This is actually an interesting question. I think it comes down to two things: 1) Frequency of use 2) Overall power requirement Calculators are kind of an i

This is actually an interesting question. I think it comes down to two things: 1) Frequency of use 2) Overall power requirement Calculators are kind of an ideal use for solar panels because people generally only use them for a minute or two at a time and often won't touch them for days. And on top of that, modern calculators use so little power it's frankly ridiculous. We're talking a couple hundred microwatts during use. In comparison, I have trouble thinking of devices with equivalent usage and power characteristics - most devices we use intermittently have pretty high power requirements while in use, especially when you consider that indoor lighting is awful for generating solar energy.