Variance of signals and noise for SNR calculations

in many calculations i have seen using variance of signals and noise for SNR calculations. just wondering is variance representative of power or somewhat equivalent to it?
please elaborate why variance is used instead of signal energy or power

hmm no replies yet
me too waiting to know the reason

variance is AC power , while mean^2 is DC power and there sum is the total power

thnx safwatonline

u got three points now :D