PVWatts Version 1 Technical 

Reference 

Aron P. Dobos 

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PVWatts Version 1 Technical 

Reference 

Aron P. Dobos 

Prepared under Task No. SS13.5030 

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Office of Energy Efficiency & Renewable Energy 

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Technical Report 

NREL/TP-6A20-60272 

October 2013 

Contract No. DE-AC36-08GO28308 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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PVWatts Version 1 Technical Reference

Aron P. Dobos

September 10, 2013

Abstract

The NREL PVWattsTM calculator is a web application developed by the National Renewable Energy Laboratory (NREL) that estimates the electricity production of a gridconnected photovoltaic system based on a few simple inputs. PVWatts combines a number

of sub-models to predict overall system performance, and makes several hidden assumptions

about performance parameters. This technical reference details the individual sub-models,

documents assumptions and hidden parameters, and explains the sequence of calculations

that yield the final system performance estimation.

Keywords: photovoltaics, PVWatts, systems modeling, solar analysis

1 Overview

This document summarizes the calculation methods used in the PVWattsTM photovoltaic system

performance model. PVWatts is based on a number of individual sub-models, each of which

has been independently documented and validated. This overview describes each sub-model in

general and attempts to show how the pieces are connected together to go from solar irradiance

input to AC power output. For a complete explanation of each sub-model, refer to the references

cited throughout.

Several “versions” of PVWatts have existed in the past several years. The version documented

here is the “standard” implementation of the hourly calculator that is available in the SAM

Simulation Core (SSC) engine, denoted as module pvwattsv1, and is used for the online PVWatts

tool (http://pvwatts.nrel.gov), the NREL PVWatts V4 web service (http://developer.nrel.gov),

and within the System Advisor Model (SAM) (http://sam.nrel.gov).

The reader may also be familiar with the former PVWatts version 2 online tool. PVWatts

version 2 was a modified version of PVWatts that was tailored to work with a particular 40 km

monthly gridded dataset in the United States. The calculations were first performed hourly using

the algorithms described in this document for a nearby site, and then translated to the specific

grid cell on a monthly timeframe by adjusting for the local irradiance and temperature. The

details of the dataset and translation algorithm are given in [6].

2 Solar Resource

The PVWatts model is an hourly simulation program that requires hourly data for one year for

two components of solar irradiance (beam and diffuse), as well as ambient dry bulb temperature

and wind speed at 10 m. The PVWatts algorithms are indifferent to the source of such data;

typical data sources are the hourly Typical Meteorological Year (TMY) version 2 or version 3

1

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3

datasets [1, 2]. PVWatts also requires that each hour be timestamped with the year, month, day,

and hour corresponding to the data line so that the sun position can be accurately calculated.

No adjustments are made for leap years or daylight savings time: hence, “hourly data” in this

context implies 8,760 data points for one year. The required data are listed in Tables 1 and 2.

Table 1. Header Information

Field Units

Latitude degrees

Longitude degrees

Time zone hours offset from Greenwich Mean Time

Site elevation meters above sea level

Table 2. Hourly Input Data Fields
 

Field Units/Values
 

Year 1950-2050

Month 1-12

Day 1-31

Hour 0-23

Direct normal irradiance (DNI) W/m2

Diffuse horizontal irradiance (DHI) W/m2

Ambient dry bulb temperature Celsius

Wind speed at 10 m m/s

Snow cover (optional, typically in TMY2 files) cm

Albedo (optional, typically in TMY3 files) 0..1

The standard implementation of PVWatts in SSC has the ability to read in 8,760 data files

in the TMY version 2, TMY version 3, and Energy Plus Weather (EPW) data file formats.

Model Inputs

PVWatts requires a minimal set of PV system specifications as listed in Table 3. Other performance parameters such as module temperature coefficients are not available to the user.

Table 3. Input Parameters

Field Units Default Value

System size kW (DC) 4

System derate fraction 0.77

Array tracking mode Fixed, 1-Axis, 2-Axis Fixed

Tilt angle degrees Site Latitude

Azimuth angle degrees 180

The specifications are used in the various submodels described in the next sections to estimate

the energy generation of the system.

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4 Sun Position

At each hour, PVWatts calculates the sun position using the algorithm described in [3]. The sun

position is calculated at the midpoint of the hour: for example, from 5 p.m. to 6 p.m., the sun

position is calculated at 5:30 p.m. to determine the solar zenith and azimuth angles. This is the

case for normal daytime hours during which the sun is above the horizon for the whole hour.

For the sunrise hour, the midpoint between the sunrise time and the end of the timestep is

used for the sun position calculation. Analogously, the midpoint between the beginning of the

timestep and sunset time is used for the sunset hour.

5 Tracking

PVWatts performs angle of incidence (AOI) (α) calculations for fixed, one-axis, or two-axis

tracking systems. Fixed systems implement standard geometrical calculations for the angle of

incidence given surface tilt β, surface azimuth γ, solar azimuth γsun, and solar zenith θsun angles,

as listed in Eqn. 1.

αf ixed = cos−1 [sin(θsun) cos(γ − γsun)sin(β) + cos(θsun) cos(β)] (1)

For one axis trackers, the algorithm documented in [7] is used. It assumes ideal tracking and

does not account for any shading. For one-axis trackers, a tracker rotation limit is adhered to,

and it is hardcoded in PVWatts with a value of ± 45 degrees.

For two axis tracking systems, the PV surface tilt and azimuth are set equal to the sun zenith

angle and the sun azimuth angle, respectively, and the incidence angle is zero.

6 Plane-of-Array Irradiance

The plane-of-array (POA) beam, sky diffuse, and ground-reflected diffuse irradiance components

are calculated using the Perez 1990 algorithm [4]. The POA beam component Ib is simply the

beam normal input multiplied by the cosine of the angle of incidence. The isotropic, circumsolar,

and horizon brightening diffuse terms are calculated from the beam and diffuse input given two

empirical functions F1 and F2 as defined and are summed to yield the total sky diffuse on the

surface Id,sky. A slight modification from the standard Perez model treats the diffuse irradiance as

isotropic for zenith angles between 87.5 and 90 degrees. The ground reflected irradiance Id,ground

is treated as isotropic diffuse with a view factor calculated from the ground with respect to the

tilted surface. The total POA incident on the module cover is the sum of the three components

(Eqn. 2).

Ipoa = Ib + Id,sky + Id,ground (2)

The albedo, or ground reflectance, is by default fixed at 0.2. However, when using TMY2

files as input that have a valid snowfall (cm) data column, and the snowfall recorded is greater

than zero, an albedo of 0.6 is chosen to account for increased ground reflected irradiance. Of

course, this assumes that the photovoltaic panels are perfectly cleaned of any accumulated snow.

When using TMY3 data as input, and valid data is found in the albedo column, the reported

albedo from the data file is used.

3

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7 Module Cover

Given the total POA irradiance incident on the module cover, PVWatts applies an AOI correction

to adjust the direct beam irradiance for incidence angles greater than 50 degrees [8] to account

for reflection losses. The correction uses the Sandia PV Array Performance Model polynomial

correction, with coefficients for glass.

The incident POA Ipoa is reduced using Eqns. 3 and 4, given angle of incidence α, beam

normal irradiance Ib, and the five polynomial coefficients for glass listed in Table 4.

f = b0 + b1α + b2α2 + b3α3 + b4α4 + b5α5 (3)

Itr = Ipoa − (1 − f) · Ib cos(α) (4)

Table 4. Module Cover Polynomial Coefficients

b0 b1 b2 b3 b4 b5

1.0 -2.438e-3 3.103e-4 -1.246e-5 2.112e-7 -1.359e-9

8 Thermal Model

PVWatts implements a thermal model to calculate the operating cell temperature Tcell using a

first-principles heat transfer energy balance model developed by Fuentes [10]. The Fuentes model

includes effects of the thermal capacitance of the module and performs a numerical integration

between timesteps to account for the thermal lag transient behavior. The thermal model uses the

total incident POA irradiance, wind speed, and dry bulb temperature to calculate the operating

cell temperature. PVWatts assumes that the system is installed at a height of 5 m, and that the

installed nominal operating cell temperature (INOCT) of the module is 45 ◦C.

The Fuentes paper discusses translation of the nominal operating cell temperature (NOCT)

measured at standard conditions (800 W/m2, 20◦C ambient) to INOCT based on mounting

configuration, but this is not used in PVWatts in the current version. Future versions of PVWatts

may include the NOCT to INOCT adjustment based on data for various mounting structures,

as suggested in [10] and [11].

9 Module Model

The PVWatts module is an adaptation of a PVFORM version 3.3 model [12, 13] that uses the

user-specified reference array power at 1,000 W/m2. The subroutine computes the DC power

from the array with a specified nameplate DC rating of Pdc0 given a computed cell temperature

Tcell and transmitted POA irradiance Itr. The array efficiency is assumed to decrease at a linear

rate as a function of temperature rise.

Itr Pdc = Pdc0(1 + γ(Tcell − Tref )) Itr > 125 W/m2 (5) 1000

The temperature coefficent γ is fixed to -0.5 %/◦C, which may be considered typical for

crystalline silicon PV modules. The reference cell temperature Tref is 25◦C.

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The code adjusts the array efficiency if the insolation is less than 125 W/m2. The adjustment arose from observations of plots of efficiency versus insolation at several Sandia PV field

sites. When insolation is less than 125 W/m2 the efficiency is adjusted down at a rate that is

porportional to that that is observed in the measured field data. This algorithm assumes that

the efficiency is zero at insolation of zero. This is not true but is a reasonable assumption for a

performance model. The effect is to reduce the overall performace of a fixed-tilt system at low

light levels.

0.008 · I2

tr Pdc = Pdc0(1 + γ(Tcell − Tref )) Itr ≤ 125 W/m2 (6) 1000

10 System Derates

The overall system derate ξsys provided by the user as a fraction between zero and one represents

losses in the system due to nameplate direct current (DC) ratings, inverter efficiency at maximum

power, mismatch, diodes and connections, DC wiring, alternating current (AC) wiring, soiling,

system availability, shading, tracking error, and aging. The default derate on power is given as

a factor of 0.77, as discussed in [9].

The system losses applied to the DC output of the system include all losses except the inverter

efficiency at maximum power ηinv. Therefore, the derated DC power output is given by

ξsys PI

dc = Pdc · (7) ηinv

The derated DC power PI is passed to the inverter model. dc

11 Inverter Model

The inverter model in PVWatts is an adaptation of the PVFORM version 3.3 subroutine that

computes AC power from the inverter system. It uses a model developed by Leeman and

Menicucci of Sandia based on efficiency changes of typical power conversion unit systems as

a function of the load on the system. These efficiency changes were determined through numerous measurements made at Sandia, and the model is determined by fitting a curve through a

set of inverter efficiency measurements ranging from inputs of 10% of full power to 100% of full

power. The efficiency equation is a third order polynomial. Between 0% and 10% a linear change

is assumed.

PVWatts assumes a nameplate DC-to-AC ratio of 1, so the inverter AC nameplate rating

Pac0 is equal to the DC rating Pdc0 of the system. An effective inverter DC input power rating

Pinv,dc0 is calculated from the full-load inverter efficiency and the AC nameplate rating Pac0.

The part-load operation fraction f is defined by the ratio of derated DC power PI to the inverter dc

DC rating Pinv,dc0.

= PI Pinv,dc0 = Pac0/ηinv f dc/Pinv,dc0 (8)

The inverter conversion efficiency at operating conditions ηop is given by Eqn. 9 whose coefficients

arise from measurements of typical inverter performance [12, 13].

ηop = 0.774 + 0.663f − 0.952f 2 + 0.426f 3 for 0.1 ≤ f ≤ 1 (9)

ηop = −0.015 + 8.46f for 0 < f < 0.1 (10)

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For either condition, ηop is constrained between 0 and 0.925, and no night time standby losses

are calculated.

As noted in the previous section, the overall system derate includes the nameplate inverter

efficiency. The nominal maximum power inverter efficiency ηinv is assumed to be 0.92. The ratio

of the specified inverter efficiency at full load ηinv and a fixed reference efficiency of 0.91 at full

load is used in addition to compute the effective inverter efficiency at the operating conditions.

The fixed reference conversion efficiency of 0.91 corresponds to the data at which the part-load

efficiency curve described above was determined. Therefore, the final AC power output is given

by

ηinv = PI Pac · (11) dc · ηop 0.91

For f > 1, the power output is clipped to the maximum AC power rating Pac0.

12 Model Outputs

PVWatts calculates several hourly outputs based on the system specifications and hourly irradiance, temperature, and wind speed data. They are summarized in Table 5.

Table 5. Hourly Calculated Outputs

Field Units

Incident POA irradiance W/m2

Transmitted POA irradiance W/m2

DC power W

AC power W

In addition, the average incident POA irradiance per day in each month is reported to the

user. For each month m, the average POA in (kW/m2/day) is given by Eqn. 12.

 0.001 · m POAh POAm = (12) number of days in month m

The hourly outputs DC and AC power are aggregated into monthly and annual energy totals

that are reported to the user.

13 Conclusion

This document provides a detailed outline of the modeling steps undertaken by the PVWatts

model. The integrated system model requires a minimal set of user-specified system specifications

and makes numerous assumptions about various performance parameters intended to yield typical

results for “common” PV system configurations and types. The small number of user inputs

makes it possible for novices to use the model and have acceptable confidence in obtaining

reasonable results.

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14 Future Work

The world-wide PV industry has progressed significantly since the inception of PVWatts. As

PVWatts remains a popular tool across a variety of stakeholders and in many cases remains the

calculation basis for incentive and rebate amounts in various localities, it is important to ensure

that PVWatts results reflect current technologies that are being deployed. For example, inverters

in 2013 are notably more efficient than the 92 % efficiency assumed. The list below indicates

potential future areas of investigation by the PVWatts development team.

•	 Update to the overall 0.77 derate default value based on analysis of large datasets of current

PV system performance

•	 Update to the inverter part-load efficiency curve based on analysis of over 1000 inverter

performance datasets

•	 Inclusion of mounting configuration specific adjustments to the INOCT in the thermal

model

•	 Ability for the user to select “premium” versus “standard” modules; this choice will affect

the module temperature coefficient γ and possibly the incidence angle modifier polynomial

coefficients bi.

References

[1] Marion, W.; Urban, K. (2012).	 User’s Manual for TMY2. Accessed September 3, 2013:

http://rredc.nrel.gov/solar/pubs/tmy2/.

[2] Wilcox, S.; Marion, W. (2008). User’s Manual for TMY3 Data Sets. NREL/TP-581-43156.

Golden, CO: National Renewable Energy Laboratory.

[3] Michalsky, J. (1988). “The Astronomical Almanac’s Algorithm for Approximate Solar Position (1950-2050).” Solar Energy (40).

[4] Perez, R.; Ineichen, P.; Seals, R.; Michalsky,	 J.; Stewart, R. (1990). “Modeling Daylight

Availability and Irradiance Components for Direct and Global Irradiance.” Solar Energy

(44:5), pp. 271-289.

[5] Marion, W.; Anderberg, M. (2000). PVWATTS	 - An Online Performance Calculator for

Grid-Connected PV Systems. Proceedings of the ASES Solar Conference, June 15-21, 2000,

Madison, Wisconsin.

[6] Marion, W.; Anderberg, M.; George, R.; Gray-Hann, P.; Heimiller, D.; (2001)	 PVWATTS

Version 2 - Enhanced Spatial Resolution for Calculating Grid-Connected PV Performance.

CP-560-30941. Golden CO: National Renewable Energy Laboratory.

[7] Marion, W.; Dobos, A. P.	 (2013). Rotation Angle for the Optimum Tracking of One-Axis

Trackers. TP-6A20-58891. Golden, CO: National Renewable Energy Laboratory.

[8] Marion, W. (2010).	 Overview of the PV Module Model in PVWatts. Presentation at the

Sandia National Laboratories PV Performance Modeling Workshop, September 22, 2010,

Albuquerque, New Mexico.

7

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.
[9] Marion, W.; Adelstein, J.; Boyle, K.; Hayden, H.; Hammond, B.; Fletcher, T.; Canada, B.;

Narang, D.; Shugar, D.; Wenger, H.; Kimber, A.; Mitchell, L.; Rich, G.; Townsend, T. (2005).

Performance Parameters for Grid-Connected PV Systems. Proc. of 31st IEEE Photovoltaics

Specialists Conference, January 2005, Lake Buena Vista, Florida.

[10] Fuentes, M. K. (1987).	 A Simplified Thermal Model for Flat-Plate Photovoltaic Arrays.

SAND85-0330. Albuquerque, NM: Sandia National Laboratories. Accessed September 3,

2013: http://prod.sandia.gov/techlib/access-control.cgi/1985/850330.pdf.

[11] California Energy Commission. (2013). “CEC PV Calculator Version 4.0.” Accessed September 3, 2013: http://gosolarcalifornia.org/tools/nshpcalculator/index.php.

[12] Menicucci,	 D. F.; Fernandez, J. P. (1988). User’s Manual for PVFORM: A Photovoltaic System Simulation Program For Stand-Alone and Grid-Interactive Applications.

SAND85-0376. Albuquerque, NM: Sandia National Laboratories. Accessed September 3,

2013: http://prod.sandia.gov/techlib/access-control.cgi/1985/850376.pdf.

[13] Menicucci, D. F. (1986). “Photovoltaic Array Performance Simulation Models.” Solar Cells

(18).

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National Renewable Energy Laboratory (NREL) 

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