Measuring electrical power
To get the maximum out of your laptop battery, it's good to know a few things about laptop power consumption.
The easiest way is to measure it using a Watt meter, like the kill-a-watt. This device measures the load a laptop draws under different conditions in Watts. It gets you in the position to know how to preserve and maximize laptop battery power, by changing the way you use your laptop.
The measuring device sits between the mains outlet and the laptop charger and measures actual load, peak load and has a build in calculator to display the costs of Watts consumed.
Have a look at a simple laptop battery test. Rate the remaining capacity of a laptop battery with a Watt meter.
How does the Watt meter work?
The device uses a shunt resistor with a very low value. The voltage drop over the shunt resistor is directly linear to the Amps or Watts that pass through. A microprocessor samples the voltage drop over the shut resistor and calculates the power passing through.
It's a 20-25 US$ device designed for loads up to 2000 Watts.Variants display voltage, amps and the frequency. The investment is multi-purpose, however not suitable for measuring high loads like A/C's .