Recommend a backpack for commuting to work

I know you said back pack, but if your bike can take a pannier rack, I'd recommend that.

For waterproof backpack, my 25 year old sporthouse is still going strong.

Otherwise, I put water sensitive bits in a bag in a bag.

I use Ortlieb panniers, they have equivalent backpack https://www.ortlieb.com/uk_en/products/backpacks#

Is it just a change of clothes? Bike packing bags are another option. https://eu.alpkit.com/collections/bikepacking (not necessarily alpkit - just there for example. I use an ortlieb bikepacking bag.)

I use an Osprey Escapist on my commute, 22km each way with a decent amount of climbing. Some days it has 2 laptops, keyboard power supplies etc and clothes, other days it just got my lunch in it.

I have been tempted to get panniers but I’d need to make adaptations as my bike won’t take them normally.

it’s a great bag also for lugging to site or travelling. I have have got an unending amount of gear in it going on an industrial site and there’s a handy helmet clip on the outside plus separate dirty and clean gears compartment.