So, let me give you some rough answers
The TDP is the maximum the CPU can eat - just as @pauliunas mentioned!
Now, the Y-Series also has a lower minimum-clock speed compared to the U-Series which also improves the power it draws (lower clocks = less power)
While I don't have any numbers on the panel, I'd just guesstimate that we have a similar power consumption to the SP4 panel which needs ~5W at 100%, so let's scale it and we're at ~3.5W. (According to notebookcheck btw their WiFi test is conducted with ~60% brightness, which is ~150nits, so I believe that would be even lower)
The metal-mesh should not have any effect on the battery to my knowledge.
WiFi does draw a bit of power - don't have any numbers on it.
The keyboard! While connected to the V (and if it was charged before) it'll just behave like a Surface Type Cover - so it'll draw battery from the tablet.
And now, what battery time can you expect?
That depends on your usage! According to notebookcheck in their WiFi test (60% screen, WiFi on, loading a new webpage every 30 seconds) the SP4 i5 goes to 7h with that. Now, multiply that with 1.25 at least (since we've got a 25% larger battery than the SP4 i5) and that's what we should see at least in the WiFi test of NBCheck - which is at least 8.5h. That is without factoring in newer WiFi cards (that should draw less power), Core M over Core i, and other improvements we made to get better runtime