I am currently using a similar solar usb charger on my setup.
It does work in that the battery charges quite quickly in bright conditions but when it is not charging the unit seems to drain power. I would say on average the battery lasts about 50% less with this attached. In the middle of summer this was fine but even now barely past the equinox I get more and more device off line warnings as the battery runs out.
Admittedly the panel is facing due east and is mounted to a window so the angle is not ideal but even so I would not expect a solar charger to draw power when not charging.
Here’s a picture of my current setup: