• In mains electricity, current and voltage are varying all the time
• This also means the power varies constantly, recall the equations for power:

• Where:
  • I = direct   current (A)
  • V = direct voltage (A)
  • R = resistance (Ω)
• The r.m.s values means equations used for direct current and voltage can now be applied to alternating current  and voltage
• They are also used determining an average current or voltage for alternating supplies
• Recall the equation for peak current:

• Therefore, the peak (maximum) power is related to the mean (average) power by:

• Therefore, it can be concluded that:

 The mean power in a resistive load is half the maximum power for a sinusoidal alternating current or voltage

Worked example: Calculating mean power

Step 1:            Write down the known quantities

Resistance, R = 40 Ω

Peak voltage, V₀ = 240 V

Step 2:            Write out the equation for the peak power and calculate

Step 3:            Calculate the mean power

• • • • The mean power is half of the maximum (peak) power

Mean power = 1440 / 2 = 720 W