I know when it is centered at 0, Vpeak/sqrt(2) gives the RMS.
When I do FFT on LTspice, it gives a peak at -17dB which is equal to 0.141254
On the FFT page, program says that it shows RMS values. But, I don't think the RMS of this wave is 0.14 W. You are also welcome to answer for this question involving LTspice.
However, main question is : I actually don't think this question is specific to power calculation, anyway, what is the method to calculate RMS of such sine waves?