7 Interpreting Measured I-V Curves
7-4
A normal I-V curve has a smooth shape with three distinct voltage regions as shown in
Figure 58.
1. A slightly sloped region above 0 V (the horizontal leg of the curve)
2. A steeply sloped region below V
oc
(the downward leg of the curve)
3. A bend or ‘knee’ in the curve between these two regions
In a normal curve, the three regions are smooth and continuous. The shape and location
of the knee depends on cell technology and manufacturer. Crystalline silicon cells have
sharper knees; thin film modules usually have more gradual knees.
The three PV model points are defined, from left to right, as follows:
• SC – First point, at the predicted short circuit current I
sc
• MP – Second point, the predicted maximum power point I
mp
, V
mp
• OC – Third point, at the predicted open circuit voltage V
oc
Interpreting I-V Curves
PV module or string performance problems will cause deviations between the measured
and predicted I-V curves. There are six distinct types of deviation, listed below and in the
following figure. A given PV source under test may show no deviation, one deviation, or
a combination of two or more deviations.
1. 1. Notches or steps
2. 2. Low Current
3. 3. Low voltage
4. Rounder knee
5. Steeper slope in horizontal leg
6. Less steep slope in vertical leg
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