Standard power supply voltage

[001]

We know that common USB charger voltage is not 5.00000 volts but 5.1-5.3 to compensate wire loss
And automotive radio PS marked '12V' in fact about 13.8V for same reason

What about 'real' voltage for 3.3/9/15/24 volts power supply?

[mariush]

No, we don't "know".

Some usb chargers have sense wire all the way to connector so that output will be 5v no matter the load
Others will just use a 5.6v zener diode as voltage reference

Car stuff is more like "whatever battery or alternator gives" with additional allowances for spikes or reverse voltage periods ... auto power is just ugly.

if you mean atx power suply voltages they're standardized , just read the atx specification, it's available on the net a site with "form factors" or something like that in name has it

[frog]

In general, power supplies should output pretty much exactly the voltage that they claim; in reality there will always be a tolerance, typically 5% or better and you'll see this in the datasheet for the device.  On high-current supplies you'll sometimes see 'sense' inputs; the purpose of these is that they can be connected to the point of load so the supply can adjust the output to make sure the load sees exactly the right voltage.

Incidentally, vehicle electrics are 13.8 because that's the normal charging voltage a 6-cell lead-acid battery.  The idea that it's 2v per cell (12v total) is an approximation and depends on the load and state of charge (etc).  You may see bench supplies designed to drive automotive equipment such as radio transceivers that specify a 13.8v output.