| Symbol | Definition | Page Links |
| $$A_i$$ | Anisotropy Index [unitless] | Hay and Davies Sky Diffuse Model, Reindl Sky Diffuse Model |
| $$AM$$ | Air Mass [unitless] | Air Mass |
| $$AM_a$$ | Absolute Air Mass [unitless] | Perez Sky Diffuse Model, Sandia PV Array Performance Model |
| $$\theta_{AOI}$$ | Angle of Incidence [degrees] | POA Beam, Angle of Incidence, Hay and Davies Sky Diffuse Model, Reindl Sky Diffuse Model, Perez Sky Diffuse Model |
| $$DHI$$ | Diffuse Horizontal Irradiance [W/m2] | Global Horizontal Irradiance, Diffuse Horizontal Irradiance, Isotropic Sky Diffuse Model, Simple Sandia Sky Diffuse Model, Hay and Davies Sky Diffuse Model, Reindl Sky Diffuse Model, Perez Sky Diffuse Model |
| $$DNI$$ | Direct Normal Irradiance [W/m2] | Irradiance and Weather – Definitions and Overview: Direct Normal Irradiance, Global Horizontal Irradiance, POA Beam, Hay and Davies Sky Diffuse Model, Reindl Sky Diffuse Model |
| $$DOY$$ | Day of Year | Extraterrestrial Radiation |
| $$E_0$$ | reference irradiance, typically 1000 W/m2 | Sandia Cell Temperature Model, Effective Irradiance, Sandia PV Array Performance Model, PVWatts |
| $$E_a$$ | Extraterrestrial Irradiance [W/m2] | Extraterrestrial Radiation, DISC Model, Hay and Davies Sky Diffuse Model, Reindl Sky Diffuse Model, Perez Sky Diffuse Model |
| $$E_b$$ | beam component of plane-of-array irradiance [W/m2] | Plane of Array (POA) Irradiance, POA Beam, Sandia PV Array Performance Model |
| $$E_d$$ | sky diffuse component of plane-of-array irradiance [W/m2] | Plane of Array (POA) Irradiance, Simple Sandia Sky Diffuse Model, Hay and Davies Sky Diffuse Model, Reindl Sky Diffuse Model, Perez Sky Diffuse Model, Sandia PV Array Performance Model |
| $$E_e$$ | effective irradiance [unitless] | Effective Irradiance, Sandia PV Array Performance Model, PVWatts |
| $$E_{POA}$$ | irradiance incident on the plane of the array [W/m2] | Plane of Array (POA) Irradiance, Faiman Module Temperature Model, Sandia Cell Temperature Model, PVsyst Cell Temperature Model, Effective Irradiance |
| $$E_{qt}$$ | Equation of Time | Simple Models |
| $$E{sc}$$ | Solar Constant | Extraterrestrial Radiation |
| $$eta_{m}$$ | efficiency of PV module | PVsyst Cell Temperature Model |
| $$f_d$$ | fraction of diffuse light used by PV module | Sandia PV Array Performance Model |
| $$GHI$$ | Global Horizontal Irradiance [W/m2] | Direct Normal Irradiance, DISC Model, Global Horizontal Irradiance, Simple Sandia Sky Diffuse Model, Reindl Sky Diffuse Model |
| $$I$$ | Current [amps] | Module IV Curve, Single Diode Equivalent Circuit Models |
| $$I_0$$ | Saturation current | Single Diode Equivalent Circuit Models |
| $$IAM$$ | Incidence angle modifier | Incident Angle Reflection Losses |
| $$IAM_b$$ | Incidence angle modifier for beam component of irradiance | Incident Angle Reflection Losses |
| $$I_D$$ | voltage-dependent current lost to recombination | Single Diode Equivalent Circuit Models |
| $$I_L$$ | light-generated current produced within PV cell | Single Diode Equivalent Circuit Models |
| $$I{M}$$ | current of module or array | Single Diode Equivalent Circuit Models |
| $$I_{mp}$$ | current at maximum power | Sandia PV Array Performance Model |
| $$I{sc}$$ | short circuit current | Sandia Model, Effective Irradiance, Sandia PV Array Performance Model |
| $$I_{sc0}$$ | | Effective Irradiance, Sandia PV Array Performance Model |
| $$I_{scr}$$ | | Effective Irradiance |
| $$I_{scr0}$$ | | Effective Irradiance |
| $$I_{sh}$$ | current lost due to shunt resistances | Single Diode Equivalent Circuit Models |
| $$K$$ | glazing extinction coefficient | Physical Model of IAM |
| $$K_n$$ | Direct Normal Transmittance | DISC Model |
| $$K_t$$ | Global Horizontal Transmittance | DISC Model |
| $$L$$ | glazing thickness | Physical Model of IAM |
| $$Long_{local}$$ | Longitude of the observer | Simple Models |
| $$Long_{sm}$$ | Longitude of standard meridian of observer’s time zone | Simple Models |
| $$M$$ | air mass modifier | De Soto Model |
| $$N_s$$ | number of cells in series | Single Diode Equivalent Circuit Models |
| $$P_{AC}$$ | AC output power | Sandia Inverter Model |
| $$P_{ACO}$$ | maximum AC power rating | Sandia Inverter Model |
| $$P_{DC}$$ | DC input power | Sandia Inverter Model |
| $$p_{in}$$ | normalized DC power | Driesse Inverter Model |
| $$p_{loss}$$ | | Driesse Inverter Model |
| $$P_{mp}$$ | maximum power point power | Point-value Models, PVWatts |
| $$P_{mp0}$$ | | PVWatts |
| $$PR_{AC}$$ | daily AC performance ratio | Performance Ratio |
| $$q$$ | elementary charge (constant) | Single Diode Equivalent Circuit Models, Sandia PV Array Performance Model |
| $$R_{av}$$ | mean Sun-Earth distance | Extraterrestrial Radiation |
| $$R_s$$ | series resistance | Single Diode Equivalent Circuit Models |
| $$R_{sh}$$ | shunt resistance | Single Diode Equivalent Circuit Models |
| $$R_{sh,ecp}$$ | | PVsyst Model |
| $$S$$ | total absorbed irradiance [W/m2] | De Soto Model, PVsyst Model |
| $$SF$$ | soiling factor; ranges from 0 (no soiling)to 1 (complete soiling) | Effective Irradiance |
| $$T_{0}$$ | reference temperature, typically 25 degrees Celsius | Effective Irradiance, Sandia PV Array Performance Model, PVWatts |
| $$T_{c}$$ | cell temperature [°C] | Sandia Cell Temperature Model, PVsyst Cell Temperature Model, Effective Irradiance, De Soto Model, PVWatts |
| $$T_{cr}$$ | | Effective Irradiance |
| $$T_{local}$$ | local time | Simple Models |
| $$T_{m}$$ | module temperature [°C] | Sandia Module Temperature Model, Faiman Module Temperature Model, Sandia Cell Temperature Model |
| $$T_{solar}$$ | solar time | Simple Models |
| $$U_{0}$$ | constant heat transfer component | Faiman Module Temperature Model, PVsyst Cell Temperature Model |
| $$U_1$$ | convective heat transfer component | Faiman Module Temperature Model, PVsyst Cell Temperature Model |
| $$V$$ | voltage [V] | Module IV Curve |
| $$V_{DC}$$ | DC input voltage [V] | Sandia Inverter Model |
| $$V_{DC0}$$ | DC voltage level at which AC power rating is achieved [V] | Sandia Inverter Model |
| $$v{in}$$ | normalized DC voltage | Driesse Inverter Model |
| $$V_M$$ | voltage of module or array [V] | Single Diode Equivalent Circuit Models |
| $$V_{mp}$$ | maximum power point voltage [V] | Sandia PV Array Performance Model |
| $$V_{oc}$$ | open circuit voltage [V] | Sandia PV Array Performance Model |
| $$V_T$$ | thermal voltage | Single Diode Equivalent Circuit Models |
| $$WS$$ | wind speed [m/s] | Sandia Module Temperature Model, Faiman Module Temperature Model, PVsyst Cell Temperature Model |
| $$Y_{fAC}$$ | AC system yield | Performance Ratio |
| $$Y_r$$ | ratio of plane-of-array irradiance to reference irradiance | Performance Ratio |
| $$alpha$$ | absorption coefficient of PV module | PVsyst Cell Temperature Model |
| $$alpha_{lmp}$$ | temperature coefficient for maximum power point current | Sandia PV Array Performance Model |
| $$alpha_{lsc}$$ | temperature coefficient for short circuit current | Effective Irradiance, De Soto Model, Sandia PV Array Performance Model, PVsyst Model |
| $$alpha_{lscr}$$ | | Effective Irradiance |
| $$beta_{Voc}$$ | temperature coefficient for module; open circuit voltage | Sandia PV Array Performance Model |
| $$beta_{Voc0}$$ | temperature coefficient for module; open circuit voltage at reference irradiance | Sandia PV Array Performance Model |
| $$gamma$$ | temperature correction for Pmp | PVWatts |
$$Delta
T$$ | temperature difference parameter | Sandia Cell Temperature Model |
| $$varepsilon$$ | bins of clearness | Perez Sky Diffuse Model |
| $$theta_A$$ | azimuth angle | Sun Position, Angle of Incidence |
| $$theta_d$$ | declination of the sun | Simple Models |
| $$theta_{el}$$ | solar elevation angle | Sun Position |
| $$theta_{hr}$$ | hour angle | Simple Models |
| $$theta_{r}$$ | angle of refraction | Physical Model of IAM |
| $$theta_Z$$ | solar zenith angle | Sun Position, Air Mass, Angle of Incidence, Simple Sandia Sky Diffuse Model, Perez Sky Diffuse Model |
| $$kappa$$ | constant | Perez Sky Diffuse Model |
| $$lambda$$ | latitude of the observer | Simple Models |
| $$tau$$ | weighted transmittance | Incident Angle Reflection Losses |