Halogen lamp: A special type of incandescent globe made of quartz glass and a tungsten filament, enabling it to run at a much higher temperature than a conventional incandescent globe. Efficiency is better than a normal incandescent, but not as good as a fluorescent light.
Harmonic content: Frequencies in the output waveform in addition to the primary frequency (usually 50 or 60 Hz.) Energy in these harmonics is lost and can cause undue heating of the load.
Head: The vertical distance that water will fall from the inlet of the collection pipe to the water turbine in a hydro power system.
Hertz (Hz): Unit of measurement for frequency. Home mains power is normally 50Hz in Europe and 60Hz in the USA. The magnitude of the current is measured in Amps.
High voltage disconnect: Voltage at which the charge controller will disconnect the array to prevent overcharging the batteries.
Hot spot: A phenomenon where one or more cells within a PV module or array act as a resistive load, resulting in local overheating or melting of the cells.
Hybrid system: A PV system that includes solar PV and some other electricity generating power source.