We demonstrate a wireless temperature sensor consisting of only a chip with an ARM Cortex-M0 and radio, a mm-scale solar panel, and one surface mount capacitor. The radio lacks a crystal reference in favor of other on-chip oscillators to reduce cost and size. Without an absolute time reference, the on-chip oscillators need to be calibrated because their frequencies vary with supply voltage and temperature. An initial optical calibration procedure is used along with temperature-based compensation. This enables the radio to properly transmit standards-compatible IEEE 802.15.4 wireless packets over a temperature range from 35.5 C to 40.0 C. The standard deviation of the temperature estimate error relative to a reference is 0.28 C. The power for all of the components, including the radio and the microprocessor, is supplied by a solar panel on a CMOS chip under 200 mW/cm2 of irradiation.