Calculating site-specific irradiation on the solar generator plane as described in Section 2.4 or 2.5 provides a far more accurate estimate of annual energy yield for a grid-coupled PV system than with the approximate guide values in Section 1.4.4. However, inasmuch as most stand-alone systems are unable to use all of the available PV installation energy, the calculation for these installations is more complex and thus will be discussed in Chapter 8.
Multiplying total annual HGa of HG by the mean annual sum performance ratio PRa allows for an approximate determination of specific annual energy yield YPa (for more on specific annual energy yield, see Section 1.4.4 and Chapter 7; for more on precise yield calculations, see Chapter 8):
Dividing by 1 kW/m2 entails no calculation effort, but merely provides the correct unit. If the annual mean value of total daily irradiation of HG is available in lieu of total annual sum of HG, to obtain the requisite total annual HGa, multiply the annual daily mean by the number of days in the year, i. e. 365. The performance ratio PR for most grid-connected PV systems ranges from around 65 to 85%. Reasonably sized installations normally achieve a PRa exceeding 70%, and good systems reach values exceeding 75%. Excellent systems at sites with relatively low summer temperatures (e. g. in the mountains) can even achieve PR values of up to around 80%. The rule-of-thumb PRa value for good PV systems in Central Europe is around 75%.
The annual performance ratio PRa can be expressed as the product of three variables (see Chapter 7):
kTa = mean annual temperature correction factor. If no exact data are available, the following
approximate estimate applies in the presence of TUa (mean annual ambient temperature in ° C) for PV systems with crystalline solar modules:
kGa = mean annual generator correction factor. Depending on tolerance, this value normally ranges
from around 75% (for systems with numerous modules with unduly low PMo, small b,
sporadic winter snow cover, problems with shading, dirt, maximum power tracking by inverters, mismatch and so on) to 90% (for new and very good systems with modules that actually deliver the nominal rated output PMo). kG values exceeding 90% are achievable if effective module power output exceeds PMo or for PV systems with solar generators that integrate uniaxial or biaxial solar tracking mechanisms.
Zwr — mean (total) inverter efficiency (European efficiency, average efficiency for radiation conditions in Europe); zWR for selected inverters is indicated in Tables 5.11 and 5.12. Modern inverters achieve zwr values ranging from around 90 to 97%, depending on size.
The radiation calculations used here are from the examples in Section 2.4.2.
HGa — 1098kWh/m2/a (and 3.01 kWh/m2/d), kGa — 86%, zwr — 96%, TUa — 9.0°C
) With Equation 2.32, kTa — 0.951 — 95.1%, while with Equation 2.31, PRa — 78.5% — 0.785
) YFa — PRa ■ HGa/(1 kW/m2) — 862 kWh/kWp/a (and 2.36kWh/kWp/d).