Duplicati backing up to Backblaze B2 costs less per year than a Backblaze Personal Backup subscription only below a dataset size you can calculate in one line, and that line is the whole decision. Personal Backup charges a flat annual fee per computer for unlimited data, so its cost curve is horizontal. B2 charges per gigabyte stored per month plus API calls and egress above the free allowance, so the Duplicati bill starts near zero and rises in a straight line. Divide the annual subscription fee by twelve times the B2 per gigabyte monthly rate, and you get the crossover in gigabytes: below it Duplicati wins every year, above it the subscription wins and keeps winning harder as you add data.
TL;DR
The 400 GB laptop owner, one MacBook holding documents, photos and a code folder: run Duplicati to B2, because a few hundred gigabytes billed per gigabyte per month sits far under any flat annual fee, and Duplicati compresses and deduplicates before you are billed.
The 6 TB media keeper with a film and music library on external drives: stay on Personal Backup, because unlimited flat pricing per computer is unbeatable at that volume and B2 charges for every terabyte forever.
The two person household with three laptops totalling 1.5 TB: run Duplicati into one B2 bucket, because the subscription is licensed per computer while a single bucket and three Duplicati jobs cover all of them under one bill.
