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Harmonic distortion
When the pattern of voltage or alternating current moves away from the sinusoidal one, there is a harmonic distortion. In fact, a non-sinusoidal pattern is composed by a basic sinusoid (of 50 Hz in the European electric grid), to which other sinusoids with multiple frequency overlap. According to the multiple of the basic one taken into consideration (of frequency f), we have second harmonic (2f), third harmonic (3f) and so on. Constructors of inverters usually supply the TDH (Total Harmonic Distortion) of the voltage or the output current; in other words, the per cent value of the relationship between the sum of all the harmonic components and the basic one.

Heat absorption
Dark metallic sheet called also picking up plate, on which tubes have been welded; some convector liquid flows in these tubes.

Hermetic seal
Being impervious to external influences. Typically associated with the sealing of a package so that oxygen, moisture, and other outside environments cannot enter the

HIT(Heterojunction with Intrinsic Thin-layer)
Bifacial PV cells made of a very thin layer of amorphous silicon placed on a high efficiency sublayer of monocrystalline silicon.

Hot spot
An undesirable phenomenon of PV device operation whereby one or more cells within a PV module or array act as a resistive load, resulting in local overheating or melting of the cell(s).

Hour angle
Angular distance of the sun from the straight line joining the sun and the earth at midday: this angle changes of 15 every hour and it is measured in daomega = 15 x (12 - ts) where ts is the unit of solar hour.

Hybrid system
A PV system that includes other sources of electricity generation, such as wind or fossil fuel generators.