We have provided a relatively simple worksheet for off-grid calculations.
It takes the integrator through the process and lays out the decisions that
need to be made, what the component parts of the worksheet means, what they
do, and what they do not do.
Determine Load
Use a wattage chart to determine the total daily electrical loads for a given
location.
Start with your desired or wanted load numbers and their operational time
component, and reduce them to your needed load and time. This will result in an
average load, although it may reflect either additional use of energy in the summer
for cooling or in the winter for heating—and additional lighting.
It would be best to develop an average annual load and then isolate the seasonal
peaks. A load study is not a guarantee of power. It is only a system energy target.
Be very clear with the system owner about what the process is and how using
additional energy will result in a need either to make up the difference with PV or
some other energy generator, or to do without at various times of the year.
If you have a generator or some other kind of backup, you can adjust the load
numbers to reduce system and battery size. Another option is to learn how to live
with less energy usage. A third way is to combine both of these; this is even better
and more satisfying.
Determine Local Solar Component
By determining a reasonably accurate monthly and annual insolation value, you
can factor in the available energy for your system design. A number of resources
exist for finding insolation values. Make sure you use data or existing information
from other locally working systems to determine the microclimate variations for
accuracy.
Determine PV System Efficiency
Your system efficiency will include de-ratings for temperature shade and other
factors related to panel selection, line and connection loss, and inverter loss.
Those factors should be somewhere between 60 and 80 percent depending upon
all of the factors above. Eighty percent is acceptable for very well-designed and
installed systems without design or installation shortcuts.
A sample is included below of a system with an efficiency of 80 percent,
which is a 20 percent loss of energy. After reviewing the sample, calculate the
system size for batteries and the array using a 60 percent efficient system that loses
40 percent of the energy as an efficiency loss. Until you have gained additional
experience and understand the finer parts of the process, your systems will
probably provide systems that are somewhere between the two.
CHAPTER 6 Standalone PV Systems 127
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