P50 value is the center/mean, and it represents the estimate that occurs with the highest probability.įigure 3: P50, P75, P90 and P99 value represented in a normal distribution Yet for the sake of simplified calculations, and also because statistically representative data is not always available, a concept of normal (Gaussian) distribution of uncertainty is used (bell-shaped curve, see Figure 1). In solar energy, distribution of uncertainty does not perfectly follow normal distribution.
Lenders and investors typically use P90 estimates to be confident that sufficient energy is generated, allowing to safely repay the project debt. Therefore, other probabilities of exceedance such as P90 (estimate exceeded with 90% probability) or P75 (estimate exceeded 75% of the time) are considered. P50 level of confidence may represent too high risk for some investors. P50 is essentially a statistical level of confidence suggesting that we expect that the predicted solar resource/energy yield may be exceeded with 50% probability. This also means that with at same probability the expectation may not be achieved. The result of the modelling is the P50 estimate, or in other words, the “best estimate”.
To assess the photovoltaic (PV) energy yield potential of a site, we run models using best available data and methods.