RTL-SDR units, $\textrm{dBm}$
It's very hard to associate the numbers you're calculating with the actual signal power at the antenna. The SDR will filter, amplify, and AGC the signal, and then it'll convert it to digital. At best, you can say that the power you're calculating is somewhat related to the true signal's power.

The power you're calculating is in linear, not logarithmic, scale. To convert to decibels, use the usual formula. However, be aware that the reference inherent when using decibels is not well specified in this case.