Electrical and Solar Fundamentals
Domain 2 tests electrical relationships and the solar-specific curve that separates PV from ordinary DC sources. You need Ohm's law, series and parallel rules, energy units, and the ability to read an I-V curve and apply temperature coefficients.
Core electrical rules
Ohm's law: V = I × R. Power: P = V × I. Energy: watt-hours = watts × hours; 1 kWh = 1000 Wh. In a series circuit, current is the same and voltages add. In a parallel circuit, voltage is the same and currents add. AC systems use root-mean-square values and may involve power factor; PV arrays produce DC before the inverter.
The PV I-V curve
Four landmarks matter: open-circuit voltage (Voc), short-circuit current (Isc), maximum-power voltage (Vmp), and maximum-power current (Imp). Pmax = Vmp × Imp. Fill factor = Pmax ÷ (Voc × Isc); a higher fill factor means a squarer, better curve. Irradiance mainly changes current; cell temperature mainly changes voltage. Voltage falls as cells heat and rises in cold weather, so cold-record design temperatures set maximum string voltage.
Worked example
A module has Voc 40 V, Isc 10 A, Vmp 33 V, Imp 9.5 A. Pmax = 33 × 9.5 = 313.5 W. Fill factor = 313.5 ÷ (40 × 10) = 313.5 ÷ 400 = 0.784, or 78.4%. Ten such modules in series have a string Voc near 400 V and a string current near Isc 10 A; three parallel strings would give about 30 A at the same voltage.
Traps
Isc is not the normal operating current; use Imp for maximum-power calculations. Nominal "12 V" modules usually have Vmp near 18 V, so do not size a controller from the label alone. Series connections raise voltage, not current. Cold temperatures raise Voc toward the inverter's maximum input; hot temperatures lower Vmp and can drop a string below the MPPT window. Do not treat AC and DC power calculations as identical when power factor is present.