You have to look at the bigger picture.
You cant run a business just for your own wage, you have expenses to consider.
Take a look at your costs - include absolutely everything - work out how much you think they cost you, per month, week or even per day.
Add on top what you would like to earn per hour, day, week, month etc
You then have some idea as to what you need to "turn over" per hour, day, week, month etc to make your business a true business.
Don't forget to add a profit on top, and if thinking about growing the business, think about costs of marketing etc too.
To give you a good idea, many of the larger window cleaning companies aim for 33 pounds + per hour !