It's 1 window with 3 panes.
Some windows have only 1 pane; some have 6 panes.
Why not charge per pane (30/50p per pane), with larger panes sub-divided into a number of 'imaginary panes', each one a 'metre square'?
It often works out about the same as the '£1 per window' method - but if most of the windows are large, or with multiple panes, you will be getting the extra money for the extra work.
Mike