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: 
You want to know the rms of a waveform \$ f(t) \$.

There are two approaches here direct application of the formula:

$$RMS = \sqrt{\dfrac{1}{T} \cdot \int\_0^T (f(t))^2 \text{d}t}$$

Where T is the period of the signal. However there is another useful function:

$$RMS = \sqrt{RMS\_1^2 + RMS\_2^2}$$

Where \$RMS\_1\$ and \$RMS\_2\$ are the RMS values two signals that when added together give \$f(t)\$.

This only works provided the two signals do not share any frequency content in common. In this case a sine wave only contains one frequency as does DC (0 Hz which is different) so you can use this formula.

The RMS of a sin wave is \$ \dfrac{\sqrt{2}}{2} \cdot V\_{pk} = \dfrac{\sqrt{2}}{4} \cdot V\_{pkpk}\$ where \$V\_{pkpk}\$ is the peak to peak voltage. The RMS of a DC signal is the same as the DC 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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