Maintenance-Free Battery - A sealed battery to which water cannot be added to maintain electrolyte level.

Majority Carrier - Current carriers (either free electrons or holes) that are in excess in a specific layer of a semiconductor material (electrons in the n-layer, holes in the p-layer) of a cell.

Maximum Power Point (MPP) - The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 volts.

Maximum Power Point Tracker (MPPT) - Means of a power conditioning unit that automatically operates the photovoltaic generator at its maximum power point under all conditions.

Maximum Power Tracking - Operating a photovoltaic array at the peak power point of the array's I-V curve where maximum power is obtained. Also called peak power tracking.

Megawatt (MW) - 1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

Megawatt-Hour - 1,000 kilowatt-hours or 1 million watt-hours.

Microgroove - A small groove scribed into the surface of a solar cell, which is filled with metal for contacts.

Minority Carrier - A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

Minority Carrier Lifetime - The average time a minority carrier exists before recombination.

Modified Sine Wave - A waveform that has at least three states (i.e., positive, off, and negative). Has less harmonic content than a square wave.

Modularity - The use of multiple inverters connected in parallel to service different loads.
Module - See photovoltaic (PV) module.

Module Derate Factor - A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.

Monolithic - Fabricated as a single structure.

Movistor - Metal Oxide Varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.

Multicrystalline - A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.

Multijunction Device - A high-efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.

Multi-Stage Controller - A charging controller unit that allows different charging currents as the battery nears full state_of_charge.