Absorber - In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).

AC - See alternating current.

Activated Shelf Life - The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

Activation Voltage(s) - The voltage(s) at which a charge controller will take action to protect the batteries.

Adjustable Set Point - A feature allowing the user to adjust the voltage levels at which a charge controller will become active.

Alternating Current (AC) - A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

Acceptor - A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.

AIC - See amperage interrupt capability.

Air mass (sometimes called air mass ratio) - Equal to the cosine of the zenith angle – that angle from directly overhead to a line intersecting the Sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the Sun is directly overhead and the radiation travels through one atmosphere (thickness).