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Design of Residential Photovoltaic Systems 141

where L CT represents the thermal capture losses:

T (
L CT = 1 + c T C − T STC)  (6.7)
P G0  
where
c T is the thermal coefficient for power
T C is the cell temperature
T STC is the temperature at STC
L CM represents nontemperature related losses (miscellaneous capture losses) that are due to

• String diodes and wiring
• Partial shading, dust, and snow on the panels
• Module mismatch
• PV inverter tracking errors (MPPT)
• Errors in irradiation measurement

There are also considered system losses that are defined as

L BOS = Y A − Y F (6.8)
which includes the total losses without the generator losses, like in the case of DC–AC conversion.
Now that all these yields and losses have been defined; the final performance indicator can be
defined, which has been named as performance ratio (PR):

PR = Y F (6.9)
Y R

giving a value of degree for approximation to the ideal case. Using this performance indicator, the
performances of PV systems of different sizes, PV cell technologies, and PV inverters and located
in different corners of the world can be compared.

6.3 CASE STUDY FOR DESIGNING A RESIDENTIAL PV SYSTEM

6.3.1 Methodology: Design Procedure
As it can be concluded from the previous sections, PV installations constitute a long-term and relatively
expensive investment. Therefore, it is essential that an exact design of a certain PV installation
is carried out. Figure 6.8 presents a simple flowchart for the design procedure of residential PV
systems. In order to be able to design a PV installation, extensive and precise information of a wide
range of parameters are required as described earlier in this chapter.
Besides all the technical aspects, there are also some financial issues to be considered for such an
investment: payback time and internal rate of return. The financial calculations can be complicated
and have many variables included. For small private PV systems on residential houses, the home-
owners often look at the investment and the simple payback time in years, which can be calculated
as the investment is divided by the yearly savings. Many homeowners wish to be self-supplied with
energy, so the PV system can be dimensioned to cover the annual electricity consumption of the
household. The average energy consumption in a typically Danish four-member family house is
approximately 4300 kWh per year. Depending on the type of feed-in tariff for the produced solar
energy, another threshold for the dimension could be the financial optimization of the system. If the
selling price for the produced solar energy is less than the purchase price, many plants are designed
to maximize the local consumption of the produced solar power.