Good topic, AlanF, as battery life has been a puzzle for many R5 owners.
We do need some way to measure and compare, but with the R5's many options, I'm not sure that just number-of-shots reveals enough. Two days ago, for example, I had time throughout the day to sit with the R5+100-500mm on a tripod--no IS. The rig was set up pointing out of our breakfast-nook window at birdfeeders, a birdbath, and loquat tree. This meant that anytime I had a chance and saw birds, I could sit on a footstool and wait for good shots.
At first I was using EFCS, but after lunch I thought to try just ES (as suggested by Whistling Wings' latest video about photographing tiny birds). By late afternoon I had taken 1040 shots and my battery still had about 40% left. Of course many of the shots were taken in bursts of 5-15, but I did spend more time than I ever had in a single day looking through the EVF. When I had to cook, do yardwork, or play with my son (home with a cold), I turned off the power.
I was using ECO mode.
(And, btw, I had my first ever issues with freeze-up! Three times, while not shooting, but just going from back-display to EVF. Very strange. All happened in the morning but not again after that.)
So, no IS, but lots of EVF and display use. I never have Wi-fi, etc. on--just what is required for capture and review.
But if I'm out walking for 90 minutes, with IS enabled, I can be down to 50% after only taking several hundred shots.
I don't think it's how many shots we take, but how we are using the body+lens. I think we can only use number of shots to determine if one body is using more than another if both bodies are configured and used in the exact same manner for the same amount of time.
Have you tried several different batteries with the same results?