I just found this on the Holosun web page:
"How do the solar optics work? – In automatic mode our solar optics automatically switch to battery power whenever there is not enough light available to power the reticle. In manual mode, the optic will always use battery power. The “solar panel” on top of our optics is actually a photo-diode array (basically a light sensor). It controls both the auto brightness adjustment and provides solar power to the product. Sometimes our customers try to test if battery power is working by covering the panel to see if the reticle will still be there. The problem with that test is that you trick the optic into thinking that you are in very low light conditions. The optic then lowers the reticle intensity down to night vision mode, which means that the user is unlikely to see the reticle in normal lighting conditions. The best way to test if the battery is taking over is to go into a pitch black room and see if you still get a reticle. (In darkness, you should be able to see the night vision intensity setting.) Since the solar panel is also the light sensor, be careful not to cover it up when testing reticle brightness. Also, keep in mind that the reticle intensity will be set to the lighting conditions of the shooter, not the target. If the target is in very different lighting conditions, you should switch the optic to manual mode and adjust the brightness to the appropriate level"