Noob's question regarding powering a Normal LED light.

I tried searching but didn’t find anything similar, sorry if this has been covered somewhere, or if it doesn’t belong here.

I am wondering if it’s possible and practical to power a normal 5W LED (Downlight) Using number of 18650s.

This is the light in question> http://www.havells.com/en/consumer/lighting/led-surface-fittings/led-surface/led-surface-lhebkjp7oz1w005.html

And power specs are >>
Wattage (watts) 5
Rated Input Voltage (V) 230
Rated Input Current (mA) 30
Power factor 0.95
Frequency Hertz 50

If it is practical then I would love to know how I can calculate the number of batteries required for this to run for (say) 6 Hours.

That light runs off AC. You could build a 12 volt 18650 battery bank and use an inverter to power it, or you could find a different light that runs off DC. I would suggest finding a different light for the sake of efficiency, or buying this one, ripping the AC driver out and putting your own in.

If this light runs on 230V AC you have to know what LED driver is build in there.
Can be 5V, 12V or COB 20-30V

If you want to power a downlight with battery better get a 3V low Vf LED like XPG3 on a star and driver for single cell lights that drives it at 1050mA, for 6 hours then 3 good 18650 are enough