The physics
In AC the power swings every cycle. A wattmeter shows the average power.
P = Vrms Irms cos φ = Irms² R
Here cos φ is the power factor. In a series circuit it equals R/Z. Only the resistance turns power into heat. A coil or a capacitor stores energy and gives it back.
cos φ = R/Z Z = √(R² + (XL − XC)²)
The voltage across R is in step with the current. A coil's or a capacitor's voltage is a quarter cycle away. So the two add like the sides of a right triangle. A choke drops volts without heat. A resistor in its place wastes I²R.
V² = VR² + (VL − VC)²