I gereally agree with Jon, Price it up exactly as you would if doing the whole job trad.
But, or at least in my neck of the woods, a rate of just £25 per hour is really low, way low!!
It does somewhat depend on what you mean exactly by 148 windows too; are there lots of big, plate glass panes? Or lots of little windows?
What is the average size of each window?
And how many of them are being cleaned inside too?
Insides of supermarket windows can be a real pain, often lots of advertising to take off, adhesive to remove, various displays to shift before you can even get at the glass and so on.
My own charge rate, as a general rule, is £1.00 per window, and on commercial work I would expect that to be worth £50 an hour or very close to it.
Ian