How to measure power consumption
Direct measurement during operation
To monitor the laptop's power consumption while it is running, unplug the AC power and run:
# watch -n1 'cat /proc/acpi/battery/BAT0/*'
This requires ACPI to be enabled. The value given is (roughly) an averge over the last minute.
Alternatively, load the tp_smapi module and run the following with AC power unplugged:
# watch -n1 'cat /sys/devices/platform/smapi/BAT0/power_now'
# watch -n1 'cat /sys/devices/platform/smapi/BAT0/power_avg'
The former provides the instantaneous power draw, and the latter is (roughly) an average over the last minute. It's OK to use tp_smapi's force_discharge function instead of physically disconnecting AC power.
Note that running on battery power may activate power-saving mechanisms, so unless you take care to deactivate them this does not reflect power consumption under AC power.
Dstat is another way to track your laptop's power consumption, run the following with AC power unplugged to relate power usage (in Watts) and battery information:
[dag@moria ~]$ dstat --time --power --battery --battery-remain 10 ----system---- power batt remai time |usage|bat0| bat0 15-06 08:53:55|0.000| 94| 4h29 15-06 08:54:05|15.74| 94| 4h30 15-06 08:54:15|15.56| 94| 4h30 15-06 08:54:25|15.52| 94| 4h30 15-06 08:54:35|15.60| 93| 4h29
Measurement for synthetic workloads
Use the Battery Life Tool Kit tool.
Measurement during suspend
To measure power consumption while the laptop is suspended, use the ACPI sleep power drain test script.
Hardware-measurement on line or ac-adapter
The best (imho) method to measure power consumption is to measure the power drawn by the laptop from the ac-adapter or the power drawn by the ac-adapter itself by a wattmeter. If you want to measure drawn only by the laptop itself, you'll have to build a little adapter.
For reducing power consumption, see How to reduce power consumption.