All the cameras use the battery all the time (circa 2% a day if you leave them in “optimal battery life mode”) and, if they have inbuilt panels and get enough sun, 300mAh will give them 2% back.
I have the EufyCam 3 (with built in panels), but one camera is in a shady place, so doesn’t get much sun.
I’ve put a panel up in a place which does gets sun, run the cable back to it, and all is good.
What I’ve noticed is that although I’m getting charge from the panel, the App “Solar Dashboard” only seems to show the charge obtained from the cameras own panel.
But I’ve had the cameras up for 142 days now, and currently have 52% charge on the camera which does get sun, and 59% on the one with the panel (which otherwise wouldn’t). The latter gets less movement too, so it uses less power