I’ve just had a look at the link to the backpack….IMO that backpack is as good as useless for working with all day. You really don’t want to be working with a backpack ( they should be renamed😁 ) on your back. When it’s full of water it’s over 25kg, that puts a big strain on your back and neck, especially when you’re doing upstairs windows. If he must go down the backpack route he’d be better with a Gardiner style one which has a carry handle. Yes it can be heavy to carry if using without a sack truck, but at least you can put it on the ground when you’re cleaning the windows.