The op amp give the rms voltage value on the resistor.
Vr = 3.86 V
Vpeak= Vr / PWM = 7.72 V
Ipeak = Vpeak / R = 15.4 A
Ppeak on resistor = R * Ipeak^2 = 118.58 W
Prms on resistor = Ppeak * PWM = 59.29 W
Irms = Ipeak * PWM = 7.72 A
Total Power = Vbatt * Irms = 64.85 W
Pmosfet = Ptot - Prms = 5.56 W
Rds₍ₒₙ₎ = (Vbatt - Vpeak) / Ipeak = 0.044 ohm
Monitoring the battery voltage , and resistor voltage, you can simply change the pwm signal to change the power.
