Here's how to estimate your laptop's power consumption at any given moment.
Step 1. Look up the power ratings of your laptop battery, in Volts (V) and Milliamperes (mAh). You'll usually find them printed on the battery itself, as well as published as part of the specs -- for the laptop (if using OEM battery) or aftermarket batteries.
Example: 14.8 V and 6,450 mAh
Step 2. Convert the above to watt-hours power rating:
Volts x Amperes = watt-hours
Example: 14.8 V x 6.45 Ah = 95.46 Wh
So, this particular Li-Ion battery can supply 95.46 W for 1 hour... or 47.73 W for 2 hours... etc.
Step 3. Have your laptop unplugged, running only on battery power, in a specific hardware / software / energy state you want to test.
Follow either 4a OR 4b.
Step 4a. IF the power gauge shows remaining time, divide that time, expressed in hours, by the power rating in watt-hours.
Example: 2h30m remaining
95.46 Wh / 2.5h &asymp 38 W
This is your instantaneous power consumption.
Step 4b. IF the power gauge shows remaining % of full capacity, then:
- note the exact time it takes the gauge to decrease by 1%
- divide the power rating in watt-hours by 100 times that time, converted to hours.
Example: 90 seconds to go from 67% to 66%
100 x 90 / 3,600 = 2.5 hours
95.46 Wh / 2.5h &asymp 38 W
This is your instantaneous power consumption.