how i can calculate power for nor2 gate in hspice?

[diod]

it is nor2 code.i need to calculate power with hspice and calculate power my self.power result in hspice is 5.8622e-4.and when i calculate power((v*v*c)/2)=(3.8*3.8*50)/2=3.61e-4.my calculate is true?
.option post
.PARAM SUPPLY=3.8
.lib 'all_mos.l' tt
.GLOBAL VDD 0
vsupply VDD 0 3.8
mpa out a x vdd pch W=10u L=1u
mpb x b vdd vdd pch W=10u L=1u
mna out a gnd gnd nch W=10u L=1u
mnb out b gnd gnd nch W=10u L=1u
cload out 0 50fF
.TRAN 50PS 10NS
.measure tran avgpwr avg p(mnA) from=1n to=3n
.measure TRAN power PARAM='iavg*3.8'
.measure TRAN iavg AVG i(mnA) FROM=1ns TO=3ns
VA A 0 PULSE 0 5 1ns 50ps 50ps 4.1ns 8ns
VB B 0 PULSE 0 5 1ns 200ps 200ps 2ns 8ns
.end
why in or gate hspice power result in simulation is more diffrent whit my calculate power.i dont khow way of my calculation is true or not.if it is true why like this diffrent?
iam waiting to your answer.
thanks