A coefficient expressing the relation between a change in a physical property and the change in temperature that causes it. Solar Panels have negative temperature coefficients which means that when the temperature rises and solar panels heat up, the power output of the solar panel decreases.The rated capacity, or power, of a solar panel (e.g. 250 Wp) is measured at 25°C. The effect of temperature on the solar panel’s power is measured by its thermal coefficient, expressed as %/K or %/°C. It denotes the % change in power for 1 degree change in Kelvin or Celsius (both are the same on a unit level) above 25°C. A negative (-) sign indicates the direction of the change. A temperature coefficient of -0.447 indicates that every 1°C increase in temperature over 25°C will cause a 0.447% decrease in power. Equally, every 1°C decrease in temperature over 25°C will cause a 0.447% increase in power.