Also, the exceptions to the insolation versus load issues above occur when
there is a greater load in the summer than in the winter. This takes place in mod-
erate to high temperature locations where summertime fan use, evaporative
cooling, or air conditioning is required to maintain comfort. The actual usage and
energy collection requirements must never be assumed and a serious discussion
with the customer must take place to eliminate uncomfortable surprises.
Here are eight steps to take to size a standalone PV system:
1. Determine budget and any planned expansion
2. Determine electric loads by developing the annual load profile
3. Determine the available solar resource for the site
4. Determine size and choice of battery type
5. Determine size and choose the type of PV array
6. Choose charge controller(s)
7. Determine size and choose inverter(s)
8. Determine wiring requirements for the system on the basis of environmental
conditions and any potential expansion
Utility-Connected System Sizing
A utility-interactive PV system does not have to provide for all daily energy load
needs. The system can be sized based on the owner’s budget and wants. When
designing a system that is tied to a grid, the following factors must be determined:
Budget and any planned expansion
Amount of space available for panels and inverter(s)
Percentage of energy load that is to be generated from the PV array
Availability of incentives
Availability of financing
Net metering interconnection rules
Regulations by and from the utility provider
A grid-connected system can be sized by dividing the daily electrical demands
(in kilowatt-hours) by the average derated system losses and peak sun hours per
day for the area. In doing so, you are taking into account the amount of energy
available as irradiance and the overall efficiency of the system, not just the panels.
You can determine the peak sun hours by consulting solar maps and various
other worksheets and tools.
For example, if you live in Lexington, Ky., and your family uses 6,000 kWh of
electricity per year, this breaks down to about 17 kWh a day. In Lexington, the
average peak sun hours are about 4.5. Divide 17 by 4.5 and this will give you the
non-derated array size, which is 3.8 kilowatts. Then divide by the system efficiency,
which we will presume for this example is 80 percent. Many integrators do not
include the derating step, which is a source of serious trouble in the long term.
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