It's not really possible to give one efficiency figure for the built-in charger because of the many modes and dynamic operation. For full details I refer you to the datasheet of the charger chip used, the BQ24040
@JimT Sorry, I could have told you. You may use a 1N4001 model (or any of the 1N400x series). You have to put that in series to the solar module, Anode of the diode at the + pin of the solar module, Kathode (the side with the ring on the diode case) to the charger module's + input. Doing that, the voltage of the solar panel will be reduced by about 0.7 V. I would add a small load too behind the diode, to cap the no-load case, like by a 1 k resistor at the chargers module input between + and GND. You can use the additional power in lead pads of the charger module for that.
I see that the module has already a built in diode, but that is a MPB120 Schottky type and reduces the Voltage only by 0.3 V.