Yup, I came here to second Arq as well. I back up to a local Windows PC which happens to be always on. Nowadays, it's very easy to enable SSH on Windows, create an account for backups, and then point Arq to that.
The only thing that got me, is that if you configure Arq to use SSH on Windows, you have to enter disk volumes as path.
For example, if you want your backups to appear on Windows in D:\BigDisk\Backups then configure Arq to back up to /D/BigDisk/Backups. That's not an Arq thing, that's how the OpenSSH server interprets paths on Windows.
The only thing that got me, is that if you configure Arq to use SSH on Windows, you have to enter disk volumes as path.
For example, if you want your backups to appear on Windows in D:\BigDisk\Backups then configure Arq to back up to /D/BigDisk/Backups. That's not an Arq thing, that's how the OpenSSH server interprets paths on Windows.