The "27 W" minimum power is the same for the Zf and my Z6 iii. But my tiny Anker PD charger has two 20w PD ports, and this does light up the orange power light near the left strap eyelet. The thin USB-A cable that came with it works fine on the Nikon.
I don't understand the "27W minimum" either.
My MH-25a came with my D780, the Nikon charger for EN-EL15 b or c, has a rated output of 8.4V--1.2A, equivalent to only 10W.
I normally charge my EN-EL15 b or c in my D780 via the USB port from my Samsung wall charger of 5V--2A, also 10W output.
However, the Nikon wall charger EH-7P has rated output of 5V--3A or 15W.
Sometimes, I even charge the D780 using the 5V USB output on my laptop.
I see that it does charge my EN-EL15c battery, but I didn't time a full charge. (I'm guessing it takes "hours" with this 20w charger). I normally charge batteries with the external charger that came with my old Z6.
I never timed the battery charging either. Charging of battery in my D780 never takes very long, probably an hour or so. However, I never drain the battery down to minimum.
According to the D780 manual, the 10W MH-25a can fully charge a EN-EL15 b (1900mAh) in 2 hour 35 minutes from minimum.
It will take a bit longer to charge a EN-EL 15c, which has a higher rated capacity of 2280 mAh.
Just like to point out the rate of charging is not determined by the "power" rating. The rate of charge is determined by the difference between the charging and residual voltage and the internal resistance of the battery. The charging rate will slow down when the voltage of the battery starts rising.
The power drawn in W, is the square of the "Voltage Difference" divided by resistance in Ohm. The actual Energy stored, in mAh x V, by the battery is the Energy, power x time, drawn less heat loss.
Also charger will limit the charging current if the residual voltage is too low to avoid over-temperature if the charging current is too high.
Also wish to point out the nominal output voltage of the EN-EL15 b or c is 7.4V. The required charging voltage will need to be higher.
Camera will accept 5V output for USB charging. I understand the camera will be able to step-up the charging voltage to higher than 7.4V.
For the battery to be charged out of the camera in a charger, the output voltage of the charger will need to be higher than 7.4 V. I don't know about other charger, the voltage output of the MH-25a is 8.4V.
The charging voltage probably should not be much higher than 9V or the over-voltage protection will kick in to stop the charging.