disabling HDMI out and 'nohz' kernel option to alter the tick interrupts from periodic clocks.
Articles I've seen also suggest that Kill A Watt measurements are pretty inaccurate at low current levels, use a bench meter if you have one available.
This article has a good few handy links in it: http://www.midwesternmac.com/blogs/jeff-geerling/raspberry-pi-zero-conserve-energy