I think that you may be a bit confused about the purpose of the iCloud Photo Library.
iCloud Photo Library is designed to keep all iOS devices and Macs that are signed into the same iCloud account, and which have iCloud Photo Library turned on, continuously sync'd. So, that any change made at any time would be automatically sync'd across all devices immediately.
So, iCloud Photo Library is a syncing service, not an archival one.
In order to accomplish what you have described, it would be far easier to import your photos from your iPhone to your Mac. You can do this in a number of ways, but the easiest way is to physically hook up the device to the Mac, and import them photos to the Photos app:
You can also set up Photo Stream on both the iPhone and the Mac, and all new photos will automatically upload and import into your Photos Library on the Mac. Videos still have to be imported manually, but photos automatically import when you have a Wifi connection on the Mac and the iPhone.
You can then back up your Photos Library on your Mac using Time Machine, or by copying it to an external drive manually.
Here are some links to articles about iCloud Photo Library that also may help you get a better idea of how it works:
iCloud Photo Library Help - Apple Support
iCloud Photo Library - Photos Help
I, personally, do not use iCloud Photo Library. I let my photos automatically import via Photo Stream to my Photos Library on my Mac, I sometimes create shared albums to send to my Mac via iCloud Photo Sharing: iCloud Photo Sharing - Apple Support, and then I import my videos on a regular basis.
I also back up my Mac Photos Library using Time Machine.
Cheers,
GB