# PV Modeling Glossary¶

This page contains nomenclature for PV-relevant terms that are used in modeling and data analysis for PV systems. Terms are grouped by category. These definitions should be helpful for anyone writing software to analyze PV systems.

The team would greatly appreciate feedback and suggestions. To comment, see the issues on the pv-terms github repo.

Version: 0.1.2

**Categories**

## Abbreviations¶

These abbreviations are used without expansion in the rest of the variables.

**ac**: alternating current**aod**: aerosol optical depth**aoi**: angle of incidence**coeff**: coefficient**dc**: direct current**dhi**: diffuse horizontal irradiance**dni**: direct normal irradiance**ghi**: global horizontal irradiance**imp**: current at maximum power**inv**: inverter**isc**: current at short circuit**max**: maximum**min**: minimum**mp**: maximum power**mpp**: maximum power point**mppt**: maximum power point tracking**noct**: nominal operating cell temperature**num**: number**oc**: open circuit**pmp**: power at maximum power**poa**: plane of array (irradiance)**ref**: reference**sc**: short circuit**stc**: standard test conditions**vmp**: voltage at maximum power**voc**: voltage at open circuit**toa**: top of atmosphere

## Optional Suffixes¶

These optional suffixes can be appended to a variable in order to further clarify which quantity the variable refers to. In the following * refers to another variable.

**Suffixes specifying the object**

***_cell**: Quantity relating to a single PV cell***_module**: Quantity relating to a single PV module***_string**: Quantity relating to a string of PV modules.***_array**: Quantity relating to a number of PV strings.***_inv**: Quautity relating to an inverter.***_site**: Quantity relating to the entire site.

**Suffixes specifying the aggregation type**

***_cumulative**: Quantity integrated over a specific length of time, usually the full lifetime of the system.***_interval**: Quantity integrated over a specific length of time, usually a shorter timespan e.g. day or hour.***_mean**: Averaged quantity***_std**: Standard deviation of quantity

**Suffixes specifying the measurement type**

***_meas**: Parameter measured from field data, e.g. voc_meas***_sim**: Parameter predicted from a model, e.g. voc_sim***_rated**: Parameter on the datasheet or as-built.***_norm**: Normalized quantity

## Location¶

These variables are associated with the location of the PV system.

**albedo**[dimensionless]: Ground reflectance between 0 and 1.**elevation**[m]: Elevation of system above sea level.**latitude**[degrees]: Latitude between -90 and 90 degrees. Positive north of the equator.**longitude**[degrees]: Longitude between -180 to 180 degrees. Positive east of the meridian.**pressure**[Pa]: Atmospheric pressure at ground level.**tz**: timezone

## Atmosphere¶

Atmosphere variables are related to weather conditions.

**absolute_humidity**[kg/m^3]: Mass of water vapor per volume of air.**airmass**[dimensionless]: airmass**airmass_absolute**[dimensionless]: absolute airmass**airmass_relative**[dimensionless]: relative airmass**dew_point**[°C]: Dewpoint temperature.**relative_humidity**[dimensionless]: The ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature, unitless.**specific_humidity**[dimensionless]: The ratio of the mass of water vapor to the total mass of moist air.**total_precipitable_water**[mm]: Depth of water in a column of the atmosphere, if all the water in that column were precipitated as rain.**wind_direction**[degrees]: Direction of origin of wind. The wind azimuth convention is defined as degrees east of north (e.g. North=0, South=180, East=90, West=270).**wind_speed**[m/s]: Wind speed measured at standard 10 meter height.

## Solar Position¶

These variables are related to the position of sun and the incidence angles with the PV array.

**apparent_solar_elevation**[degrees]: Refraction-corrected solar elevation angle.**apparent_solar_zenith**[degrees]: Refraction-corrected solar zenith angle.**solar_azimuth**[degrees]: Azimuth angle of the sun in degrees East of North**solar_elevation**[degrees]: Elevation angle of the sun.**solar_zenith**[degrees]: Zenith angle of the sun.

## DC Output¶

These variables are related to the DC side of a PV power plant.

**current_dc**[A]: DC current.**energy_dc**[kWh]: DC energy.**imp**[A]: Current at the maximum power point. [Deprecated/alternates:*i_mp*]**isc**[A]: Short circuit current. [Deprecated/alternates:*i_sc*]**pmp**[W]: Power at the maximum power point. [Deprecated/alternates:*p_mp*]**power_dc**[W]: DC power.**vmp**[V]: Voltage at the maximum power point. [Deprecated/alternates:*v_mp*]**voc**[V]: Open circuit voltage. [Deprecated/alternates:*v_oc*]**voltage_dc**[V]: DC voltage.

## AC Output¶

These variables are related to the AC side of a PV power plant.

**current_ac**[A]: Root-mean-squared AC current on a single phase inverter.**current_ac_a**[A]: Root-mean-squared AC current on phase a of a three phase inverter. Use _b or _c for the other phases.**energy_apparent**[kWh]: Integral of apperent power over time.**energy_reactive**[kWh]: Integral of reactive power over time.**energy_real**[kWh]: Integral of real power over time.**frequency**[Hz]: AC frequency.**frequency_a**[Hz]: AC frequency on phase a. Use _b, _c for other phases.**power_ac**[W]: Cycle-averaged AC power on a single phase inverter.**power_ac_a**[W]: Cycle-averaged AC power on phase a of a three phase inverter. Use _b or _c for the other phases.**power_apparent**[VA]: Cycle-averaged AC apparent power.**power_factor**[dimensionless]: Power factor.**power_factor_a**[dimensionless]: Power factor on phase a of a three phase inverter. Use _b or _c for the other phases.**power_reactive**[VAR]: Cycle-averaged reactive power.**power_real**[W]: Cycle-averaged AC power.**power_real_a**[W]: Cycle-averaged AC power on phase a.**voltage_ac**[V]: Root-mean-squared AC voltage on a single phase inverter.**voltage_ac_a**[V]: Root-mean-squared AC voltage on phase a of a three phase inverter. Use _b or _c for the other phases.

## Inverter¶

These parameters are related to the operation of inverters.

**current_inv_ac_max**[A]: Maximum inverter AC output current.**current_inv_dc_max**[A]: Maximum inverter DC input current.**efficiency_inverter**[dimensionless]: Inverter efficiency. Real AC power out divided by DC power in.**efficiency_inverter_cec**[dimensionless]: Nominal inverter efficiency using CEC standard.**efficiency_inverter_eu**[dimensionless]: Nominal inverter efficiency using European standard.**num_mppt_circuits**[dimensionless]: Number of MPPT circuits.**power_inv_ac_max**[W]: Maximum inverter AC output power.**temperature_inv_operate_max**[°C]: Maximum inverter operating temperature.**temperature_inv_operate_min**[°C]: Minimum inverter operating temperature.**total_harmonic_distortion_max**[dimensionless]: Maximum total harmonic distortion of inverter AC output.**voltage_inv_ac_max**[V]: Maximum inverter AC output voltage.**voltage_inv_ac_min**[V]: Minimum inverter AC ouptut voltage, in volts**voltage_inv_ac_nominal**[V]: Nominal inverter AC output voltage.**voltage_inv_dc_max**[V]: Maximum inverter dc input voltage.**voltage_inv_mppt_min**[V]: Minimum array voltage for inverter to engage maximum power point tracking.**voltage_inv_start_min**[V]: Minimum inverter input DC voltage to start operation, also called the inverter start voltage.**voltage_mppt_max**[V]: Maximum array voltage for inverter to engage maximum power point tracking.

## Irradiance¶

These variables denote irradiance in units of W/m^2.

**dhi**[W/m^2]: Diffuse horizontal irradiance.**dni**[W/m^2]: Direct normal irradiance; irradiance taken on a plane normal to direction of the sun.**dni_toa**[W/m^2]: Direct normal irradiance at the top of the atmosphere; dni if the atmosphere did not exist. Also sometimes called extraterrestrial irradiance.**effective_irradiance**[W/m^2]: Irradiance reaching the module’s cells, i.e., in the plane of array, reduced by soiling and reflections, adjusted for spectrum.**ghi**[W/m^2]: Global horizontal irradiance.**ghi_toa**[W/m^2]: Global horizontal irradiance at the top of the atmosphere; ghi as if the atmosphere did not exist.**poa_toa**[W/m^2]: Plane-of-array irradiance if the atmosphere did not exist.**poa_diffuse**[W/m^2]: Total diffuse irradiation in plane. sum of ground and sky diffuse.**poa_direct**[W/m^2]: Direct/beam irradiation in plane.**poa_global**[W/m^2]: Global irradiation in plane. sum of diffuse and beam projection.**poa_ground_diffuse**[W/m^2]: In plane ground reflected irradiation [Deprecated/alternates:*gti*]**poa_sky_diffuse**[W/m^2]: Diffuse irradiation in plane from scattered light in the atmosphere (without ground reflected irradiation)**clearness_ghi**[dimensionless]: Horizontal clearness index, clearness_ghi = ghi/ghi_toa**clearness_dni**[dimensionless]: Normal clearness index, clearness_dni = dni/dni_toa**clearness_poa_global**[dimensionless]: Clearness index in plane-of-array, clearness_poa_global = poa_global/poa_toa**clearness_poa_direct**[dimensionless]: Clearness index in plane-of-array, clearness_poa_direct = poa_direct/poa_toa**fraction_beam**[dimensionless]: Fraction of irradiance in beam component, beam_fraction = 1 - dhi/ghi**fraction_diffuse**[dimensionless]: Fraction of irradiance in diffuse component, beam_fraction = dhi/ghi

## PV System¶

These variables are related to the operating current, voltage and power of PV cells, modules or arrays.

**aoi**[degrees]: Angle between direct beam component and surface normal, between 0 and 90.**height_array**[m]: Height above ground of the bottom edge of the module for a fixed tilt system.**axis_azimuth**[degrees]: Azimuth of axis of rotation for a single-axis tracking system (e.g. North=0, South=180, East=90, West=270).**height_axis**[m]: Height above ground of the axis of rotation for a single axis tracker.**axis_tilt**[degrees]: Orientation of axis of rotation for a single-axis tracking system relative to horizontal. 0 = horizontal.**date_time**[YYYY-MM-DD HH:MM:SS]: Date and time of measurement interval**ground_coverage_ratio**[dimensionless]: A value denoting the ground coverage ratio of a tracker system which utilizes backtracking; i.e. the ratio between the PV array surface area to total ground area. A tracker system with modules 2 meters wide, centered on the tracking axis, with 6 meters between the tracking axes has a gcr of 2/6=0.333. If gcr is not provided, a gcr of 2/7 is default. gcr must be <=1.**modules_per_string**[dimensionless]: Number of modules in series in each string.**parallel_strings**[dimensionless]: Number of parallel strings in an array of modules.**spectral_mismatch**[dimensionless]: The ratio of power produced by a PV cell with a particular spectral distribution of light divided by the power produced by a reference spectrum.**surface_azimuth**[degrees]: Surface azimuth angles. The azimuth convention is defined as degrees east of north (e.g. North=0, South=180, East=90, West=270)**surface_tilt**[degrees]: Surface tilt angle. The tilt angle is defined as degrees from horizontal (e.g. surface facing up = 0, surface facing horizon = 90)**temperature_air**[°C]: ambient dry bulb temperature.**temperature_cell**[°C]: Temperature of the cell.**temperature_module**[°C]: temperature of the module.**tracker_type**: Tracker type, can be ‘fixed’, ‘single-axis’ or ‘two-axis’.

## Module Parameters¶

Module parameters are primarily used to describe the electrical characteristic of a PV module.

**alpha_imp**[A/°C]: Temperature coefficient of current at maximum power point and reference conditions.**alpha_isc**[A/°C]: The temperature coefficient of short-circuit current at reference conditions. [Deprecated/alternates:*alpha_sc*]**alpha_isc_norm**[1/°C]: alpha_isc/isc_ref. The temperature coefficient of short-circuit current at reference conditions divided by the short-circuit current at reference conditions.**band_gap_ref**[eV]: Energy bandgap at reference temperature. 1.121 eV for crystalline silicon. EgRef must be >0. For parameters from the SAM CEC module database, EgRef=1.121 is implicit for all cell types in the parameter estimation algorithm used by NREL. [Deprecated/alternates:*Egref,EgRef,Eg_ref*]**band_gap_temperature_coeff**[1/K]: The temperature dependence of the energy bandgap at reference conditions in units of 1/K.**beta_vmp**[V/°C]: Temperature coefficient of voltage at maximum power point and reference conditions.**beta_voc**[V/°C]: Temperature coefficient of open-circuit voltage at reference conditions.**beta_voc_norm**[1/°C]: beta_voc/voc_ref. Temperature coefficient of open-circuit voltage at reference conditions divided by the open-circuit voltage at reference conditions.**cells_in_series**[dimensionless]: Number of cells in series in a PV module.**diode_factor**[dimensionless]: Diode ideality factor.**fill_factor**[dimensionless]: Fill factor of the PV pmp/(voc*isc)**gamma_pmp**[W/°C]: Temperature coefficient of power at maximum power point and reference conditions. [Deprecated/alternates:*gamma_pdc*]**gamma_pmp_norm**[W/°C]: Normalized temperature coefficient of power at maximum power point and reference conditions, equal to gamma_pmp/pmp_ref. [Deprecated/alternates:*gamma_pdc*]**imp_ref**[A]: Current at maximum power point at reference conditions.**irradiance_ref**[W/m^2]: Reference irradiance. Typical value is 1000 W/m^2. [Deprecated/alternates:*irrad_ref*]**isc_ref**[A]: Short circuit current at reference conditions. [Deprecated/alternates:*i_sc_ref*]**module_area**[m^2]: Module aperture area. The area inside the frame for a rectangular module.**module_area_active**[m^2]: Total area of cells in a module.**module_area_external**[m^2]: External dimensions of a module.**nNsVth**[V]: The product of the usual diode ideality factor (n, unitless), number of cells in series (Ns), and cell thermal voltage.**nNsVth_ref**[V]: The product of the usual diode ideality factor (n, unitless), number of cells in series (Ns), and cell thermal voltage at reference conditions. [Deprecated/alternates:*a_ref*]**noct**[°C]: Nominal operating cell temperature.**photocurrent**[A]: Photo-generated current. [Deprecated/alternates:*I_L*]**photocurrent_ref**[A]: Photocurrent at reference conditions. [Deprecated/alternates:*I_L_ref*]**pmp_ref**[W]: Power at maximum power point at reference conditions. [Deprecated/alternates:*p_mp_ref*]**resistance_series**[Ohm]: Series resistance. [Deprecated/alternates:*R_s,Rs*]**resistance_series_ref**[Ohm]: Series resistance at reference conditions. [Deprecated/alternates:*R_s_ref,resistance_series*]**resistance_shunt**[Ohm]: Shunt resistance. [Deprecated/alternates:*R_sh,Rsh*]**resistance_shunt_ref**[Ohm]: Shunt resistance at reference conditions. [Deprecated/alternates:*R_sh_ref*]**saturation_current**[A]: The dark or diode reverse saturation current. [Deprecated/alternates:*I_o*]**saturation_current_ref**[A]: The dark or diode reverse saturation current at reference conditions. [Deprecated/alternates:*I_o_ref*]**temperature_ref**[°C]: Reference temperature. Typical value is 25 °C. [Deprecated/alternates:*temp_ref*]**vmp_ref**[V]: Voltage at maximum power point at reference conditions. [Deprecated/alternates:*v_mp_ref*]**voc_ref**[V]: Open circuit voltage at reference conditions. [Deprecated/alternates:*v_oc_ref*]**voltage_thermal**[V]: Thermal voltage per cell, equal to k_B*T/q where k_B is the boltzman constant (in J/K), T is the temperature (in Kelvin) and q is the electron charge (in Coulombs).