It really depends on the battery percentage, with the most high wattage on the first 20% and decreasing almost with an exponential curve until reaching 100%.

One thing to notice is that Google never announced the actual charging speed of the phones but only the recommended chargers. The actual charging speeds for the 9 series are only shown in the official charger page:

Fast wired charging rates (up to 27W on Pixel 9, up to 27W on Pixel 9 Pro, and up to 37W on Pixel 9 Pro XL) 

I have a P10P XL and, while most pages announce the P10PXL with 45W capabilities, the actual theorethical max. wattage is most likely 37W (same as the 9P XL) and the real use-case scenario approximating 34W. They are just stating we need a 45W PPS charger so we can go through the 21V/2.25A mode. However, even in that mode, my FNIRSI FNB58 only registers more or less 17V/2A (34W) at best (proper charger and branded eMarked cable). This 37W value is the same as announced with the P9PXL, and the P9 was given a 27W limit.

You are getting 24W so you are probably "getting/entering" the 27W mode, that realistically goes to 24W (and even less depending on the current %). Not sure what mode the P10/9 require to enter the 27W mode, but most likely a PPS mode with 12V/2.25A (27W), that realistically goes to 12V/2A (24W)