An easy way to calculate the power needed to drive the speaker uses Paverage = VRMS * IRMS, or Paverage = Power (Watts) = Voltage squared/ Resistance (Ohms). This uses only the resistive part of the impedance.
Watts RMS actually doesn’t exist. The voltage in the formula refers to RMS Voltage (explained below), but the calculated power is average power and it is called Watts.
AC Voltage and Current cycles from 0 to the positive peak voltage, and back through 0 to the negative peak voltage. Since it isn’t accurate to get the voltage or current value by measuring AC Voltage at an instantaneous point, RMS is used as a way to define the effective value of an AC (changing with time) voltage or current.
RMS is not the average, the average of an AC signal is zero because the negative and positive peaks cancel each other out.
V RMS is = 0.7 * Vpeak
Vpeak is = 1.4 * VRMS
Here is a good way to picture it : A lamp connected to a 6 VDC shines with the same intensity as a lamp connected to a 6 VRMS supply. If you connect the lamp to a 6 Vpeak supply, then it is only getting 0.7 *6 VRMS = 4.2 VRMS (same as 4.2 VDC). This example came from here.
A DVM shows VRMS values on the digital readout. This is useful because it makes a nice comparison to a DC Voltage such as for a battery.
An oscilloscope shows the entire cycle, and exhibits the Voltage peaks, both negative and positive.