Lower limit to power consumption

$I$ bits of Information
No. configurations $\Omega=1$
Entropy $S=\log \Omega\ k = 0$
Perform calculation, destroys information
$0$ bits of Information
$\Omega=2^{I}$
$S=\log \Omega\ k = I \log{2}\ k$
Remove entropy to restore computer to original state
$0$ bits of Information
$S=0$		$S\ge I \log{2}\ k$

Typically entropy removed via heat $Q$ (consumes energy). $Q = S T$ $Q \ge I \log{2}\ kT$. I.e. at least $\log{2}\ kT$ used per bit of information lost ($2.8\ 10^{-21}$ Joules at room temperature).
Cf. low power pentium $5\ 10^{-11}$ (300 MHz 0.5 Watt)