lib中internal_power值为什么是负的？

今天看library时看到的这个问题。
比方说nand2的cell
----------------------------------------------
pin(A1)
...
...
internal_power
...
rise_power()
index(xx,xx,xx,xx)
value(-xxx,-xxx,-xxx,-xxx)
---------------------------------------------
了解过delay的负值问题。但是这里的功耗值为负不太懂是怎么回事。不知道有没有朋友碰到或者了解这个问题。
谢谢啦

Each entry in the lookup table is calculated by measuring the current drawn from the power supply during a detailed spice simulation and subtracting any current used to charge the output load. In other words all of the energy that is not consumed charging up the output load is considered internal energy. Note that some of the internal power is negative. This is simply due to how we account for energy. We can either assume all energy is consumed only when the output node is charged and no energy energy is consumed when the output node is discharged, or we can assume half the energy is consumed when the output is node is charged and half the energy is consumed when the output node is discharged. In this case the library is using the second approach, and the way the total energy is averaged across both transitions can result in either the rising or falling internal power to be negative.