You must be doing something wrong...
...because my N95 has plenty of RAM to play videos at the same time as it runs the energy monitor; not a lot of point in having one otherwise, is there?
It uses about three watts. However, note that:
a) the energy monitor itself burns about a third of a watt (which is an order of magnitude more than the normal quiescent power of the N95 as measured with my handy dandy milliwatt meter.
b) running two apps at once is disproportionately hard on the battery in these devices due to the cache pollution side effects; ARM1136JF-S doesn't have that much cache, and all code segments are cacheable in SymbianOS, so the RAM interfaces will be working much harder with the energy monitor running than it would be otherwise.
Measured figures with a meter:
Typical power dissipation of an N95-3 playing AAC audio at 320 kbits/sec into 32 ohm headphones at maximum volume is around 300 mW with display illuminated and 200 mW with it dark, while on a rather weak UMTS HSDPA cell (-= 30 mW for GSM/Edge), Bluetooth on (probably <10 mW- was lost in the noise), WLAN off (depends on your power setting, but generally double the setting's ERP), and with no USB connected (+= 100 mW or so for the USB line drivers).