Ohm's Law

lm-sensors can monitor the CPU voltage, and the current used, so it can derive the power (in watts) by calculating watts = voltage * current.

On my system I can obtain the power drawn from the battery from

cat /sys/class/power_supply/BAT0/power_now
9616000

On Thinkpads if the tp_smapi module is loaded, the file is

cat /sys/devices/platform/smapi/BAT0/power_now

The value seems to be in µW, though. You can convert it with any tool you're comfortable with, e.g. awk:

awk '{print $1*10^-6 " W"}' /sys/class/power_supply/BAT0/power_now
9.616 W

In case you cannot find the location within the sysfs file system, you can search for it:

find /sys -type f -name power_now 2>/dev/null

Additionally, the package lm-sensors may be used to determine the system power usage on some machines:

# sensors power_meter-acpi-0
power_meter-acpi-0
Adapter: ACPI interface
power1:      339.00 W  (interval =   1.00 s)

If the sensor is digital one you should have in the datasheet a "Conversion Time vs Temperature" graph. In the datasheet also you can see the current consumption in shutdown mode (0.5 - 1 uA) and active mode (10 - 12 uA). Depending on how often you measure the temperature you can compute the power consumption.

Once lm-sensors is installed you need to reach for your terminal:

type

sudo sensors-detect

just press ENTER for everything it suggests (shown in Uppercase)

At the end it will ask you whether to add what it finds to /etc/modules. If you are happy with the findings type "yes".

More information about lm-sensors and how to tailor it for your system can be found on the lm-sensors installation wiki page

Typing

sensors

will display the values for the sensors detected previously.

e.g.

acpitz-virtual-0
Adapter: Virtual device
temp1:       +55.0°C  (crit = +90.0°C)

Do you currently have any compute in0 statements in your /etc/sensors.conf, /etc/sensors3.conf or /etc/sensors.d/*.conf that would apply to sensor chip nct6798-isa-0290? If you have, comment them out and run sensors --set as root.

Then look at the value again.

According to my old notes (probably scribbled up from a datasheet found who-knows-where in the internet), the Nuvoton NC6798D's voltage inputs have a range of 0 .. 2.048 V with a 8-bit accuracy, and a number of inputs have a built-in 2x divider to extend the range. These inputs with the range doubler are in2, in3, in7, in8 and in9, possibly also in0. That would mean that the lowest significant bit in each voltage register would signify a change of either 8 or 16 mV, depending on whether the input has the divider or not.

The voltage sensors with the built-in divider are integral to the chip, and have default designations:

• in2 = AVSB
• in3 = 3Vcc (normal 3.3V rail)
• in7 = 3Vsb (stand-by 3.3V input)
• in8 = Vbat (CMOS battery voltage)
• in9 = Vtt (processor memory controller voltage?)

It sort of looks like something may be applying an extra 2x divider to your in0 value. If so, adding something like

chip "nct6798-isa-0290"    # add this if it does not already exist
    compute in0 @*2, @/2   # then add this line _after_ the previous one

to your lm-sensors configuration should fix it.