You can see the current discharge rate of your battery without any additional tools. Open a terminal with Ctrl+Alt+T and enter this:
This gives you something like this:
Now enter this command, substituting
BAT1 if necessary:
You will get output similar to this:
capacity state: ok
charging state: charged
present rate: 0 mA
remaining capacity: 3395 mAh
present voltage: 12393 mV
present rate line is what your looking for (my PSU is plugged in right now, so the discharge rate is zero).
If you want to see how the rate changes, e.g. per second, do this:
watch -n 1 cat /proc/acpi/battery/BAT1/state
This gives your the output from above, but refreshed every 1 second. Terminate with Ctrl+c.