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Contract No. DE-AC36-08GO28308

SAM Technical Review 

Committee Final Report

Summary and Key Recommendations 

from the Onsite TRC Meeting Held 

April 22-23, 2013

N. Blair, A. Dobos, S. Janzou, P. Gilman, 

J. Freeman, and L. Kaffine

Milestone Report

NREL/TP-6A20-58785

August 2013
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC.

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

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SAM Technical Review 

Committee Final Report

Summary and Key Recommendations 

from the Onsite TRC Meeting Held 

April 22-23, 2013

N. Blair, A. Dobos, S. Janzou, P. Gilman, 

J. Freeman, and L. Kaffine

Prepared under Task No. SS13.5010

Milestone Report

NREL/TP-6A20-58785

August 2013
NOTICE

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Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

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Foreword

The System Advisor Model (SAM) is a broad and robust set of models and frameworks for 

analyzing both system performance and system financing. It does this across a range of 

technologies dominated by solar technologies including photovoltaics (PV) and concentrated 

solar power (CSP). 

The U.S. Department of Energy (DOE) Solar Energy Technology Program requested the SAM 

development team to review the photovoltaic performance modeling with the development 

community and specifically, with the independent engineering community. The report 

summarizes the major effort for this technical review committee (TRC).
iv This report is available at no cost from the 

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Acknowledgments

The authors would like to thank the appropriate parties for the onsite TRC meeting. Particularly, 

this includes the following committee members (external to DOE): 

Industry 

• Ben Bourne (SunPower Corporation) 

• Jeff Newmiller (DNV KEMA Energy & Sustainability)

• Monali Joshi (Black & Veatch Corporation) 

• Drew McMahan (Luminate)

• Walt Novash (Johnson Controls, Inc.) 

• Charlie Dearie (McCalmont Engineering)

• Tim Pfannenstiel (AWS Truepower)

National Laboratories

• Josh Stein (Sandia National Laboratories). 
v This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

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Executive Summary

In 2012, the DOE Solar program and NREL completed a plan for the Laboratory’s next three 

years of research, development, and deployment for system modeling. The overall goal is to 

improve system modeling accuracy and risk assessment via research into improved data and 

algorithms. The systems modeling activity aims to make robust models available to various 

audiences, thereby improving the industry characterization of risk and improving bankability 

across all markets (residential, commercial, and utility). 

In April 2013, the systems modeling team at NREL hosted an onsite technical review committee 

(TRC) to invite discussion and guidance from the industry on how to best prioritize research and 

development in PV systems modeling with a focus on improving the flagship System Advisor 

Model (SAM) tool. The full day meeting revealed general agreement regarding several key areas 

for improvement to the tool. These categories of SAM enhancements are highlighted and briefly 

explained below, with an indication of how they might be achieved with DOE support through 

modified and expanded agreement deliverables and milestones. 

Some aspects of the current research and development (R&D) agreement were thought by the 

TRC to be of limited value, and although these are listed , they are likely to be deemphasized 

going forward. It is worth noting that numerous key improvements suggested by the TRC were 

already in the three-year development plan, but a reprioritization is suggested to reflect the 

current needs of the community. 

Six key areas of improvement were identified:

1. Integrated 3D shading modeling and visualization 

2. Module model enhancements and ability to integrate measured module performance data 

directly from test measurements

3. Inverter model flexibility

4. Weather data input flexibility and associated tools 

5. Validation reports, PV technical documentation, and loss tree improvements 

6. PVWatts documentation and usage guidance 
vi This report is available at no cost from the 

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at www.nrel.gov/publications.

Table of Contents

1 Introduction ........................................................................................................................................... 1

1.1 TRC Participants........................................................................................................................... 2

1.2 Agenda .......................................................................................................................................... 2

2 Key Outcomes and Recommendations.............................................................................................. 4

2.1 Integrated 3D Shading Scene Modeling and Visualization........................................................... 4

2.2 Module Model Enhancements and Integration of Measured Module Performance Data ............. 4

2.3 Inverter Model Flexibility ............................................................................................................. 5

2.4 Weather and Solar Data Input Flexibility...................................................................................... 6

2.5 Validation Reports, Documentation, and Loss Diagram Improvements....................................... 6

2.6 PVWatts Documentation and Usage Guidance............................................................................. 7

3 Detailed PVWatts Feedback ................................................................................................................ 8

4 Detailed SAM Feedback ..................................................................................................................... 10

4.1 General User Interface ................................................................................................................ 10

4.2 Resource Inputs........................................................................................................................... 10

4.3 Derates, Performance Adjustments............................................................................................. 11

4.4 Utility Rates & Electric Load...................................................................................................... 12

4.5 Simulation Options (p50/p90, Stochastic, Parametrics).............................................................. 12

4.6 Simulation, Results, and Outputs Options (Plots, Cash Flows, Loss Trees)............................... 12

4.7 Financing, Incentives, Depreciation, Costs................................................................................. 12

4.8 Documentation/Help, User Support, Training, SAM API, SAM SDK....................................... 12

4.9 PV system design needs not being met - particularly in the areas of layout options, shading 

options/models, and module/inverter models.............................................................................. 13

4.10 SAM Validation Needs (What would make users more confident?) .......................................... 15

4.11 CSP and CPV Modeling Feedback ............................................................................................. 15

Appendix A. SAM TRC Feedback and Prioritization Notes from the White Board/Paper Sheets..... 16
1 This report is available at no cost from the 

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1 Introduction

In 2012, the DOE Solar program and NREL completed a plan for the Laboratory’s next three 

years of research, development, and deployment for system modeling. The overall goal is to 

improve system modeling accuracy and risk assessment via research into improved data and 

algorithms. The systems modeling activity aims to make robust models available to various 

audiences, thereby improving the industry characterization of risk and improving bankability 

across all markets (residential, commercial, and utility). The proposal is divided into three parts 

(in decreasing level of resources required to complete these enhancements): 

1) Validation of System Advisor Model (SAM)/PVWatts and related Web services to 

improve acceptance by the solar industry and gather valuable insights to improve and 

interact with a technical review committee of users and potential users

2) Transfer R&D results from across systems integration to the solar analysis 

community via development of the successful DOE-sponsored SAM tool 

3) Refactor simple system algorithms to better inform the risk and improve the 

bankability of residential systems and provide technical updates to the PVWatts system 

model. 

Within the validation task (Task 1), a description of the use and value of a technical review 

committee (TRC) for SAM was noted, including the following language:

Create a TRC made up of independent engineers, other lab modelers (from Sandia in 

particular), the solar finance community, and utility engineers.

• This TRC will meet in person with the SAM team annually (starting in the early second

quarter of year one); several shorter Web-based meetings will also be held

• The TRC will have several outcomes for the SAM team:

o Education of the TRC members on the current and planned capabilities of SAM 

o Development of what members would consider as convincing “validation” of 

SAM and “bankability” of SAM results

o Impart to the SAM team the reasons that other tools (specifically PVSyst) are 

considered more “trustworthy” than SAM 

o Assistance for the SAM team in feedback on the model

• The TRC will reconvene annually in person and more than once per year via Web to reevaluate progress toward SAM improvements to meet their needs and the needs of the larger 

industry 

• Integrate the TRC with Sandia’s modeling workshop (if logistically possible) which has been 

pushed to FY13 and reports the outputs of the TRC to the PV Modeling Collaborative 

website.
2 This report is available at no cost from the 

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at www.nrel.gov/publications.

Unfortunately, due to staffing issues and delays from FY12 milestones, the first TRC meeting 

was not held until April 2013. However, the goals described in the planning document text 

above should be achieved going forward. 

1.1 TRC Participants

The TRC provided very useful feedback with participation from all individuals invited. In 

person contributions were particularly helpful in the discussion.

Industry 

• Ben Bourne (SunPower Corporation) 

• Jeff Newmiller (DNV KEMA Energy & Sustainability)

• Monali Joshi (Black & Veatch Corporation) 

• Drew McMahan (Luminate)

• Walt Novash (Johnson Controls, Inc.) 

• Charlie Dearie (McCalmont Engineering)

• Tim Pfannenstiel (AWS Truepower)

Sandia National Laboratories 

• Josh Stein

NREL

• Nate Blair

• Aron Dobos 

• Steven Janzou 

• Paul Gilman 

• Leah Kaffine

• Janine Freeman

• Sarah Kurtz

• Pam Gray-Hann (logistics)

1.2 Agenda

The goal of the SAM TRC was to receive structured feedback on a variety of SAM topics of 

greatest interest to this particular audience. The TRC was queried in advance to identify those

areas and topics of most interested. The organizers received a response from most members and 

adjusted the final schedule accordingly (noted below). In response, the agenda dedicated more 

time to priority topics. For example, items with 15 minutes on the agenda received either 1 or 0 

votes from the TRC committee members. For example, we could assume that this audience 
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at www.nrel.gov/publications.

would not be interested in utility rates, while for other users that capability represents a key value 

in using SAM. 

Agenda for SAM TRC

Monday Afternoon 

11:30-12:00 PM Lunch and introductions 

12:00-12:30 PM SAM Team introduction, planned updates and improvements 

12:30- 1:15 PM PVWATTS new version demo, planned work, and feedback

1:15 to 5:00 PM Specific topics for feedback

1:15- 1:45 PM General user interface

1:45- 2:15 PM Resource inputs 

2:15 – 2:30 Break

2:30- 3:30 PM Derates, performance adjustments 

3:30- 3:45 PM Utility rates and electric load 

3:45- 4:15 PM Simulation options (p50/p90, stochastic, parametrics) 

4:15- 4:30 PM Simulation, results, and outputs options (plots, cashflows, loss trees) 

4:30- 4:45 PM Financing, incentives, depreciation, costs 

4:45-5:15 PM Documentation/help, user support, training, SAM API, SAM SDK 

Tuesday Morning

8:00- 10:15 AM PV System design needs not being met

• Layout options 

• Shading options/models 

• Module/inverter models 

10:15-10:30 AM Break

10:30- 11:15 AM SAM validation needs (What would make users more confident?) 

11:15- 11:30 AM CSP and CPV modeling feedback

11:30- 12:30 PM Prioritization of improvements and wrap-up
4 This report is available at no cost from the 

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2 Key Outcomes and Recommendations

This section categorizes and describes the primary TRC recommendations for SAM and 

PVWatts modeling efforts at NREL.

2.1 Integrated 3D Shading Scene Modeling and Visualization

- Ability to define a 3D scene of buildings, trees, and other obstructions, and estimate 

shading impacts on a PV array over the course of a year 

- Ability to properly model row-to-row self-shading in one-axis tracked arrays, including 

diffuse light degradation on some backtracked arrays 

Historically, the SAM team has avoided integrating 3D shading scene modeling capabilities into 

the tool under the assumption that this could be done with other tools, even if that effort was 

cumbersome. However, discussion with the TRC and at the subsequent Sandia PV modeling 

workshop in Santa Clara indicated very broad support for the integration of such capabilities into 

SAM. Several industry stakeholders showed how their use of 3D visualization tools helped them 

estimate shading impacts, allow interactive visual review of a proposed system design, and have 

an iterative check throughout the modeling process. There was strong general agreement that 

current tools and methods were inadequate and were a significant stumbling block for efficiently 

developing project proposals.

Several approaches are possible for 3D visualization of a PV system installation and calculation 

of the shade. It is recommended that the Lab consider partnering with commercial software 

vendors that have similar tools and investigating what levels of integration might be possible 

with SAM. Additionally, the SAM team will investigate the techniques for implementing such a 

capability from scratch with SAM using standard, well documented methodologies for 3D shade 

calculations. The actual path will be chosen based on cost, performance and integration with 

SAM. Either path reflects a potentially major effort, but the feedback was very clear that 

improvement in this area was critically important for SAM to be more broadly accepted. 

In a related area, one-axis tracking in SAM currently does not derate the diffuse light due to rowto-row shading effects. Furthermore, if backtracking is disabled, SAM assumes that the rows are 

far enough apart to avoid shading altogether. This is typically an invalid assumption as more 

developers may tolerate some level of shading to increase system packing factors. 

Consequently, SAM must be modified to enable self-shading calculations for one-axis tracking 

to really make the back-tracking feature work as desired. The TRC indicated that these values 

would be important, and that this aspect be approached using a straightforward, area-weighted 

approach. 

2.2 Module Model Enhancements and Integration of Measured 

Module Performance Data

- Ability to edit module performance parameters directly and easily 

- Ability to tune the module model given measured IV curves and related data 

- Ability to automatically import IEC-61853 test matrix data and calibrate the module 

models
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- Ability to import or otherwise manipulate .pan files used by PVSyst 

- Continue to maintain a current module database 

The most important piece of any photovoltaics modeling software is how the module itself is 

modeled. While SAM provides several best-in-class module models and frequently updates its 

module databases for the CEC and Sandia models, the TRC members agreed that a key area for 

improvement is the ability for users to enter module performance parameters directly into the 

tool. This involves being able to take measured specifications from a third-party performance 

test and tune the SAM module models to accurately reflect the measured performance across the 

whole irradiance-temperature performance space. In addition, new modules used in large scale 

systems frequently do not appear in the module databases immediately, so there is a need to be 

able to manually enter performance specifications.

The TRC expressed that there is generally “standardization” in the industry to accept the IEC61853 test matrix for PV modules. The recommendation is to augment the single-diode model 

representation in SAM to be flexible enough to accurately characterize measured module 

performance. This requires a research effort to determine which additional parameters are 

required in the model to enable it to match specifications. A mix of physical parameters and nondimensional tuning parameters may be required. 

Given additional module model flexibility, there is a need to be able to automatically and 

accurately estimate model parameters from the given data. The concept is for SAM to 

automatically calibrate its module model to the set of measured performance data provided; if 

only one or two IV curves are available, SAM would know how to best utilize those to predict 

overall performance. If the IEC-61853 test matrix results are available, SAM would know how to 

import those results and automatically calibrate the model. Currently it is not possible to do this 

with either module model (CEC, Sandia) in the SAM tool. An extension of this would be to 

interpret PVsyst .pan files that appear to be common in the industry; however, it is not proposed 

to make a PVsyst-specific import option because that would imply that the underlying models 

are identical, which could not be guaranteed. 

The SAM team had identified some of these issues and has several aspects of these features on 

the current plan, and proposed to augment and shift milestones based on TRC feedback to best 

address the stated needs in a timely and robust fashion. 

2.3 Inverter Model Flexibility

- Ability to manually edit Sandia inverter performance parameters directly and easily 

- Ability to manually enter inverter efficiency curves based on measurements

- Ability to import or otherwise manipulate .ond files used by PVsyst

- Continue to maintain a current inverter database

General agreement existed among TRC members that the SAM inverter model implementation 

was not flexible enough to accommodate many usage scenarios. Frequently, a specific inverter 

was not included in the default Sandia model database, and the single point efficiency model was 

insufficient to properly characterize system performance.
6 This report is available at no cost from the 

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The SAM team proposes to augment the current Sandia model to allow users to easily enter their 

own performance parameters, as well as add a new inverter model based on a piece-wise linear 

or spline-fitted efficiency curve derived from measured part-load efficiency data points provided 

by the user. This general purpose inverter model method will deliver much greater flexibility. 

The inverter-related enhancements agreed to in the three-year project plan regarding staging and 

other large system effects were not considered to be a high priority among TRC members. 

2.4 Weather and Solar Data Input Flexibility

- Ability to import sub-hourly data and select what timescale at which the SAM model 

runs

- Ability to enter measured plane-of-array (POA) data directly into the model

- Ability to enter solar and weather data in CSV column-data formats rather than be 

required to use TMY2 or TMY3 file formats, especially when the data itself is not a 

TMY specification

- Built-in tool with capability to analyze, visualize, merge, and backfill measured data 

streams for producing SAM-capable input files

Extended options and additional flexibility for entering solar and meteorological resource data 

into SAM was a consistent theme among TRC members. In particular, the ability to directly 

enter measured plane-of-array (POA) resource data was a highly sought-after capability that 

comes with several challenges due to the nature of the PV models that were designed to utilize 

multiple irradiance components. This represents a research opportunity to determine the best 

technique for entering POA data into SAM consistently across all PV module models. 

A sub-hourly data input capability was also requested, along with the ability to run SAM at a 

timestep different from the data input. For example, a user could enter one-minute data, but have 

SAM run simulations at five- or ten-minute steps, and aggregate accordingly for the multitude of 

hourly output data vectors. The SAM project plan included this idea of but will likely be 

accelerating it at least via the interface more immediately. A key aspect of this extension would 

require designing a flexible standard SAM data input file format that: 1) is minimalistic, 2) is 

easy to edit with Excel, 3) works across all time step inputs, 4) is well documented, and 5) has 

integrated tools in SAM for manipulating, visualizing, and automatically backfilling missing 

data. This solar resource data analysis tool could be similar to the capabilities available to the 

wind program via the commercial Windographer tool. 

2.5 Validation Reports, Documentation, and Loss Diagram 

Improvements

- Augment validation reports against measured system performance data with intermodel comparisons 

- Provide detailed technical manuals for the PV performance models

- Enhance the loss tree reporting system in SAM to show more details similar to PVsyst

The TRC indicated that the SAM validation reports in progress under Task 1 were valuable 

contributions to the knowledge base and would help SAM gain acceptance for PV systems 

modeling. Interestingly, the group had not found similar documentation and validation reporting 
7 This report is available at no cost from the 

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at www.nrel.gov/publications.

for PVsyst, but that did not diminish its use. Regardless, the TRC suggested that inter-model 

comparisons, particularly to PVsyst for the same validation datasets, would be a useful outcome. 

In addition to the SAM validation reports, a comprehensive PV Modeling Technical Manual for 

SAM was suggested as an important step toward achieving full transparency and subsequent 

acceptance of the SAM model implementation. The SAM team has already started on this work, 

and it intends to add its completion as a project milestone for year two. 

The primary guidance with regards to the visual loss tree was that to be accepted, the loss tree in 

SAM needs to be much more detailed. Several TRC members described how the PVSyst loss 

tree is used as a design tool as well as a final document to include in reports. The SAM team will 

implement the outcome of the efforts of Sandia and others to define standard terminology for the 

losses and system derates. However, in the interim, the SAM team will attempt to modify their 

work structure to replicate the PVSyst loss terminology for better consistency. It should be noted 

that calculating the losses appropriately will require reformulating significant parts of the PV 

performance models to enable the calculations to be made during the simulation process. 

2.6 PVWatts Documentation and Usage Guidance

The TRC was generally very pleased with the demonstration of the updated PVWatts online tool. 

However, the main concerns were that users were not adequately warned of PVWatts limitations 

and the level of uncertainty in its predictions. Suggestions included having more comprehensive 

documentation of the tool as well as the underlying calculations, and adding disclaimers and 

caveats on the results pages. In particular, adding a watermark or similar cautionary text to the 

final PVWatts report would help educate users as to the appropriateness of a PVWatts 

calculation for a particular purpose. Moreover, the proposed work in year two to add P50/P90 

inter-annual variability analysis to PVWatts was a much welcomed capability. The PVWatts 

team will also provide additional links and guidance to SAM and non-DOE tools to encourage 

enterprising PVWatts users to migrate to more sophisticated tools in situations where detailed 

systems performance analysis is warranted.
8 This report is available at no cost from the 

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3 Detailed PVWatts Feedback

The new beta version of the online tool for PVWatts, which is available at 

http://pvwattsbeta.nrel.gov was also demonstrated at this event, with several screen captures of 

that demonstration provided below. The TRC provided feedback even though they aren’t 

necessarily the key PVWatts user group (even though all members were familiar with it). 

The comments received are listed in Appendix 2. There was a consistent theme in the 

suggestions: providing the user with more understanding of the limitations of PVWatts. The 

team had already included some of this thinking in the three-year plan. However, the response 

from the SAM TRC encouraged the team to raise this in priority. Specifically, they suggested or 

supported current plans for:

• Generally including uncertainty in the output values

• Outputting a range of production values, giving guidance as to how to achieve the 

higher end of the range, and explaining how to access SAM for more in-depth analysis

• How to avoid implying that impossible configurations are possible on a roof 

• Disregarding module power coefficients 

• Warnings when system sizes exceed the assumptions used in the model

Figure 1. PVWatts beta system information input page 
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Figure 2. New PVWatts beta results view
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4 Detailed SAM Feedback

The team received high-level and detailed feedback from the TRC in a number of areas. To make 

this discussion more structured for the reader, the authors have summarized the results by agenda 

area as much as possible. Certain items cross into other areas, so the entire set should be 

reviewed collectively. For each recommendation, the team has initially indicated how this would 

impact the three-year work plan. A separate document and spreadsheet will indicate to DOE 

proposed changes to milestone language and language in the LPDP technical narrative 

document. The expectation is that a broader audience might be interested in this feedback. 

4.1 General User Interface

The agenda began with basic user interface feedback, which is generally easy to provide and also 

allows the SAM team to ascertain where the TRC was likely to focus their interest when 

reviewing the user interface. 

The SAM team started this section by describing the concern that the SAM user (for PV) is 

almost immediately faced with the choice of selecting a “PV module model” which can be 

problematic. The TRC generally agreed that can be confusing and supported the plan to let the 

user select an existing module name and allow SAM to select the best module model for that 

module name. 

There were other comments on how to make SAM more streamlined. The approach used by 

TurboTax came up as a related idea to indicate which inputs had deviated from the default values 

for the project. Finally, the discussion centered around the suggestion to make common inputs 

more prominent by pushing seldom-used inputs to options chosen by selecting an “advanced’ 

button and/or adding text to the UI indicating “this isn’t very common” or “this is rare”. 

In general, the TRC liked the fact that SAM has great visualization options and can do 

parametric analysis easily.

As the idea of streamlining the module/inverter modeling interface was in the existing work plan, 

pursuing these ideas likely would not lead to modifications of milestones. The team will 

investigate indicators for when the SAM inputs have been changed from the default values.

4.2 Resource Inputs

There was a robust conversation about resource data, reflective of the TRC makeup as well as 

the changing world of PV project development. Several of the key highlighted ideas (ranked 

highly by individual TRC members at the end) were grouped into this category. Additionally, 

this same area of discussion came up several times in the subsequent Sandia Modeling Workshop 

in California.

Specific requests in this area include:

• Sub-hourly data capability

• As already addressed in the SAM multi-year plan, the use of sub-hourly data with SAM. 

While this could go down to rates in seconds, the goal here is to modify SAM to include 
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existing sub-hourly data and, more importantly, allow SAM to run with sub-hourly time steps 

and handle the various output values correctly 

• As stated above, the existing project plan included this idea but will likely be accelerating it, 

at least via the interface, more immediately 

• Greater flexibility in irradiance input options for all models (i.e., SAM use of POA). Here, 

the primary and immediate driving issue is the use of POA data. This is how data is typically 

measured and is critical if the team intends to use Locus Energy as a near-term validation 

partner.

• More flexibility with resource input; possibly a standard SAM PV input file with reduced 

variables. The key idea is to input sub-hourly data at various time levels and minimize

useless data streams. The current SAM PV models use only a third or less of the available 

data within a TMY3 file

• Ability to input one’s own long-term or multiple resource datasets for P50/P90 analysis. This 

can already be done in SAM but likely could have improved access and/or documentation 

• Nomenclature: “TMY3” is being used where not appropriate because the “format” of the file 

often gets confused with the content. A user can create a TMY3-formatted file that has a 

specific year’s worth of data. Creating a SAM-specific solar data format would be one way to 

improve this. 

4.3 Derates, Performance Adjustments 

This area is a primary research effort area for both NREL and Sandia. The SAM team eventually 

intends to incorporate the output of several other NREL projects, including those being led by 

Sarah Kurtz. Additionally, Sandia is also working extensively on the bankability/rate issue. 

The Sandia-organized PV modeling workshop in also focused heavily on this topic, with a 

special half-day session at the end of their meeting. 

From the SAM TRC, the primary guidance was that to be accepted, the loss tree in SAM needs 

to be much more complex. The SAM team learned that the PVSyst loss tree is used as a design 

tool as well as a final document to be included. 

The SAM team will implement the resulting efforts of Sandia and others to define standard 

terminology for the losses/derates. However, in the interim, the SAM team will likely modify 

their work structure to more significantly replicate the PVSyst loss terminology for better 

consistency.

Of the items listed below, a number of them are relatively easy and the SAM team will make 

have made progress on several of these by the next release. The most difficult issue will be the 

calculation of these values within the SAM construct and the calculation of snow and soiling 

based on available data.

• Nomenclature: “Snow & Soiling” loss?

• More intuitive snow cover input- number of days (Summary NOAA data: x days with >5”) 
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• Single curtailment input (MW)

• Apply degradation to capacity vs. performance 

• Nomenclature- losses & efficiencies on scale of 100% versus coefficients

• Availability gets distributed over entire year- this is a problem for comparison on a monthly 

or hourly basis 

• Soiling: Provide Addie. Kimber’s original and follow-up papers, link to references in 

interface. Basically, allow users to easily access collective wisdom about soiling.

4.4 Utility Rates & Electric Load 

For this section, the SAM team described the various options available and how SAM accesses 

these items within the interface. These features were new to the TRC, as few of them had used 

them from within SAM for their work. The SAM team will continue to work with other 

stakeholders that use these capabilities extensively to perform analysis at NREL and for 

universities.

4.5 Simulation Options (p50/p90, Stochastic, Parametrics) 

In this area, the team rolled out some of our general plans, notably the idea of combining 

stochastic SAM capabilities with the idea of doing p50/p90. This approach was well received 

and the team was able to educate them more broadly on the issue. 

4.6 Simulation, Results, and Outputs Options (Plots, Cash Flows, 

Loss Trees)

Related to derates and general PV improvements, the TRC indicated that they used the standard 

outputs and that they used the standard reports. They typically use other financial tools but like 

having the option to use this as a first cut. 

4.7 Financing, Incentives, Depreciation, Costs

For this section, the SAM team described the various options available and how SAM accesses 

these items within the interface. The TRC members learned how to utilize these SAM 

capabilities for their work. The SAM team will continue to work with other stakeholders that 

use these capabilities extensively to perform analysis at NREL and for universities.

4.8 Documentation/Help, User Support, Training, SAM API, SAM SDK

The SAM team reviewed the current plan and activities related to these topics. Several 

organizations were very interested in the SAM SDK and hadn’t yet known about it. Almost 

everyone said they had used the online videos to get started on SAM. They all felt that, while 

there was a lot of information in the documentation, it would be helpful to pull it out separately

for a specific audience. The SAM team asked if the current plan to provide a relatively high-level 

PV technical manual would be helpful, and that was strongly encouraged. Key suggestions for 

these areas included:

• Links to whitepapers and other references at the relevant location in the SAM interface

• PV technical user guide 
13 This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

• Additional granularity in pop-up help menus to indicate the location on the page

• SAM SDK webinar

• (From the Sandia modeling workshop, a related suggestion) make the libraries of information 

available within SAM more readily available, such as the Sandia Module model coefficient 

library

The SAM team has already added an SDK webinar to the plan and has started the process of 

modifying the SAM website to make it easier for users to download related data items like the 

libraries separately from the main SAM tool. The team will continue to provide technical 

support through a variety of mechanisms. 

4.9 PV system design needs not being met - particularly in the areas 

of layout options, shading options/models, and module/inverter 

models

This section of the agenda received a majority of votes from the TRC members before the 

meeting and they were very interested in providing feedback in this area. Again, the team started 

by describing current plans for improving module and inverter models and system layout. 

Several of those were embraced and several were discouraged. The items that were most desired 

and those currently planned that received the least support are highlighted below. 

Major Suggestions (in terms of likely effort)

• Visualization & 3D shading calculations 

o Ability to display shading at a given date/time 

o Inter-row shading for backtracking (i.e., First Solar)

Comment: this is a new request for the SAM team, but it received very broad support. The use of 

this visualization as a first cut at shading, for a visual review of the design, and as an iterative 

view throughout the modeling process was not something we had fully grasped. A valid 

question is “how best to do this?” Initially, partnering with programs that have existing tools and 

then reviewing any standards in the literature for appropriateness (for example, export formats 

from AutoCAD) will likely be helpful. The team also will investigate the options for 

constructing our own tool. Both of these seem like potentially major efforts, but the feedback 

was very clear that this was critically important to allow SAM to be generally accepted. 

Backtracking in SAM currently has the issue that diffuse light isn’t degraded if there is shading 

from other modules. Additionally, if backtracking is turned off, SAM assumes that the rows are 

far enough apart not to have shading. This is a poor assumption as more developers forgo using 

backtracking. SAM uses a similar assumption for two-axis tracking as well. The team would 

need to add self-shading to at least one-axis tracking to really make the backtracking feature 

work as desired. The TRC indicated that these values would be important and could be 

approached in a more simplistic area-weighted approach, as done by PVSyst. 
14 This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

• Module model enhancements to utilize other module data directly and represent 

performance across the performance space better 

• Ability to “read in” PAN files used by PVSyst

• Ability to edit module coefficients/information directly and easily

• Ability to input one’s own module based on efficiency curves, user data, etc. 

• Methods to integrate 61853 test data 

• Maintain a current module database 

• Selection by PV module vs. model, but maintain option to switch to other model

Comment: The team plans to modify and simplify the SAM PV interface so that users (especially 

new users) aren’t immediately faced with the decision of which module model to use. This 

enhancement will present a list of modules or the option to import or construct custom modules. 

This plan was received positively even though TRC members would like the ability to change 

the option back from, for example, Sandia to CEC. 

• Light induced degradation loss option and/or modifying existing models to include light 

induced degradation more directly.

Comment: This issue and the related modeling have also arisen at the recent Sandia Modeling 

Workshop in California. 

Minor Suggestions (in terms of likely effort)

Comment: Each of these items will not be commented on individually, but instead be reviewed

for difficulty and implemented based on efficiency. The most complex item on this list is 

probably focusing SAM on enabling design via several smaller changes. However, all of the 

suggestions can readily improve someone’s user experience and so they are worthy of 

consideration. 

• Monthly albedo values (hourly is too refined, but annual is not flexible enough)

• Generally modifying SAM with a focus on enabling design rather than just energy 

estimation. An example of this would be improved AC/DC sizing feedback/guidelines

• No one uses isotropic model - remove or hide

• Separate thermal models from module models and evaluate which thermal models to 

have 

• No one uses NOCT - hide it

• Visual representation of which defaults have been changed

• Visual representation of which sections a user has visited

• Backtracking inputs - module width and tracker center-to-center versus current inputs

• PV subarrays - when tracking options are highlighted, tilt and azimuth should be 

grayed out 
15 This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Currently planned items that were discouraged:

• Building on the current FY12 capability to have multiple sub-arrays within a SAM system 

feeding a single inverter, allowing the SAM user to have multiple sub-arrays feeding 

either a single inverter or a bank of staged inverters.

Comment: It seemed that this particular activity wasn’t very highly valued by the TRC given that 

they aren’t sure the data is readily available and the use of this system topology was not very 

commonly used. A modular structure for large systems was much more typical than staged 

inverters.

4.10 SAM Validation Needs (What would make users more confident?)

Again, here the team reviewed our current validation efforts and asked specifically what kinds of 

graphs/charts/metrics they would like to see that would validate SAM. The TRC responded with 

several comments:

• Specifically, they were interested in seeing more inter-model comparisons as well as 

comparisons to real data. They felt that comparisons between SAM and PVSyst would be an 

important step. This in the plan already, but it was good to hear it was considered valuable.

• The idea of documenting the primary steps that SAM goes through to model a PV system 

would be important. 

4.11 CSP and CPV Modeling Feedback

This discussion, while short, was generally illuminating. Some members of the TRC felt quite 

strongly that CPV wouldn’t likely be a widely deployed commercial technology in the near 

future, and so we should focus less effort there in the near term. 

On the topic of CSP, two reviewers indicated that SAM was the premier solar modeling tool 

available for CSP. No one seemed to make concrete suggestions for CSP.
16 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Appendix A. SAM TRC Feedback and Prioritization 

Notes from the White Board/Paper Sheets

The following bulleted list contains a summary of the final TRC meeting section, the 

prioritization of suggested improvements by the committee members. All improvements that 

were suggested during the two-day meeting are listed, grouped by category. In the final section 

of the TRC meeting, participants were asked to rank their top five priorities. The highlighted 

bullet points indicate the top priorities as ranked by the participants. Green highlighting indicates 

that the item received multiple votes in the number one or two spot, and yellow highlighting 

indicates the rest of the items that received a top five priority by the TRC. 

PV Watts 

• Improve user understanding of limitations of PVWatts 

o Output a range of production values and give guidance for how to achieve higher 

end of range; link to SAM for more in-depth analysis 

o How not to imply that impossible configurations are possible on a roof 

o Module power coefficients not taken into account 

o Warnings when system sizes exceed the assumptions used in the model

• Improved PVWatts default assumptions for tilt angle- not based on latitude? (consult 

Dunlop) 

• Check ohmic effects in PVWatts (reference Jeff Newmiller)

• Ability to input a two slope array on a roof. 

Resource Inputs

• Sub-hourly data capability

• Greater flexibility in irradiance input options for all models (i.e., SAM use POA)

• More flexibility with resource input, possibly a standard SAM PV input file with reduced 

variables

• Input individual long-term or multiple resource datasets for P50/P90 analysis

• Ability of SAM to evaluate timestamps

Results/Outputs

• Update output report with more information, specifically improved loss tree 

o Break out module losses 

o Specify gain from transposition

o Break out inverter losses: clipping, temperature, threshold

• Probabilities of Exceedance 

o P50/P90 analysis using global NASA dataset 

o Variable uncertainty (P75,P90,P99, etc.) ability
17 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

o Evaluate just on resource AND on combination resource/energy 

• Ability to filter hourly modeled data by a given parameter 

• Parametric analysis: highlight large differences or outliers

Support/Documentation

• Validation activities

o Inter-model (i.e., compare to PVSyst, etc.) 

o Work with Sandia validation algorithm 

o Model “edge” cases as well as normal cases

• Links to whitepapers and other references at the relevant location in SAM 

• Target development one sector at a time (i.e., residential, then commercial, then utility)

• PV technical user guide 

• Additional granularity in pop-up help menus telling users where they are on the page.

Models & Algorithms

• Visualization & 3D shading calculations 

• Module/Inverter files 

o Ability to use PAN and OND files

o Ability to edit module/inverter files

o Ability to input one’s own inverter based on efficiency curves, user data, etc. 

o Implement piecewise inverter curve calculation

• Tool to fit 61853 test curves 

• Maintain a current module/inverter database

• Apply degradation to capacity vs. performance 

• Bring back the “No financing” option 

• Soiling: Provide Addie. Kimber’s original and follow-up papers, link to references in 

interface. Basically, allow users to easily access collective wisdom about soiling.

• Combine Monte Carlo and probability of exceedance analyses 

• Ability to handle SMA “above nameplate” inverters 

• Inter-row shading for backtracking (i.e., First Solar)

• Monthly albedo values 

• Separate thermal models from module models and evaluate which thermal models to include 

o No one uses NOCT, hide it 

• Availability gets distributed over an entire year; this is a problem for comparison on a 

monthly or hourly basis 
18 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

• Tilt angle optimization tool (with options based on what optimizing for)

• Backtracking algorithm doesn’t work. 

PV Design Interface

• Simplify: reduce options, hide advanced settings to make more user friendly (designate tasks 

for the “super user” or functions that might be considered rare) 

• Ability to display shading at a given date/time

• Improvements in using SAM as a PV design tool (not just evaluation) 

o Example: AC wiring calculations 

o Example: Separate inverter/transformer specifications and losses

• Improved AC/DC sizing feedback/guidelines 

• Selection by PV module vs. model, but maintain option to switch to other model

• Fix confusing SAM layout 

• Visual representation of which defaults have been changed 

• Visual representation of which sections have been visited

• Single curtailment input (MW)

• Mechanism for users to provide feedback as to what’s missing

• Nomenclature- “TMY3” being used where not appropriate 

• Nomenclature- losses and efficiencies on scale of 100% versus coefficients

• Light induced degradation loss option 

• Date updated should display for OpenEI data in SAM 

• Backtracking inputs- module width and tracker center-to-center vs. current inputs 

• PV subarrays- when tracking options highlighted, tilt and azimuth should be grayed out 

• Nomenclature: “Snow and Soiling” loss?

• More intuitive snow cover input- number of days (Summary NOAA data: x days with >5”) 

• Isotropic model rarely used - remove or hide.