Solar PV Performance Analysis: 1. Checking Data

Before we start our performance analysis we need to check the availability and completeness of site data. Typically any solar asset has some form of monitoring software which can be used to visualise and download data from site.

The Performance Ratio (PR) is one of the easiest ways to evaluate site performance of any capacity, the formula is as follows:

To calculate a PR we need at least:

1. Production --> Meter(s) or all inverters data

Meter data is the most accurate measurement of energy delivered to the injection point. There may be several different meters on the site depending on the size of the asset. For each day or period of measurement, we need to know the total energy delivered to each injection point.

If somehow meter data is not available, we can substitute in inverter data. Sum of inverter data is typically about 1-2% higher than meter data (because there are AC cable losses to the injection point), so inverter data should never be used to bill a client but can be used to estimate site performance.

2. Irradiation --> Some form of irradiance data

The data should be complete without gaps. E.g. in the image below the pyranometer (irradiance) data has gaps, so we can’t use it to calculate a performance ratio. In this case you could (1) either use pyranometer data from a nearby site as a rough estimate or (2) use satellite data.

We should combine the PR calculation with a check of availability, i.e. what % of the site is available to produce energy. The availability can be calculated at the inverter and/or string level and ideally should always be 100%.

To check site availability at the string level we need in addition:

3. String current per inverter

String current data is needed to know if strings are not producing or if there is a string with a looming insulation resistance (RISO) fault which could soon cause the inverter to trip (or worse).

In summary:

Analysis	Best Data Source	Alternative Data Source 1	Alternative Data Source 2
Inverter Availability	Inverter Power	-	-
String Availability	String Current Data	-	-
Performance Ratio	Meter Output	Inverter Output	-
Performance Ratio	Pyranometer Data	Satellite Data	Nearby Site Pyranometer Data

Now assuming we have enough data, we can move on to checking site availability (inverter and string) in the next post.