Rated vs. actual power usage of a linear fluorescent lamp
Also consider that many LEDs can bring up the power quality problem. LEDs are bringing higher harmonics to the system, and cutting those out may need designing a low-pass filter. [1]

If you want to read more about Power Factor correction, you can look at [2].

Also your electricity provider may want to measure the power readings from time to time if it's a company building, and if it discovers that you're flooding the grid with harmonics high fees may be brought up on your company, as you're decreasing grid power quality.

For AC circuits use P=U* I* cos(phi) to measure the real power. If you want to measure the cosinus(phi) you can measure the real power(P), then measure RMS values of current and voltage, and from that calculate the apparent power (S) by multiplying I(RMS) times U(RMS). Cosinus phi would be a P/S. There are also measuring instruments that can measure cos(phi) by them own.

[1]http://pe.org.pl/articles/2012/11a/61.pdf

[2]https://www.ecnmag.com/product-release/2011/08/power-factor-correction-techniques-led-lighting