So, I have an 18W fluorescent fixture with non-Philips preheat ballast that is labeled for 18W fluorescent lamp. But, when I tested it with watt meter, it says that it uses 39,7 watt. So my question is, does preheat ballast use more electricity than it's rated or because I'm using a non-Philips ballast that maybe it's for 40W tubes but labeled 18W and I'm over-ballasting it?

Inductive ballasts are relatively inefficient. It indeed likely uses about 40 watts. They can, however, specify a lower consumption. This has to do with the power factor. Back in the day, the standard analog electricity meter with the slowly spinning disk, did not pick up the consumption by inductive loads like old school ballasts, so you didn't pay for the power that you kind-of are usung. They would only 'see' the 18w of resistive load the lamp takes.

This power factor stuff is why all professional lighting installations with inductive ballasts include a capacitor to bring the power factor closer to normal. Because although the meter doesn't see the load, the generator in the power station most certainly does, and your wiring does too. We used to have a tanning bed with 10x 100w fluorescent in it, but no capacitors. Even though it only was specced at 1000w, it heated up its mains cord like crazy because apparently the designers thought 'Huh, about 5 amps, we can send that through the cheap thin 6 amp mains cord' while the actual power consumption was closer to 10 amps if you include the inductive load.

Modern digital meters *do* pick up on loads with weird power factors. It's al a very complicated story, but in the end it boils down to 'ignore it if you have an analog electricity meter, try and improve it if you have a digital meter and gotta pay for the extra power usage'.

The reason why inductive ballasts are used and not just resistors, is that the heat production of a resistive ballast would be huge, and you can't use the inductive kickback to ignite the lamp.

A similar thing - but in reverse because the load is capacitive - can happen with LED drivers with a capacitive dropper circuit. If you put enough LEDs with capacitive dropper on the circuit with the fluorescents with inductive ballast, you end up compensating the inductance with the capacitance and the power company is happy again.

So, I have an 18W fluorescent fixture with non-Philips preheat ballast that is labeled for 18W fluorescent lamp. But, when I tested it with watt meter, it says that it uses 39,7 watt. So my question is, does preheat ballast use more electricity than it's rated or because I'm using a non-Philips ballast that maybe it's for 40W tubes but labeled 18W and I'm over-ballasting it?

I think 18W is the rated lamp wattage and it doesn't include ballast losses. Also watt meters might be inaccurate when used with inductive or capacitive load.