M

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.