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Now
is the time to become your own Power Producer.
We can help with all of your power system needs.
 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.
TO DETERMINE THE PV ARRAY
SIZE FOR A GRID CONNECTED SYSTEM
| Step 1 |
FIND YOUR
MONTHLY AVERAGE ELECTRICITY USAGE FROM YOUR ELECTRIC BILL |
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This will be in
kilowatt-hours (kWh). Because of 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 here. 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/5h-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 Sept 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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To determine possible
array size/inverter combinations, use the chart here.
This chart shows inverter
and module combinations for common modules used in grid
connected systems. Fore a given inverter and module
combination, the chart shows the acceptable number of
series strings of modules and the number of modules per
string for temperatures between 14°F and 104°F.
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
104°F or the minimum temperature is higher than
14°F. In locations where the minimum temperature is
lower than 14°F, the maximum number of modules per string
may be lower. String sizes followed by *** may have
some derating in full sun.
The lower section of the
chart shows inverter models that are designed to work on
3-phase AC voltage systems that have 208 VAC line to line,
or 277 VAC line to neutral. SMA 1800 and 700 watt
inverters operate on 120 VAC, which can be used line to
neutral on 240 VAC single phase and 208 VAC 3-phase
systems.
In this 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 and dirt on the modules, will affect the
system's actual output.
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