How to calculate RMS power of an offsetted sine wave
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.