Human disadvantage versus automation

One underlooked factor in the man vs. machine debate is 'uptime'.  A computer server or an industrial robot may have an uptime of for arguments sake 99%.  A week has 24*7=168 hours.  But a human being only works 8*5=40 hours.  And with 5 weeks vacation a year, 1 in every 10 days they are not there.  Sick leave adds another 2 weeks off a year.  Then there is maternity leave, and a growing issue is long term disability leave.  And I almost forgot statuatory holidays.

Finally a big time business leader I read said the average worker, 'works' 3 hours out of the 8 hour shift. 

So say a big box grocer is installing automatic checkout machines.  Its not just replacing one worker.  The store may be open 7am-10pm, 7 days a week.  That is 105 hours a week.  And would take at least 2.5 full time workers to cover.  And more like 3-4 once you count all the factors I mentioned.

An added and even less appreciated factor is the reduction in management costs to deal with this.  There is accounting, human resources and layers of management needed, to administer human related issues.  When you replace enough workers, you can also get rid of a great deal of the administration above them.

Humans also require facilities, such as more washrooms, office space, parking space, security systems and so on.  All of this can be reduced as the workforce gets reduced.