I am not sure of the full specifications - only that it is part of their Dacta range - the eLab sets specifically.
According to the Lego Site:
The solar cell consists of seven (7) solar cells and a diode. They are connected in serial so the total volt = 7 x 0.4 V (a total of approximately 2.8 V).
Lego also offer the following cautions for its use:
Although the standard Lego Motor (with 5225, RCX etc) runs at 9v, it can be run from these lower voltages. Combine this set with the Lego Capacitor and a few other components - and you can build a Lego BEAM Robots example.
Thanks to Peeron for set and part information.
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.