The General Idea Is Good
The general notion of making your backups inaccessible to malware is solid. There are multiple ways of going about doing that and it doesn't necessarily have to involve turning your backup drive off.
So it isn't a bad idea to make the drive, or at least the specific directories where the backups are stored, inaccessible.
Malware Masquerades as You
Here's the thing about malware. If you get malware on your computer, it gets to run with all the same permissions that you have, since it is running in your security context.
If you, personally (or any other user on your network) have write access to your backups, and you get malware, that malware also has write access to your backups. So, for example, ransomware could encrypt your backups and make them useless to you.
Run Your Backup Job In a Separate Service Account
If your backup job is running from a different account, and that account is the only one with write access to the backups, then even if you inadvertantly install ransomware, it cannot change your backups. This is regardless of whether the backup drive is turned on or off.
Also consider that if you just power your backup drive down abruptly, you could lose data that is in cache on the drive, but hasn't been written permanently to disk yet.
And, finally, consider that if you do have malware and your account has write access to the backup directories, then your backups become vulnerable when the drive powers up to run the backups, even though the drive is powered down for part of the day.
Either way, using a service account for running your backups, and denying write access to your backup repository to every other account including your own (or especially including your own) is a good practice.