Those made between wires with the same polarity (positive or negative), and cause current to be additive while voltage remains the same.

A building design that uses structural elements of a building for heating and cooling, without the use of mechanical equipment; requires careful consideration of the local climate and solar energy resource, building orientation and proper window sizing, overhangs, and thermal mass for storage.

A system designed to collect, store, and distribute solar energy without the aid of mechanical or electrical devices.

Passive or natural ventilation optimizes natural convective air flow to distribute warm, cool, or fresh air throughout the house. Properly sized and placed doors, operable windows, and skylights can maximize cross-ventilation and air movement

Refers to the period of time when a maximum amount of power supply is needed from the utility. Typically, peak-demand periods reflect higher electricity rates than non-peak demand periods.

A popular impulse turbine runner--a wheel that receives the jet(s) of water, arresting its motion to convert kinetic energy into mechanical power that drives an alternator, generator, or machine. Pelton wheels are made with a series of cups or “buckets” cast on the edge of a rotating disk.

A financial incentive which pays money for electricity generated, usually designed as payment per kilowatt-hour for a set number of years.

A generating device with permanent magnets (usually ferrite or NdFeB material) in its rotating part (magnet rotor). The changing pattern of magnetism through its fixed coils (stator) creates electricity as the rotor spins. The speed (for a given voltage) can only be adjusted by adjusting the magnet rotors while stationary.

Consists of a harness, a rope or cable, anchor point(s), and specific attachment hardware, and is one way workers who are exposed to vertical drops of 6 feet or more can meet OSHA fall-protection requirements.

PEV can refer to either a personal electric vehicle or, more commonly, a plug-in electric vehicle. Personal electric vehicles are generally small, low-speed single-user products; plug-in electric vehicles can include full-sized, full-featured cars and motorcycles.

A measure used to determine the acidity, neutrality, or alkalinity of a solution--a measurement used for testing propylene glycol.

A plug-in hybrid (PHEV) is a hybrid drive system that allows recharging of the batteries by conventional plug-in charging (rather than exclusively being charged by the gasoline motor, or regenerative braking).

The basic physical process through which a PV cell converts sunlight into electricity.

Systems that transform light from the sun directly into electricity, without any moving parts; also known as solar electric or PV systems.

A method of generating electrical power by converting solar radiation into DC electricity using semiconductors that exhibit the photovoltaic effect.

A method of governing that relies on changing the pitch angle of the blades to reduce their aerodynamic effectiveness and reduce efficiency, and thus to limit speed and production. Blades can be pitched toward feather or stall, reducing or increasing the angle of attack of the blade.

Refers to any car with an electric drivetrain, full electric, or gasoline-electric hybrid. It is most often used to distinguish between hybrids that do and do not allow plug-in recharging, or for vehicles like the Chevy Volt--branded as primarily an electric vehicle, rather than a hybrid.

Testing for polarity means confirming that the positive and negative conductors are in the correct locations; a polarity check needs to be done at each electrical box with all DC sources before commissioning a system.

PV cells made by a casting silicon in a mold; once solidified, it creates many crystals within the ingot.

Water that is drinkable; domestic water.

The instantaneous rate at which electricity is transferred by an electric circuit and is a measurement of work; expressed in watts (W). It can be derived from Ohm's Law by multiplying current and voltage (W = V x A).

The ratio of a module's wattage to its surface area; modules with a higher power density are more efficient.

1 kW = 1,000 W; 1 MW = 1,000 kW; 1 GW = 1,000 MW; 1 TW = 1,000 GW

A power purchase agreement (PPA) is a contract between a producer and a purchaser, lasting between 15 and 20 years, where the purchaser buys energy, and sometimes capacity, from the producer.

A gauge in antifreeze systems used to monitor system pressure.

A reaction hydro turbine that uses a propeller-type runner, through which the water flows axially. This is the type of turbine that can handle the largest flows and the lowest heads.

Requires utilities to purchase power from non-utility generators, including distributed generation such as roof-mounted grid-direct PV systems.

An assembly containing a system pump(s), control, valves, air relief, and sometimes a heat exchanger and expansion tank as a single unit.

The Public Utility Regulatory Policies Act (PURPA) requires utilities to purchase power from non-utility generators, including distributed generation such as roof-mounted grid-direct PV systems.

A group of PV modules or panels wired and mounted together to produce specific levels of current and voltage.

The simplest type of PV system, consisting of only a PV module and a DC load.

Consists of PV cells wired together to produce a desired voltage and current when exposed to sunlight.

According to the National Electrical Code: “a collection of modules mechanically fastened together, wired, and designed to provide a field-installable unit."

Also known as photovoltaic systems or solar electric systems, transform light from the sun directly into electricity--without any moving parts.

Pulse width modulated (PWM) devices regulate battery charging by switching the PV array on and off hundreds or thousands of times a second to taper the array output current.