How much energy a PV module delivers depends on several factors, such as local weather
conditions, seasonal changes, and installation of modules. As we already discussed in
Chapter 18, PV modules should be installed under the optimal tilt angle in order to
achieve best year-round performance. However, if the PV system is only used during a
specific period, the tilt angle needs to be optimized for that specific period. In fact the
power output during winter is much less than the annual average, and in the summer
months the power output will be above the average. Figure 20.2 shows the average global
horizontal irradiance of the world in kWh/m
2
/day, which is equivalent to the daily
equivalent sun hours (ESH). We mainly see that the insolation decreases with latitude.
However, other variations are visible because of regional climates.
Figure 20.2: The average global horizontal irradiance of the world given in kWh/m 2 /day, which is equivalent to the
daily equivalent sun hours (ESH). (Data taken from [157].)
As we already discussed in Chapter 5, solar cells are usually characterized with the
AM1.5 spectrum, which is normalized such that it has a total irradiance of 1,000 W/m
2 .
Hence 1 equivalent sun means a solar irradiance of 1,000 W/m
2
. When solar irradiation
data are available for a particular location, the equivalent sun hours can be determined.
Figure 20.3 shows the annual irradiation for the Netherlands at an optimally tilted plane
with a tilt of 36°. For example, in Delft (Netherlands) the average annual solar irradiation
at a horizontal plane is 999 kWh/m
2 [158]. For optimally tilted modules the annual solar
irradiation is a bit higher with 1,146 kWh/m
2
[158]. With the value from the AM1.5
spectrum, we hence find 999 equivalent sun hours at a horizontal plane and 1,146
equivalent sun hours at optimal tilt. When we take the length of the year (365.25 days)
into account, we calculate the average number of daily sun hours to be 2.7 h and 3.1 h for
horizontal and optimal tilt, respectively. For the rest of the discussion, we will use a value
of 3 h of sun per day.
conditions, seasonal changes, and installation of modules. As we already discussed in
Chapter 18, PV modules should be installed under the optimal tilt angle in order to
achieve best year-round performance. However, if the PV system is only used during a
specific period, the tilt angle needs to be optimized for that specific period. In fact the
power output during winter is much less than the annual average, and in the summer
months the power output will be above the average. Figure 20.2 shows the average global
horizontal irradiance of the world in kWh/m
2
/day, which is equivalent to the daily
equivalent sun hours (ESH). We mainly see that the insolation decreases with latitude.
However, other variations are visible because of regional climates.
Figure 20.2: The average global horizontal irradiance of the world given in kWh/m 2 /day, which is equivalent to the
daily equivalent sun hours (ESH). (Data taken from [157].)
As we already discussed in Chapter 5, solar cells are usually characterized with the
AM1.5 spectrum, which is normalized such that it has a total irradiance of 1,000 W/m
2 .
Hence 1 equivalent sun means a solar irradiance of 1,000 W/m
2
. When solar irradiation
data are available for a particular location, the equivalent sun hours can be determined.
Figure 20.3 shows the annual irradiation for the Netherlands at an optimally tilted plane
with a tilt of 36°. For example, in Delft (Netherlands) the average annual solar irradiation
at a horizontal plane is 999 kWh/m
2 [158]. For optimally tilted modules the annual solar
irradiation is a bit higher with 1,146 kWh/m
2
[158]. With the value from the AM1.5
spectrum, we hence find 999 equivalent sun hours at a horizontal plane and 1,146
equivalent sun hours at optimal tilt. When we take the length of the year (365.25 days)
into account, we calculate the average number of daily sun hours to be 2.7 h and 3.1 h for
horizontal and optimal tilt, respectively. For the rest of the discussion, we will use a value
of 3 h of sun per day.