Maybe the right thing to do is to measure from ideal room temperature? From zero doesn't make any sense but setting an anchor temperature makes sense.
This is done when people rate the efficiency of home heating: SCOP is a function of the heat pump's ability to hit a particular temperature, for instance.
I'd guess the baseline temperature on the tube should be 21C maximum. Percentages don't make sense here, but 7C over the target temperature (for instance) is pretty bad in those terms. I'd be surprised if TfL hadn't set that somewhere.
Also worthy of note is that it sounds like the tube is a prime source of heat for a district heating system. Win win, perhaps.
This is done when people rate the efficiency of home heating: SCOP is a function of the heat pump's ability to hit a particular temperature, for instance.
I'd guess the baseline temperature on the tube should be 21C maximum. Percentages don't make sense here, but 7C over the target temperature (for instance) is pretty bad in those terms. I'd be surprised if TfL hadn't set that somewhere.
Also worthy of note is that it sounds like the tube is a prime source of heat for a district heating system. Win win, perhaps.