Answer:
The average solar calculator produces between 50 and 100 microwatts. A typical solar cell might produce 20 - 40 microamps at between 2 and 3V.
In practical terms, this is an incredibly small amount of electricity. You could easily generate that much electricity with a small hand generator.
Graphing calculators (which are never solar) use far more electricity (around 0.3 watts or 300,000 microwatts, which is aronud 4,000 times as much). This is why these calculators are never solar. To use solar cells they would need a solar panel 4,000 times bigger than the one on the non-graphing calculators, and that would be too big to fit on the calculator!