It won’t show what the usage of the free Google photos is.
I’m thinking try a Google gdpr data export of your data , if it includes the photos you can get some idea hopefully.
Edit;
Includes your photos, so should be good to go, depending on your internet connection and hard drive space.
Honestly, if i did this i wouldn’t have anywhere to store my pictures lol there is too many. The good news is all my current pictures are safe. It’s just trying to calculate what i will need for the future 
You can go to the site, deselect everything except photos, start the download and see what the file size is and cancel maybe.
I’ve just seen, it lets you transfer direct to onedrive , instead of downloading which is a nice feature, and may work for you.
Ahh, gotcha. My phone must be set up to sync with original files, then, instead of high res.
The export suggestion sounds like a great idea. You can select what you export so can select to export photos only, hopefully (I did an export of my Google Music data the other week because I wasn’t sure what I had in there; nothing, as it turned out. Quickest Google export I ever ran (most times it takes so long I forget I started it and so forget to download the export before the time is up 
)).
I highly rate OneDrive.
A few months ago I was looking at my old Google account and noticed there were some photos on there from 2011. I deleted them all after downloading and I still had a few GB of storage used. I literally had deleted everything and yet more things kept appearing in photos such as wallpapers I had literally never seen in my life.
OneDrive beats it by just acting as a cloud drive, with things like Word and Photos on top but the drive itself is there under it all. You can easily see what files you have and where they are etc. I feel like that with Google your files can be spread across many products, not at all easily organised etc.
For example your photos don’t appear in Google Drive, but count towards your drive limit? It’s just an odd way of doing things.
From the article linked above:
The company will send alerts and warnings when you begin to approach that cap. Google is also putting new storage management tools into Google Photos, including a tool that makes it easier to find and delete photos you might not want anyway, like blurry images or screenshots.
Google is also going to show a more useful “personalized estimate” of how much longer a storage tier will last in terms of time instead of gigabytes. It estimates each user’s average uploads over time to guess how much longer they’ll be able to use their current tier.
“Never say never” 
On a more serious note, I can understand them, probably costs piled up over time and it became very difficult to justify the “unlimited” cap from a business perspective. I would have hoped for something more than 15GB though
I don’t think it’s pot stirring at all. Google got people hooked on the service and now that people are dependent on it (many people aren’t so savvy to have 3-4-5-10 backups of the same photos), Google will happily ask them to pay for continued use if they don’t want to
a) go through the additional faff of downloading the photos, with messed up metadata, and upload them somewhere else
b) have their memories split up between different services
A scummy move that can’t be defended at all
And there’s the little * because we all know Google changes their mind every day so it might just take the existing storage in a year or two when everyone forgot
But when Monzo charge for previously free features, that’s fine and entirely forgivable? lol
No, I never said that. And if you maybe read my other comments on this forum I have long lost all sympathy for both companies
The email that Google sends informing you of the change in policy includes a link to a customised estimate of when they think you will exceed 15GB
Perfect, thanks for that. It would be nice if they let us see that before they change the storage rules but i guess its better than nothing. At least they have kept our existing pictures exempt. That’s nice of them
I don’t think I’ve ever seen a Google Photos ad before. I wonder if it has anything to do with the free tier going away soon
Shame the actual video seems to have been taken down.
