I noticed that with the panel covered, I still have somewhat warm water, although it's not the same as when it was uncovered.
Attention, even though there are holes visible in the photos for tying, it does not include straps and this made it difficult for me to secure it properly and prevent it from coming off with the wind. The best solution I found was using luggage elastics ("octopus").
At least it was a very economical choice, otherwise I might have returned it when I saw that it didn't have straps.

I bought it to protect the solar panels from the sun during the summer. The panel with the cover raises higher temperature compared to the other panel that is exposed to the sun without a cover. Maybe it's because I am plastic. Maybe it's suitable for winter, but I don't think it's suitable for summer, at least for me.