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A Gaide to PhotoVoltaic (PV) System Design and Installation - Page 3
Article Index
A Gaide to PhotoVoltaic (PV) System Design and Installation
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2.2.2. Shade Structure
An alternative to roof mounting is to mount the system as a
shade structure. A shade structure may be a patio cover or
deck shade trellis where the PV array becomes the shade.
These shade systems can support small to large PV
The construction cost with a PV system is a little different
than for a standard patio cover, especially if the PV array is
acts as part or the entire shade roof. If the PV array is
mounted at a steeper angle than a typical shade structure,
additional structural enhancements may be necessary to
handle the additional wind loads. The weight of the PV array
is 3-to-5 lbs./ft2, which is well within structural limits of most
shade support structures. The avoided cost of installing roof
brackets and the associated labor could be counted toward
the cost of a fully constructed patio cover. The overall cost of
this option will likely be higher than roof mounting, but the
value of the shade often offsets the additional costs. Other
issues to consider include
• Simplified array access for
• Module wiring, if visible from
underneath, must be carefully
concealed to keep the installation
aesthetically pleasing
• Cannot grow vines, or must be
diligent about keeping it trimmed
back from modules and wiring
2.2.3. Building-Integrated PV Array (BIPV)
Another type of system displaces some of the
conventional roofing product with buildingintegrated
PV modules. Commercially available products currently include roof slates (similar to
masonry roofing) and standing seam metal roofing products. Special attention must be paid to
ensure that these products are installed properly and carry the necessary fire ratings.
Figure 2 Patio Cover or Deck Shade
Figure 3 Building-Integrated Installation
PV Installation Guide
June 2001 Page 8
Dimensional tolerances are critical and installation requirements must followed precisely to avoid
roof leaks.
2.3 Estimating System Output
PV systems produce power in proportion to the intensity of sunlight striking the solar array surface. The
intensity of light on a surface varies throughout a day, as well as day to day, so the actual output of a solar
power system can vary substantial. There are other factors that affect the output of a solar power system.
These factors need to be understood so that the customer has realistic expectations of overall system output
and economic benefits under variable weather conditions over time.
2.3.1. Factors Affecting Output
Standard Test Conditions
Solar modules produce dc electricity. The dc output of solar modules is rated by manufacturers under
Standard Test Conditions (STC). These conditions are easily recreated in a factory, and allow for consistent
comparisons of products, but need to be modified to estimate output under common outdoor operating
conditions. STC conditions are: solar cell temperature = 25 oC; solar irradiance (intensity) = 1000 W/m2
(often referred to as peak sunlight intensity, comparable to clear summer noon time intensity); and solar
spectrum as filtered by passing through 1.5 thickness of atmosphere (ASTM Standard Spectrum). A
manufacturer may rate a particular solar module output at 100 Watts of power under STC, and call the
product a “100-watt solar module.” This module will often have a production tolerance of +/-5% of the rating,
which means that the module can produce 95 Watts and still be called a “100-watt module.” To be
conservative, it is best to use the low end of the power output spectrum as a starting point (95 Watts for a
100-watt module).
Module output power reduces as module temperature increases. When operating on a roof, a solar module
will heat up substantially, reaching inner temperatures of 50-75 oC. For crystalline modules, a typical
temperature reduction factor recommended by the CEC is 89% or 0.89. So the “100-watt” module will
typically operate at about 85 Watts (95 Watts x 0.89 = 85 Watts) in the middle of a spring or fall day, under
full sunlight conditions.
Dirt and dust
Dirt and dust can accumulate on the solar module surface, blocking some of the sunlight and reducing
output. Much of California has a rainy season and a dry season. Although typical dirt and dust is cleaned off
during every rainy season, it is more realistic to estimate system output taking into account the reduction due
to dust buildup in the dry season. A typical annual dust reduction factor to use is 93% or 0.93. So the “100-
watt module,” operating with some accumulated dust may operate on average at about 79 Watts (85 Watts x
0.93 = 79 Watts).
Mismatch and wiring losses
The maximum power output of the total PV array is always less than the sum of the maximum output of the
individual modules. This difference is a result of slight inconsistencies in performance from one module to
the next and is called module mismatch and amounts to at least a 2% loss in system power. Power is also
lost to resistance in the system wiring. These losses should be kept to a minimum but it is difficult to keep
these losses below 3% for the system. A reasonable reduction factor for these losses is 95% or 0.95.
Dc to ac conversion losses
The dc power generated by the solar module must be converted into common household ac power using an
inverter. Some power is lost in the conversion process, and there are additional losses in the wires from the
rooftop array down to the inverter and out to the house panel. Modern inverters commonly used in residential
PV power systems have peak efficiencies of 92-94% indicated by their manufacturers, but these again are
PV Installation Guide
June 2001 Page 9
measured under well-controlled factory conditions. Actual field conditions usually result in overall dc-to-ac
conversion efficiencies of about 88-92%, with 90% or 0.90 a reasonable compromise.
So the “100-watt module” output, reduced by production tolerance, heat, dust, wiring, ac conversion, and
other losses will translate into about 68 Watts of AC power delivered to the house panel during the middle of
a clear day (100 Watts x 0.95 x 0.89 x 0.93 x 0.95 x 0.90 = 67 Watts).
2.3.2. Estimating System Energy Output
Sun angle and house orientation
During the course of a day, the angle of sunlight
striking the solar module will change, which will affect
the power output. The output from the “100-watt
module” will rise from zero gradually during dawn
hours, and increase with the sun angle to its peak
output at midday, and then gradually decrease into
the afternoon and back down to zero at night. While
this variation is due in part to the changing intensity of the sun, the changing sun angle (relative to the
modules) also has an effect
The pitch of the roof will affect the sun angle on the module surface, as will the East-West orientation of the
roof. These effects are summarized in Table 1, which shows that an array on a 7:12-pitch roof facing due
South in Southern California gives, for example, the greatest output (correction factor of 1.00), while an East
facing roof at that same pitch would yield about 84% of the annual energy of the South facing roof (a
correction factor of 0.84 from Table 1).
Table 2 is intended to give a conservative estimate of the
annual energy expected from a typical PV system, taking
into account the various factors discussed above.
These values are for annual kWh produced from a 1-kilowatt
(1kW) STC DC array, as a simple and easy guide. If the
system includes battery backup the output may be reduced
further by 6-10% due to battery effects.
Example: A 4 kWSTC solar array (as specified under STC
conditions) located in the Los Angeles area at a 4:12 pitch
and facing southeast should produce at least 5343 kWh of
electric energy annually (1406 kWh/kW x 0.95 x 4 kW =
5343 kWh). The typical residential customer in that area
uses about 7300 kWh annually1, meaning such a PV system
could produce at least 75% of the total energy needed by such a
typical home. And if energy efficiency measures were taken by the owner to reduce the overall electrical
consumption of the home, the percentage could approach 100%. Note that the low end of the range was
used to calculate the actual savings. It is wise to be conservative when making performance claims.
Net metering has recently been extended to time-of-use customers yielding a potential additional value of
20-30% for the PV electricity generated by the system. With this net time-of-use metering, the homeowner
would cover almost their entire electric bill and only have to pay the monthly metering charge.
1 Actual residential electrical energy usage varies dramatically from one home to the next. It is best to use the previous
two years of energy bills to determine actual energy consumption for a particular home. Energy consumption in California
can vary from 3,000 kWh/year for a very minimal user to 25,000 kWh/year for a large home with heavy electrical usage.
Table 1: Orientation Factors for Various
Roof Pitches and Directions
Flat 4:12 7:12 12:12 21:12 Vertical
South 0.89 0.97 1.00 0.97 0.89 0.58
SSE,SSW 0.89 0.97 0.99 0.96 0.88 0.59
SE, SW 0.89 0.95 0.96 0.93 0.85 0.60
ESE,WSW 0.89 0.92 0.91 0.87 0.79 0.57
E, W 0.89 0.88 0.84 0.78 0.70 0.52
Table 2: Annual Energy Production
by City per kWSTC array rating
CITY kWh/kWstc (range)
Arcata 1092 - 1365
Shasta 1345 - 1681
San Francisco 1379 - 1724
Sacramento 1455 - 1819
Fresno 1505 - 1881
Santa Maria 1422 - 1778
Barstow 1646 - 2058
Los Angeles 1406 - 1758
San Diego 1406 - 1758
PV Installation Guide
June 2001 Page 10
2.4. Installation Labor Effort
Installation effort is very sensitive to specific house layouts and roofing type. An experienced crew can
install a 2 kW non-battery PV system in two-to-four person-days. Systems with large solar arrays are
relatively less effort per watt of power and kWh of energy than smaller systems because the installation of
the inverter and other hardware required by all PV systems is spread over more solar modules. Systems
with battery backup are more labor intensive than non-battery systems because of the additional wiring
required for wiring the critical load subpanel. A battery system can add 50-100% to the time required for the


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