The world's first solar-rechargeable tuner is the TC-1S from TASCAM. The TC-1S charges its battery from a bank of solar cells on the front panel in just 25 minutes, and a USB input is also available for quick-charging. From a full charge, the battery works for six hours of constant use and seven years of standby charge in a dark place. It's wrapped in a shock-proof silicone cover that's available in six colors and keeps the unit protected.
The TC-1S has a built-in microphone as well as a 1/4" input for guitar and bass tuning. Its lightning-fast pitch detection is displayed on a wide bargraph display for instant feedback. Four display modes are available, including fine pitch display for 1-cent accuracy and strobe tuning animation. The tuner can be calibrated to an external source such as a piano or other hard-to-tune instrument.
Included with the TC-1S is a nylon strap and clip. Snap it to your guitar or bass gig bag and it will stay charged all the time.
At a price under $40, every planet-conscious musician is going to need a TC-1S.