Is that running constantly for 24 hours or what she actually used making several trips over a 24 hour period?
Might not be so far off actually. In the early days, running at full speed, and making 8 trips in 24 hours. Reduce that to 1 trip a day and throttle her back, like she did in her later years. OK so I (or the place where I read it anyway) was a bit out, but perhaps 40 tonnes? I'm not going to do accurate maths to find out.