power calculation

Hello. everyone. I am doing the simulation in Cadence(IC6.1.6). I need to calculate the power provide by the source. 

In the simulation, I use both the vdc and vpulse in the libary analogLib with conductive load.

I used two ways to get the power.

1.rms

 With the tran analysis, I measure the voltage and the positive terminal of the vdc and vpulse to get the current waveform.

Then I use the calculator to get voltage and current with the 'rms' function. Then the power = voltage(rms)*current(rms).

2.Average

Still with same way above.

But I use the calculator to get V and I with 'average' function. Then the power = voltage(ave)*current(ave).

I can see there is a big difference in these two results, sometime up to 20% or even more.

I can guess there are some reasons related to the phenomena.

It's negative and  positive current flowing into the vdc or vpulse source. Negative current means the current provided by the source. And this part of power is the part I need to calculate. And the positive part is the power that are not provided by the source. Then I should negelect. Though in my case, there is not much influence in my results. Because the positive loss is quite small.

Another reason is that the real source should contain resistor. When I use the ideal source, it didn't contain with the resistor. I can see a extremely large  current peak, like 1KV. 

I have searched so much material to verify which one is right. But it seems different people have different evident to support their ideas.

It really confuses me.