| UTILITY INTER-TIE
(GRID-TIE) PV SYSTEM DESIGN Budget, roof
dimensions and other site-specific factors often call for custom system
design. If you are planning
to mount your array on a roof, decide which module best fits into the
available roof space, taking into consideration obstructions such as
chimneys, plumbing vents and skylights. See
solar module section for
dimensions of modules. A grid-connected PV system consists of PV modules,
output cables, module mounting structures, AC and DC disconnect switches, inverter(s), grounding equipment and metering system.
The Worksheet below will help you decide what size PV array
would be required to eliminate your electric bill. This will be the largest
system that would be cost effective to install. A smaller system can reduce
part of your bill, or eliminate higher cost electricity in locations that
have progressively increasing rates as consumption increases. Use this
information and the amount of available space to get a rough idea of your PV
array size.
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TO DETERMINE THE PV ARRAY SIZE FOR A
GRID CONNECTED SYSTEM |
|
Information Required |
Fill in Amounts |
| Step 1 |
FIND YOUR MONTHLY
AVERAGE ELECTRICITY USEAGE FROM YOUR ELECTRIC BILL |
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This will be in kilowatt-hours (kWh). Due to air conditioning,
heating and other seasonal usage, it is a good idea to look at
several bills. You can add the typical summer, fall, winter and
spring bills and divide by four to find the average monthly usage.
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| Step 2 |
FIND YOUR DAILY AVERAGE
ELECTRICITY USE |
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Divide the monthly average number of kWh use by 30 (days).
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| Step 3 |
FIND YOUR LOCATION’S
AVERAGE PEAK SUN HOURS PER DAY |
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See the chart and listings on pages 12 and 13. For example, the
average for California is 5 peak sun hours
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| Step 4 |
CALCULATE THE SYSTEM
SIZE (AC WATTS) TO PROVIDE 100% OF YOUR ELECTRICITY |
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Divide your daily average electricity use by the average sun hours
per day. For example, if the daily average electricity use is 30
kWh, and the site is in California, then the system size would be:
30 kWh / 5 h = 6 kW AC
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| Step 5 |
CALCULATE THE NUMBER OF
PV MODULES REQUIRED FOR THIS SYSTEM |
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Divide the system AC watts in Step 4 by the CEC watt rating of the
modules to be used, then divide by the inverter efficiency, usually
0.94 and you get the total number of modules required. (Round this
number up)
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Use the below chart to determine possible array
size/inverter combinations
This chart shows inverter and module combinations for common modules used
in grid connected systems. For a given inverter and module combination, the
chart displays the acceptable number of series strings of modules and the
number of modules per string for temperatures between 14ºF and 104ºF. Where
the inverter will support more than one string of modules, the chart shows
the number of modules that can be used with multiple strings. Sizing is
accurate in locations where the maximum temperature is lower than 104ºF or
the minimum temperature is higher than 14ºF. In locations where the minimum
temperature is lower than 14ºF, the maximum number of modules per string may
be lower. String sizes followed by *** may have some derating in full sun.
In the chart, the line labeled CEC Watts is the expected output of the
modules at normal operating temperature, in full sun. The line below this,
labeled Ratio, is the output at operating temperature divided by the label
rating of the module. Modules with a higher ratio give you more actual
output power per rated watt.
The approximate power output of a system in full sun will be the number
of modules times the CEC rating of the modules times the inverter efficiency
from second column on the table. Other factors, such as high or low
temperature, shading, array orientation, roof pitch and dirt on the modules,
will affect the system’s actual output.
String Sizing Tool
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Permissible number of
modules per string - 14ºFMinimum Temp. - 104ºF Maximum Temp. |
|
Inverter |
Module |
Shell |
Evergreen |
Kyocera |
Mitsubishi |
|
SQ165 |
SQ175 |
EC-110-GL |
EC115-GL |
EC120-GL |
KC175 |
MF170 |
|
Brand & Model |
CEC Efficiency |
CEC Watts |
149.1 |
158.3 |
98.4 |
103.1 |
107.6 |
154.9 |
152.5 |
|
CEC % of nameplate |
0.904 |
0.905 |
0.895 |
0.897 |
0.897 |
0.885 |
0.897 |
|
SMA SWR1800U |
91.5% |
one string |
6 to 8 |
5 to 8 |
11 to 16 |
11 to 16 |
11 to 16 |
8 to 12 |
7 to 11 |
|
two strings |
6 |
5 to 6 |
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SMA SWR2500U |
93.0% |
one string |
9 to 12 |
9 to 12 |
18 to 24 |
18 to 24 |
18 to 24 |
13 to 16 |
12 to 17 |
|
two strings |
9 |
9 *** |
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SMA SB3800U |
94.5% |
one string |
8 to 10 |
8 to 10 |
16 to 20 |
16 to 20 |
16 to 20 |
12 to 15 |
10 to 14 |
|
two strings |
8 to 10 |
8 to 10 |
16 to 20 |
16 to 20 |
16 to 18 |
12 to 13 |
10 to 13 |
|
three strings |
8 to 9 |
8 |
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SMA SB6000U |
94.5% |
one string |
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two strings |
9 to 12 |
9 to 12 |
18 to 24 |
18 to 24 |
18 to 24 |
13 to 18 |
12 to 17 |
|
three strings |
9 to 12 |
9 to 12 |
18 to 21 |
18 to 21 |
18 to 20 |
13 to 14 |
12 to 14 |
|
four strings |
9 to 11 |
9 to 10 |
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FRONIUS IG2000 |
93.5% |
one string |
7 to 8 |
6 to 8 |
12 to 20 |
12 to 20 |
12 to 20 |
8 to 14 |
8 to 14 |
|
two strings |
5 to 7 |
5 to 7 |
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three strings |
5 |
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FRONIUS IG3000 |
94.0% |
one string |
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|
12 to 20 |
12 to 20 |
12 to 20 |
8 to 15 |
8 to 14 |
|
two strings |
5 to 8 |
5 to 8 |
12 to 14 |
12 to 14 |
12 to 14 |
8 to 9 |
8 to 9 |
|
three strings |
5 to 6 |
5 to 6 |
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four strings |
5 |
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FRONIUS IG4000 |
94.0% |
one string |
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two strings |
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12 to 20 |
12 to 20 |
12 to 20 |
8 to 13 |
8 to 13 |
|
three strings |
6 to 9 |
6 to 8 |
12 to 13 |
12 to 13 |
12 to 13 |
8 |
8 |
|
four strings |
5 to 6 |
5 to 6 |
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five strings |
5 |
5 |
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FRONIUS IG5100 |
94.5% |
one string |
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two strings |
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|
12 to 20 |
12 to 20 |
12 to 20 |
8 to 15 |
8 to 14 |
|
three strings |
8 to 9 |
7 to 9 |
12 to 17 |
12 to 17 |
12 to 17 |
8 to 11 |
8 to 11 |
|
four strings |
6 to 9 |
5 to 8 |
12 to 13 |
12 to 13 |
12 to 13 |
8 |
8 |
|
five strings |
5 to 9 |
5 to 6 |
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six strings |
5 to 6 |
5 |
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seven strings |
5 |
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|
XANTREX GT 2.5 |
94.0% |
one string |
8 to 12 |
7 to 12 |
16 to 24 |
16 to 24 |
15 to 24 |
11 to 18 |
10 to 17 |
|
two strings |
8 to 9 |
7 to 8 |
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XANTREX GT 3.0 |
94.5% |
one string |
8 to 12 |
7 to 12 |
16 to 24 |
16 to 24 |
15 to 24 |
11 to 18 |
10 to 17 |
|
two strings |
8 to 11 |
7 to 10 |
16 to 17 |
16 |
15 |
|
10 to 11 |
|
three strings |
|
7 |
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XANTREX GT 3.3 |
94.5% |
one string |
8 to 12 |
7 to 12 |
16 to 24 |
16 to 24 |
15 to 24 |
11 to 18 |
10 to 17 |
|
two strings |
8 to 12 |
7 to 11 |
16 to 18 |
16 to 17 |
15 to 17 |
11 |
10 to 12 |
|
three strings |
8 |
7 |
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String sizes
followed by *** may have some derating in full sun.
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