To design for five days to recharge it, the batteries will require upsizing the
array by a fifth to 2,448 DC watts or more.
Remember, this is at full sun. During storms or cloudy weather, you may be
in very limited insolation conditions over time. As a result, you will need to
include all of the factors discussed above.
Always discuss an expansion plan with the customer. You can add on to the
panels and inverter section of the system, but you cannot expand the batteries
without endangering the whole battery array.
Cable Conversion and Adjustment Losses
One part of determining the PV array production is calculating voltage losses.
Electrical wiring that connects the PV array, battery, and load is responsible for
some of those deficits, as are poor connections and terminations. Cables and all
other components lose voltage through resistance. The conversion from solar
energy into usable current for equipment and appliances decreases electricity
production. The battery causes losses as well.
Cable Losses
Picking the right size cable eliminates the majority of electricity yield losses. Most
off-grid systems are designed with an expected 3 to 5 percent loss for the cable
from the array to the charge controller, as well as cable loss from the battery to the
charge controller. In total, voltage loss (V
L
) is expected to be around 6 percent.
For high performance systems, use 1 to 2 percent wire loss calculations
adjusted for summer temperatures.
Battery Conversion Losses
Calculating the transformation of chemical energy into electrical energy is hard
without knowledge of many variables. But as a rough guide, you can assume a
system will lose 10 to 20 percent of its energy as it goes from PV panels to batteries.
Mismatching Losses
During PV system operation, voltages do not always correspond as expected. PV
modules even in the same production batch will vary in voltage, current, and
power. With PV modules, the problem is in the variation of voltage and current.
Without testing and properly matching strings, this difference can be around
10 percent. By testing each string and making sure that the modules are operating
in the same voltage and current ranges, you can minimize the mismatch.
Remember that the inverter and charge controller, especially if you are using an
MPPT unit, are looking at the voltage. If the strings have different voltages, the
MPPT will not properly adjust to its optimum value.
The poorest performing module in the string will drag the rest of the string
down to its level. Multi-string mismatch outputs will cost more than the average
of those strings. It will also cheat your customer out of energy production.
130 ADVANCED PHOTOVOLTAIC INSTALLATIONS
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