Both! Hard to say as I've never hooked it up to a proper MPPT to verify, but I suspect my DC-DC is only 80% efficient at best. The panel is '40W' by Chinese standards, so really probably only 35W. I also have a long run of wire through the frame to where it connects to the battery and it's only 18AWG so again, not the best between all those things.
That being said, in optimal conditions it can charge around 100Wh+ per day. Considering the battery is only ~700Wh (was 850Wh, but 400 cycles and 5+ years of heavy use has degraded it), and the bike can manage 12Wh/km, that's around 8.3km additional range per day. If the battery were completely flat it could full charge back up from solar in about a week under optimal conditions. I've never tested that though.
Anyway, the point of this thread is the aero...and I gotta say, while I wasn't expecting it, the benefits of the aero this panel gives on this bike when riding are fantastic. It prevents my body acting like a wind sock, it helps keep my hands warmer in winter, and it deflects the rain (which I usually try and avoid) away from my displays and torso.
I have had a few other panels on in the past with a 15W being my first try, then 30W, and now for the last 3 years or so this 40W. If I need to replace this one I might try a 50W which is slightly longer. The challenge then becomes fitting it so that I don't kick it with my feet, and low enough to see over.
I have also considered getting two panels and mounting them into a V shape. Unsure how that kind of setup would fair though...I am not gentle on things!