Question: [![enter image description here](https://i.stack.imgur.com/qe9JA.png)](https://i.stack.imgur.com/qe9JA.png)

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

[![enter image description here](https://i.stack.imgur.com/0mP38.png)](https://i.stack.imgur.com/0mP38.png)

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?**

Answer: 
As the signal is not totally recognizable in the first figure, I'll assume your sine wave equation to be \$ f(t) = 0.25 + 0.25\,sin(5000\,\pi\,t)\$ and \$f(t)\$ has Watt unit.

Thus, the RMS value is given by \$\sqrt{\dfrac{1}{T}\int\_{t\_0}^{t\_0+T} f(t)^2dt}\$ being T the period of the function, which I have assumed to be \$\approx .4\,ms\$. Thus, \$T = .4\,ms\$ and as the function has its peak at \$\approx .5\,W\$ I have thus reached the equation given above. Figuring out the integral value we end up with :

\$\sqrt{\dfrac{1}{0.0004}\int\_{0}^{0.0004}(0.25 + 0.25\,sin(5000\,\pi\,t))^2dt} = 306.18\,m\,W\$ as the approximate RMS value.
I am assuming the waveform has a peak value of 500mW and an average of 250mW.
The rms value of dc portion is 250mW, same as the average value.
For the 1kHz ac part, rms is:
$$ \frac{500-250}{\sqrt{2}} = 177mW
$$

So the net rms will be:
$$ \sqrt{DC rms^{2} + AC rms^{2}} = 306mW
$$
To get rms value from LTSpice, CTRL + left-click the waveform name at the top of the screen (waveform name- I(speaker)\*V(V\_speaker).)
$$
\
\
$$
FFT should give 20\*log(ac\_rms) which is:

$$20\*log(0.177)=-15dB \ at \ 1kHz $$
This is pretty close to what you have got. So looks like your simuation is giving the right results.

That being said, I am not sure if rms value of power has any significance. For most purposes we use average power only.

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