This article covers the four key measurements used in professional PV diagnostics: open circuit voltage (Voc), short circuit current (Isc), isolation resistance (Riso), series resistance (Rs) and system impedance. Safety note: Troubleshooting a PV system may be a complex and. Testing a PV system correctly requires understanding which electrical parameters matter and why. The measurements can be used to calculate the power output. Resistance evaluations assist in identifying defective solar cells, 2. Understanding resistance aids in. How to calculate electrical resistance? To calculate the electrical resistance of your solar panels, that is, what resistance their materials have to the passage of electrical current, you will have to multiply the coefficient of resistivity by its length and divide by its area.