Hello,
for the case there should be a power outage, will the entire blockchain be downloaded again or only the changes (since the outage)?
Thank you!
Most commonly a power outage results in irreversible corruption of data bases relevant to blockchain copy on you disk thus IBD will be inevitable fact of life if you don't have the backup.
I have backup 2TB SSD disk (which I synch on regular monthly base) for such cases. Thus should the outrage happens I will copy the content of this disk to SSD which is attached to my bitcoin node.
I think UPS or replacement battery is cheaper than 2TB SSD and also a better solution. Losing power can lead to data loss and silent corruption all over system, not only Bitcoin.