This model is a bit of an enigma since there is little information about it and the solar cell is large for the year of introduction, since there had been development of more efficient cells.
Perhaps, if you want to add a
solar cell to a credit-card sized calculator and need to make it a folding type, then you might as well use a cell which takes up the whole of the side for maximum sensitivity.
The folding design and the photocell are the same as that of an early Teal model (the Photon III), but the keys and display are different. The year when the EL-825 was introduced is given as 1981,
and, from observation of examples of this model appearing on the auction site eBay, it may have just been sold in part of the U.S.A.
If you have any further information about this please get in touch with us.
In 1976 Sharp had introduced the first calculator with solar cells, the EL-8026 "Sun Man". However, this was not purely solar powered because the
calculator used rechargeable batteries and the solar cells were on the back of the calculator and were used just to recharge the batteries.
The early solar cells were still inefficient. This one requires quite bright light before the display appears and the calculator operates. Soon, with improvements in the technology (and also
in the power requirements of the electronics), the size of the solar cells dropped to a fraction of this one.
It is interesting to see how the designers have managed to produce a credit card calculator with this large size of solar cell by folding the calculator in the middle.
Also see the TEC 205G below, and also the Sharp EL-8026 and Teal Photon, two of
the first solar powered calculators, and the Sharp EL-826, a contemporary calculator with a more efficient solar cell.