I have an FT-857D and have messed with the idea of that rig at 50W output with the Bioenno 12V 15Ah battery.
My 12AH Bioenno battery lasts for two hours of constant use (CW/SSB) with the FT-891.
I have some power usage numbers from 22 park activations this year.
All CW, all on Wednesdays for 2.5 to 3 hours, TT Eagle at max output (90w on battery voltage, 1.2A rcv, 15A xmit). I call CQ the whole time I'm at a park at 24-25 wpm (slowing down when answered by slower ops), and for 1 hour (1900-2000z) I operate the CWops CWT, which is constant CW at 30 wpm. All these park activations have been between 130 and 226 CW contacts.
I have one of those inline DC power monitors BUT I only use it for charging the battery at home. It has Powerpole connectors and since every connection makes a slight voltage drop, I don't use it when on the air. A heavy 4 ft DC cable connects the radio to a 40 Ah LiFePO4 battery.
The most power it has taken to CHARGE the battery after an activation is 21.0 Ah, the least has been 17.0 Ah. The lesser Ah was a 2.5 hour outing, still CQing the whole time. Sometimes during the 1 hour CWT I have worked as many as 112 stations (twice), usually it is around 90, and sometimes 70 or so.
A 20 Ah LiFePO4 battery would be near or at it's limit for most of my park activations.
Glenn AE0Q