I can fit my laptop plus charger and mouse, work documents, a change of clothes, and lunch stuff all in one big pannier. But I don't always necessarily like all that weight on the one side of my bike so sometimes will break it up between the backpack and the pannier. You don't have to commit to one specific way of doing things all the time. The more tools you have at your disposal the more you'll dial in what's right for you in each situation. Sometimes I don't carry anything but my tool kit, pump, and water. I've never had a trunk bag or a handlebar bag.