I think the specifications represent the range for the proper working of the device. So the 100-240 V should actually mean that the device is able to work fine if the voltage lies within this range. And related to your query about the power consumed, the question arises what part of the power are you actually concerned about, is it the average, instantaneous,active or reactive ?
But I guess this might help you,
for a sinusoidal wave,
$$V(t)=V\sin(\omega t), I(t)=I\sin(\omega t)$$
thus, $P=V(t)\times I(t)=V\times I\times \sin(\omega t)^2$, where $V$ and $I$ are the r.m.s. values.
Accordingly using $V_{rms}=\frac{V_{given}}{1.414}$ and similarly for $I$, and putting $\omega = 2 \pi\times f$, where $f$ represents the frequency.