[morzh]

If needs to be measured very accurately, the voltage across the resistor can be measured on AC, then on DC, then the results are squared, summed and the square root extracted. That willbe the RMS of the voltage. Before extracting the root, same sum of the squares times 300 ohm is the power dissipated.

Again, only possible with true RMS DMM, else the averaging DMM can gve anywhere 10 to 40% error on complex AC forms.