20 mW average maximum available power supply for headphones. Add some for the circuitry and radio, I bet you can manage at 500mW.<p>1368 W&#x2F;m^2 solar energy on earth on average, or 0.137 W&#x2F;mm^2 = 137 mW&#x2F;mm^2. Let&#x27;s assume 15% solar cell efficiency, so you&#x27;ll get 20 mW&#x2F;mm^2 out.<p>500mW &#x2F; 20 mW&#x2F;mm^2 = 25 mm^2. That&#x27;s how large cells it be exposed to the sun.<p>Cover both the headphones and cables, and you&#x27;ll be able to charge any batteries enough to survive the night.