How to calculate the voltage used to light up the emergency light from the given details of manual..?
Since the device has a wide operating voltage it is likely that it has a switched-mode universal power-supply.

We can do some maths and work out the current at 18 W for both 110 V (North America) and 230 V (Europe) power.

For 110 V, \$ I = \frac {P}{V} = \frac {18}{110} = 0.163 \ \text A \$.

For 230 V, \$ I = \frac {P}{V} = \frac {18}{230} = 0.078 \ \text A \$.

The 0.1 A rating on the PSU is normally the worst case at 90 V in your case. We can see that there is something wrong here so it may be that the manufacturer has erred or has given some sort of average current since once charged it will trickle-charge the battery and the power consumed will be very low.