It is totally up to you what your body is worth to you. For me, it was very important to buy a backpack that feels good to carry even with 15 Kilos on my back since I have back problems. 15 kilos are easily achieved with technical equipment such as laptop, hard drives, power bank, kindle, wireless speaker and leads etc. I went to an outdoor equipment store and have well-advised. I tried 4 different backpacks. My choice was the Deuter Aircontact 45+10 Trekkingbackpack. It felt very comfortable and has no hard plastic or metal parts (which makes it easy to repair anywhere in case something rips according to the shop assistant). I tried the backpacks on filled with 10-kilo ballast and walked up and down to floors on the stairs and I totally recommend you try that too.
There are many more tips on the Deuter website such as:
(Note: I am not paid by Deuter for this post)