SMA Solar Technology AG 9 Glossary
User Manual SDesign-BA-BEN120432 89
The temporal average is therefore zero, known as reactive power, which "shuttles back and forth" in
the lines. Many electrical loads need a phase-shift between current and voltage so that the power
distribution grid always feeds reactive power in addition to active power.
Specific energy yield
The specific energy yield is the amount of energy typically fed into the power distribution grid over a
year compared to the size (peak power) of the PV array. The result is shown in kWh/kWp and gives
a first estimate of whether the PV plant supplies an energy yield typical for the respective location.
Since this value is also influenced by the structural characteristics of a PV plant (e.g. orientation of the
PV array), it is generally not suitable for comparing PV plants to each other.
STC (Standard Test Conditions)
In order for a performance comparison of different PV modules to take place, standard test conditions
are used; these standard test conditions are also called STC:
• Solar irradiation 1 000 W/m
2
• Temperature 25°C
• AM 1.5 (AM= Air Mass, a relative measurement for the way that light covers the distance
through the earth's atmosphere to the earth. The air mass on the equator is AM=1 and approx.
1.5 in Europe)
String
Describes a group of electrical series-connected PV modules. Normally, a PV plant is made up of
multiple strings.
String inverter
Decentralised inverter concept where the PV array is divided up into individual strings. The individual
strings are connected to the power distribution grid through their own string inverters. This greatly
simplifies installation and reduces the yield losses which can be caused by manufacturing deviations
or variations in shadowing on the PV modules.
Typical PV voltage
The typical PV voltage is the MPP voltage of a PV module that cell temperature corresponds to the
NOCT (normal operation cell temperature = cell temperature at irradiation of 800 W/m
2
, ambient
temperature of 20°C and wind speed of 1.0 m/s). The NOCT is also often shown on the fact sheet
of the PV modules.
Unbalanced load
The unbalanced load is the maximum output difference whose may occur between any two phases
of a PV plant feeding into the three-phase power distribution grid. In the planning phase of a PV plant,
this value refers to the distribution of nominal AC power from single-phase inverters to the three phases
of the power distribution grid at the feed-in point.
