Products like Kill-A-Watt show how much power appliances are using. It’s good way to measure your computer’s power consumption — when it’s plugged in. But what if I want to know how much power my laptop is using when I’m not plugged in?

Turns out it’s fairly easy to get battery info. Running cat /proc/acpi/battery/BAT0/state should return the current discharge rate and total battery capacity.

The trouble is that the “current rate” field sometimes switches between mW and mA. I have no idea why this is the case. Oh well, we can easily convert mA to mW with the formula: $latex watts = amps times volts $.

The end result is this conky friendly bash script:

[bash]
#!/usr/bin/env bash
# Print the current power consumption in watts.
# The script pulls the power consumption info
# from /proc/acpi/battery/BAT0/state
#
# Usage: power.sh [battery_num]
#
# Add to conky:
# ${execi 10 ~/conkyscripts/power.sh 0}
#

# Default is BAT0
BATTERY=0
if [ $1 ]; then
BATTERY=$1
fi

# Sometimes the "present rate" is returned in milliwatts,
# but sometimes it is in milliamps. If it’s in milliwatts,
# we just convert to watts and return. Otherwise we
# convert to watts with the formula:
# Watts = Amps * Volts

UNIT=`cat /proc/acpi/battery/BAT$BATTERY/state |
grep "present rate:" | awk ‘{ print $(NF) }’`

if [ $UNIT == "mW" ]; then
MILLI_WATTS=`cat /proc/acpi/battery/BAT$BATTERY/state |
grep "present rate:" | awk ‘{ print $(NF-1) }’`
e cho $[ $MILLI_WATTS / 1000 ].$[ ($MILLI_WATTS % 1000) / 100 ]
else
MILLI_AMPS=`cat /proc/acpi/battery/BAT$BATTERY/state |
grep "present rate:" | awk ‘{ print $(NF-1) }’`
MILLI_VOLTS=`cat /proc/acpi/battery/BAT$BATTERY/state |
grep "present voltage:" | awk ‘{ print $(NF-1) }’`
POWER=$[$MILLI_AMPS * $MILLI_VOLTS]
echo $[ $POWER / 1000000 ].$[ ($POWER % 1000000) / 100000 ]
fi
[/bash]