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Solar Installation Guidebook - System Design Considerations
This section of the guidebook provides an overview of some of the major technical aspects that should be considered when developing a PV project.
Solar in Philadelphia

The potential for solar generation in Pennsylvania is relatively consistent across the Commonwealth although it is higher in the Philadelphia region than the Pittsburgh region. This is partly because Philadelphia has many reflective surfaces and receives about 9% more solar radiation on an annual basis than Pittsburgh as is graphically illustrated in Figure 3.2.1

When Philadelphia became designated a Solar America City in 2008, it set a goal to install 2.3 MW of solar generation capacity by 2011, which will be enough to provide electricity to more than 350 households. In 2009, the entire state had 4 MW of installed solar capacity2. To meet this commendable and aggressive goal, local solar contractors and developers must have a clear understanding of Philadelphia’s Permitting requirements and unique attributes of the City that might impact project development such as:

  • Complying with local rules and regulations and in the right order (refer to Section 4 of this guidebook for a detailed description of the process flow and requirements)
  • Historical buildings: Solar collecting devices installed within designated historical buildings, structures, sites, objects and districts must be reviewed and approved by the Philadelphia Historical Commission3
  • Structural ability of buildings to accommodate additional weight of PV modules because of high anticipated snow loads: Residential flat roofs may present additional challenges when anchoring typical stand-offs and tying into roof rafters that are several inches below the roof deck. A thorough analysis will be required to ensure a code-compliant installation.
  • Within the PECO service territory, there are several secondary network distribution systems that may limit the ability and capacity of PV systems installed. (See Section 4.3 for more information about PECO requirements.)
  • Prevailing Wage Law: The state’s Prevailing Wage law, which was established in 1964, sets $25,000 as the threshold above which Prevailing Wages must be paid to workers, if any state (i.e. taxpayer) money is involved. The Prevailing Wage law applies to projects of all classes, including residential and small commercial.4 Projects that use state rebate funds would require prevailing wages to be paid to workers for projects over $25,000 and will usually increase the labor costs of those projects.

Efficiency First

Before considering a solar photovoltaic system for a building (particularly a residential building), energy-efficient measures should be incorporated first. Energy-efficiency isnot only a cost-effective means of reducing carbon emissions (reducing energy usage thereby reducing the need for electric generation) but a home may also not need as large a PV system to meet its loads. Weatherization and conservation are low hanging fruit to achieve both energy and dollar savings. Figure 3-3 shows the profitability ofenergy efficiency upgrades and highlights the top 5 efficiency measures with the greatest return on investment:

  • 1 – High efficiency lighting & fixtures
  • 2 – Duct Sealing
  • 3 – Energy Star® clothes washer
  • 4 – Programmable thermostat
  • 5 – Water heater tank wrap

Additional measures include replacing older refrigerators and inefficient room air conditioning units which can also be very cost effective. Implementing energy efficient measures are much more cost effective and less capital intensive than installing a solar PV system, and therefore should be done first or in concert with installing solar PV.

These measures are contrasted with other familiar investments such as 30-year bonds, money market accounts and dividends on common stocks to give perspective of the positive return on investment these actions can provide.

There are also many opportunities for efficiency within commercial buildings. Advancement in lighting, air conditioning, and control technologies has resulted in large increases in efficiencies. A PV system will have a synergistic effect when installed in conjunction with energy-efficiency measures for any building type.

Contact the following groups for more information about energy efficiency recommendations:

  • Energy Coordinating Agency
  • Delaware Valley Green Building Council
  • Factors Impacting PV system performance

    There are many variables that affect the output of a PV system. Some of these factors are discussed in Section 3.5 Estimating PV System Performance of this document. The key factors that have the greatest impact on how a PV system performs are described here.

    Sunlight Intensity (Irradiance)
    The performance/output of a PV system is relatively proportional to sunlight intensity. Therefore these systems can generate electricity even on cloudy days. Greater amounts and duration of sunlight increase system performance. Sunlight intensity is called irradiance, which is measured in watts per square meter (W/m2). In summer, when the sun is nearly directly overhead, its irradiance at the surface of the Earth, at sea level, is approximately 1000 W/m2. This irradiance is defined as ―full‖ or ―peak sun,‖ and it is the standard irradiance for testing and rating PV modules. At peak sun conditions, roughly 70% of the sun that is incident at the top of the atmosphere penetrates to the surface of the Earth. If PV modules are mounted perpendicular to the sun’s rays, it is possible to receive close to peak sun nearly every sunny day at noon for much of the continental United States.5

    Over a one year period, PV modules in Philadelphia will receive the most perpendicular sunlight when the PV array is facing due South and is tilted at an angle that is slightly less than the latitude of the region (for Philadelphia, 5 degrees less than the local latitude – 39 degrees – is recommended). However, tilting the PV array at or below the angle of latitude is not always the best solution for a project.

    • Some rooftops may not be able to structurally withstand the additional forces imposed (wind, snow, etc.) as the tilt angle of a PV array increases.
    • PV arrays tilted at lower angles will not need to be spaced as far apart to avoid shading from adjacent rows, therefore a greater quantity of modules can be installed. This may be desirable if a property has limited space. There are several modeling tools that can estimate the energy output of a PV system based on proposed design parameters (i.e. quantity of modules, electrical characteristics of specific PV technologies, tilt, local climate, etc.). These tools can help determine the best configuration that provides the best possible kilowatt-hours generated per area for the project.

    Shading Shading portions of a PV array will have the most adverse effect on the system’s performance. It is important to determine during the site assessment if a potential location for the PV array will be shaded, especially between the hours of 9 a.m. and 3 p.m. This is important, as the output of PV modules may be significantly impaired by even a small amount of shading on the array.

    A careful assessment using an hourly computer simulation program is necessary to determine the benefits of westerly orientations. A minimum of six hours of unshaded operation is important for best system performance.

    Figure 3-4 shows the location of the sun in the Philadelphia sky relative to a particular point (as noted by the Solar Azimuth on the x-axis). For example, at 12 noon on December 21st, the sun will be due south and at an angle (relative to the horizon) of approximately 25o. On this same day at 3pm, however, the sun will be at an approximate angle of 12o and will be located slightly southwest in the sky. These angles should be used to determine how far apart rows of PV modules should be located to ensure adequate energy generation during the winter months and minimize shading (when the sun is the lowest in the sky).

    However configuring a PV array to avoid shading 100% of the time is not always possible given space constraints both on building rooftops and on open land. For PV arrays that consist of multiple rows of PV modules and array structures, the losses that occur when one row shades an adjacent row should be accounted for when estimating the performance of the PV system.6 (Accounting for these losses is discussed further within this guidebook in Section 3.5 Estimating System Performance.)

    Next to shading, orientation of the PV array is one of the more important aspects of the site assessment. Facing the PV array due South is ideal; however, slight deviations can also be feasible. For example, an unshaded PV array with a tilt of 35 degrees and facing +/- 45 degrees away from due South (SE or SW) will still receive 92% of the annual solar radiation as compared to the PV array facing due south.

    Fully understanding what the orientation will be at construction must be understood very early in the project. Often the roof tilt is used as the orientation of a residential rooftop system due to the improved aesthetics of a parallel standoff roof mounted array rather than an array that is tilted at an angle greater than the roof tilt. Most roof orientations are not the most ideal for the array orientation so the impact of a less than optimal orientation must be understood prior to solidifying the system orientation.

    Other Factors
    There are several other factors that affect the performance of a PV system that should be communicated to potential system owners so they have realistic expectations of how their system will perform and the resulting economic benefits they can expect over time. These include:

    • PV module nameplate DC rating: PV modules are rated under specific conditions that are easily recreated in a factory and allow for consistent comparison of products known as Standard Test Conditions (STC). This value can be used to determine the DC power rating of the PV array. Since PV modules produce direct current (DC) electricity, their rating is designated in terms of watts DC at STC. For example a 100 watt PV module will have an output of 100 watts when conditions are identical to STC (cell temperature 25 degrees Celsius, solar irradiance 1000 W/m2, air mass 1). If these conditions are not present, then the actual output will be different based on the remaining factors below.
    • Temperature: PV module output power reduces as module temperature increases. The change in output varies based on the electrical characteristics of the specific PV technology.
    • Dirt and dust: can accumulate on a PV module surface, blocking some of the sunlight and reducing the output. However, this tends to be more of a problem in the Southwestern part of the U.S. (the Philadelphia region receives sufficient rainfall to minimize this impact).
    • Mismatch and wiring losses: There are inconsistencies in performance from one module to another (mismatch) and typically results in at least 2% loss. There are also losses associated with the resistance in system wiring which can be minimized by increasing the size of the wire.
    • DC to AC conversion losses: The DC electricity generated by PV modules must be converted to AC electricity to match requirements of common building loads. This is accomplished by an inverter. Currently, inverters have peak efficiencies of 92-98% (higher efficiencies are typically associated with larger inverters used in commercial and utility-scale projects).

    Note that in most all cases, the actual DC power generated will be less than the nameplate rating of the PV modules; however, when the ambient temperature is very cold and the sky is very clear, which sometimes occurs in the months of October or February, it is possible for the PV modules to generate power above their nameplate rating.

    There are also other factors used in determining how much the DC power of a PV system will be derated as it is converted into AC power (also known as the derate factor). The derate factor and its impact on system performance is examined in Section 3.5 Estimating PV System Performance in this document and includes a range of typical losses associated with each factor.

    Other Factors to Consider When Designing a System

    There are other variables that affect the design, cost and constructability of a PV system. In addition to optimizing the output of a PV system, other considerations that designers and installers should take into account include:

    • Space available for the PV array (roof or ground): Optimize the use of space by configuring the PV system for an acceptable shading derate factor (can be based on the Ground Cover Ratio (see call-out box “Shading Derate Factor and Ground Cover Ratio as a Design Tool” for details). Tilting a PV array at latitude is not always the best solution particularly in space constrained areas.
    • PV array string sizing. The number of PV modules that comprise a basic circuit (the building block of the PV array) should be determined based on the maximum system voltage expected by the system. The maximum system voltage is based on the lowest expected ambient temperature for the region7. (Note: One source for obtaining this value is the ASHRAE Handbook – Fundamentals. For the City of Philadelphia, this value is -15 degrees Celsius.8)
    • Ability for the existing electric service to accommodate a PV system: If the PV system delivers electricity to a building via a breaker in the existing service panel, the breaker size (as determined by the maximum current output from the PV inverter) is limited based on the rating of the panel 9 according to the electric code. Often times older homes may require upgrades to their existing electric service in order to accommodate a new PV system.
    • Current energy usage: Based on current net metering regulation in Pennsylvania, a PV system should not be designed to generate more annual energy than the building uses otherwise the cost-effectiveness of the system will be reduced.
    • Structural integrity of the roof : For rooftop applications, determine if the roof is able to accommodate the additional weight of the PV array and how the system should be attached to the existing structure.
    • Project Budget: Whether it’s a homeowner, business owner or commercial or industr ial user, the owner’s bottom line for a budget will have the most impact on the system size.

    Estimating PV System Performance

    There are several computer modeling tools that will estimate the performance of a PV system in a particular region of the country and the world. Some of the more commonly used tools in the industry for grid-connected PV systems are listed below:

    • PV Watts: Version 1 – allows users to select a location from a map or text list of predetermined locations throughout the world. Version 2 allows users to select any location in the United States by selecting a site on a 40-km gridded, interactive map. (Developed by the National Renewable Energy Laboratories)
    • Solar Advisor Model: Combines a detailed performance model with several types of financing (from residential to utility-scale) for most solar technologies (Developed by the National Renewable Energy Laboratories and Sandia National Laboratory)
    • Solar Design Studio: Simulates PV system operation based on user selected climate and system design and provides information on likely system power output and load consumption, necessary backup power during the operation of the system, and the financial impacts of installing the proposed system (Developed by the Maui Solar Energy Software Corporation)
    • PVSYST: Software package for the study, sizing, simulation and data analysis of complete PV systems (Developed by the University of Geneva) This software is oriented towards architects, engineers, and researchers

    It is important to thoroughly understand how these tools operate and the limitations of their results based on the users inputs.

    (Note: For a more comprehensive list of computer software tools, visit NREL’s Solar Technology Analysis Models and Tools

    The performance of a PV system depends on many variables as was discussed earlier. When estimating the energy savings that a PV system will achieve for a customer, it is better to not ―over-sell‖ the performance. Many of the performance modeling tools that are available allow the user to input the amount that the DC output of a PV array will be reduced (or derated due to losses in the system) by the time the power is delivered to loads. This derate factor should not be used as a hard-fast value to estimate the amount of power or energy that a PV system will generate. Instead it is recommended that contractors provide an output range that the actual expected performance will fall within.

    PV Watts provides a default derate factor based on losses associated with the system parameters it models. Although some designers consider it a conservative estimate, it is considered by others to be a reasonable estimate. Based on local experience, when comparing solar site assessment results and the actual annual performance of hundreds of PV installations in Southeastern Pennsylvania, an overall derating factor of 0.80 has been found to be a conservative value (not including shading impacts). This means that in this area, one can expect to get 80% of the output of a system. Table 3-1 summarizes the system parameter losses that determine the default derate factor.

    Some tools (such as Solar Design Studio, SAM and PVSYST) will model the output of the PV system based on the electrical characteristics of specific equipment such as PV modules and inverters. This is helpful when evaluating PV modules of different electrical characteristics (such as temperature coefficients for voltage, current and power). Ambient temperature has a greater impact on the output and performance of some modules than others, which can be helpful (and often necessary) when completing the electrical design.

    System Commissioning

    The commissioning process is well-accepted in the building industry and is typically applied to entire buildings. The process begins at project inception (during the pre-design phase) and continues through the life of the facility.

    The main objective of a commissioning plan is to assure the safe and orderly handover of the unit from the constructor to the owner, guaranteeing its operability in terms of performance, reliability, safety and information traceability. According to the Building Commissioning Association, ―The commissioning process includes specific tasks to be conducted during each phase in order to verify that design, construction and training meet the owner’s project requirements.‖ 10 Additionally, when executed in a planned and effective way, commissioning normally represents an essential factor for the fulfillment of schedule, costs, safety and quality requirements of the project.

    Translating this to the PV industry allows a means to formalize quality control of installed PV systems. It ensures that systems have been installed in a safe manner and will be high performing. Commissioning encourages integrators to be responsible for their installations and facilitates project closeout.

    Although the initial start-up of a PV system is often referred to as ―commissioning the PV system,‖ the final commissioning event should occur after all inspections have occurred (utility and per local code) and permits are signed off. However, the commissioning plan provides details throughout the development of the system about the events, requirements and timeline.

    Verifying the Actual Performance
    compared to the Expected Performance Determining whether a PV system is performing as expected is one of the most important aspects of commissioning a PV system. There are several ways of accomplishing this, including web-based tools and calculators such as PVWATTS. Other available tools include performance calculators that account for electrical rate schedules and provide financial analyses. There are also calculations that can be performed in the field based on simple measurements to adjust the output based on actual conditions at the site (irradiance, temperature, assumed derate factors, etc.)

    Part of the commissioning process should include some basic tests in the field. Before starting up a PV system for the first time, the following tests should be conducted to establish baseline data about the PV system:

    • Open Circuit Voltage and Polarity Test: determines if each string is producing the expected voltage and if polarity of the strings (+) and (-) at each combiner box is correct
    • Conductor Insulation Test (also known as a ―Megger‖ test based on the megohmmeter device used): verifies there are no leakage currents between the conductors and earth.
    • Ground Continuity Test: ensures continuous ground continuity between PV module frames and mounting hardware.

    There are also calculations that can be performed in the field based on simple measurements to adjust the theoretical power output based on actual conditions at the site (irradiance, temperature, assumed derate factors, etc.).12 The theoretical or calculated power can then be compared to the actual power output indicated by the inverter. Also, the DC current and DC voltage values can be measured to verify the inverter displayed power output is correct.

    Thoroughly commissioning the PV system and documenting the results will also provide reassurance to the owner that their system is performing to their expectations. The commissioning results can be a useful resource for troubleshooting potential problems in the future.

    Safety Concerns

    Photovoltaic systems produce direct current (DC) power from sunlight. Some grid-connected PV arrays use hundreds of modules connected in series and parallel to produce large amounts of power. Operating voltages may exceed 600 volts dc and currents at the subfield level may be hundreds of amperes.13 Working safely with PV systems requires a fundamental understanding of electrical systems coupled with common sense. There should always be a licensed electrician employed on any solar PV installation project to perform some if not all of the electrical work. Some of the interlocking plug-in wiring can be safely carried out, but it is essential that proper lockout/tagout procedures are followed when making the final wiring connections for a grid-tied system. There are the usual subtle hazards, as well. These include nicks, cuts, and burns from sharp or hot components. As such, gloves should be used when handling anything that might be sharp, hot, rough, or that might splinter. More importantly, special insulated gloves and eye protection should be used when testing or working with the electrical parts of the PV system.

    There is always the possibility of dropping tools or materials on either oneself, someone else, or on sensitive equipment or materials. Dropping tools across battery terminals is an especially dangerous hazard

    Other safety measures that should be observed include:

    • OSHA Regulations
    • Safety in Attics
    • Working Space for Electrical Systems
    • How Photovoltaic Systems Work, and Associated Safety and Testing Issues
    • Fire code safety Fire code requirements in Philadelphia, based upon OSHA regulations, require that a three foot perimeter around the roof edge is maintained on residential units for Fire Department access. For non residential roofs, a 6 ft. perimeter needs to be maintained.

    Required Project Coordination

    Once a site has been deemed suitable for a PV installation, the following coordination must occur:

    • Assemble the project team
    • Prepare Preliminary Design Plans
    • Coordinate with PECO to ensure the proposed design can be accommodated by PECO lines
    • Submit appropriate applications for State Solar Sunshine Program (if applying for a rebate), the Philadelphia Department of Licenses and Inspections (L&I), and PECO. See Section 4 for more details.
    • Await results of submittal review from all entities before proceeding further
    • Upon approval from all entities, begin constructing the PV system
    • Arrange for appropriate inspections and PECO meter installation
    • Finalize the start up and acceptance testing as part of the Commissioning Plan
    • Place the PV system in operation
    • Ensure the owner understands and is trained regarding how the system will operate and transfer commissioning test results, O&M manuals and contact numbers.

    Section 4 of this guidebook details the steps that solar contractors must follow to ensure compliance with all local codes, regulations and requirements.