Real-time estimation techniques are presented to estimate solar irradiance and photovoltaic (PV) module temperature simultaneously from maximum power point condition. An algebraic equation which is function of PV output voltage and current measurements is utilised to estimate.
To check your panel, you'll need a few tools: a multimeter to measure voltage (meters from Gardner Bender and Fluke fared well in our testing, see “ Digital Multimeters,” April 2004), a clamp-on DC ammeter (Klein CL120, $55 is an economical choice), a DIY resistance device.