Wafer - A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.

Watt - The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

Waveform - The shape of the phase power at a certain frequency and amplitude.

Wet Shelf Life - The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

Window - A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.

Wire Types - See Article 300 of National Electric Code for more information.

Work Function - The energy difference between the Fermi level and vacuum zero. The minimum amount of energy it takes to remove an electron from a substance into the vacuum.

Z

Zenith Angle - the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).