Key metrics used in the solar energy industry

Monitoring a photovoltaic system (and any other system in general) is needed to ensure continuous optimal operation. It helps lower the impact of possible faults, it can be used to compare and benchmark the system’s performance to other systems or to itself over another period of time. In my other article I talk about some of the ways photovoltaic systems can be monitored, and what are the benefits from it.

In order to empirically quantify aspects of the system, various metrics are used, and this article is focused on describing their meaning, where are they applicable and how they can be calculated.

Specific yield – kWh/kWp

Specific yield (kWh/kWp) is one of the most commonly used performance metrics for solar systems of all sizes. It’s used to compare different locations, to analyze different designs or to assess the health of an array.

Specific yield (or simply “yield”) refers to how much energy (kWh) is produced for every kW peak of module capacity over the course of a typical or actual year. For example, if you have 300 kW of photovoltaic modules peak capacity, and your systems yields 450 MWh of energy in one year, your metric is:

450 MWh / 300 kWp= 1500 kWh/kWp

Performance ratio

The performance ratio is the ratio of the actual and theoretically possible energy output. It is largely independent of the orientation of a PV plant and the incident solar irradiation on the PV plant. For this reason, the performance ratio can be used to compare PV plants supplying the grid at different locations all over the world.

The performance ratio is a measure of the quality of a PV plant that is independent of location and it therefore often described as a a quality factor. The performance ratio (PR) is stated as percent and describes the relationship between the actual and theoretical energy outputs of the PV plant. It thus shows the proportion of the energy that is actually available for export to the grid after deduction of energy loss (e.g. due to thermal losses and conduction losses ) and of energy consumption for operation.

Self-sufficiency and self-consumption rate

These metrics apply to systems where the energy is consumed by the owner of the system. Hybrid systems where some part of the produced energy is self-consumed and part of the energy is fed to the grid also exist, so interpretation of this metric should be carefully made.

Self-sufficiency refers to how much of the consumed energy, came from the photovoltaic system. It is a ration between self-consumed produced energy of the system and the total consumed energy. It is important metric in the case where the energy consumption is higher than the actual production of the system (or when the consumption curve does not match the production curve).

Self-consumption rate is calculated as the percentage of produced electricity that is self-consumed. It is equal to the ratio of self-consumed electricity and produced* electricity.

*In the case where energy can not be fed back to the grid, when the energy requirement is lower than the possible production, inverters can lower the energy production, so it matches the energy draw, that there will be no energy fed back to the grid. In this case, the self-consumption rate may be very close to 100%, but that is not a relevant metric, because it doesn’t take into account the discarded energy by the inverters.

Here is an example of graph taken from the Solar Data Collector monitoring dashboard, illustrating a case where the energy production never exceeds the energy consumption, although it could, if the energy draw was higher.

Global Horizontal irradiance- GHI

The radiation from the sun reaching the earth’s surface can be represented in a number of different ways. Global Horizontal Irradiance (GHI) is the total amount of shortwave radiation received from above by a surface horizontal to the ground. It is used to characterize the energy potential of a location and is independent of the system’s specification.

Plane of array irradiance – POA

The radiation from the sun reaching the surface of the PV modules. It is dependent on the placement of the PV modules, such as angle, azimuth, nearby obstructions and horizon shadings (such as high mountains).

The metrics above can be aggregated over a certain period of time, usually one year. A full year is used to take into account seasonal changes and patterns. But sometimes, it is useful to compare the metrics between seasons, or even between hours of the day. The consumption curve can be improved to match the production curve, so the self-consumption rate is increased, for example.

The following metrics bellow are constant throughout the lifetime of the system (unless upgrades or changes take place), and are determined in the planning phase of the system:

DC to AC ratio

The ratio of how much DC capacity (the quantity and wattage of solar panels) is installed to the inverter’s AC power rating is called the DC-to-AC ratio, or DC load ratio, over-sizing ratio or overloading ratio, etc. For example, a 120-kWdc array with a 100-kWac inverter has a DC-to-AC ratio of 1.2. It often makes sense to oversize a solar array, such that the DC-to-AC ratio is greater than 1. In other words, one would adjust the number of solar panels so the DC capacity divided by the AC output is greater than one. This allows for a greater energy harvest when production is below the inverter’s rating, which it typically is for most of the day.

Ground coverage ratio (GCR)

The Ground Coverage Ratio (GCR) is the ratio between the PV modules area and the total ground area. A higher GCR means PV modules are more densely installed at the site, which maximizes the land usage, but it can have negative impact on the energy yield, due to inter-row shading. The rows of modules will be installed closer relative to each other, and one row can shade another, in the early and late hours of the day.

Some of these metrics are easy to calculate, but some require models for simulation and advanced data processing techniques. Solar Data Collector will soon launch a platform that automates the calculation of the metrics above (and many others), that integrates with the data from the inverters to provide easy to use dashboards and tools for monitoring of photovoltaic systems of any size. In the basic case, it doesn’t require additional hardware and can be setup in less than 30 minutes. Contact us if you want to try the platform for free, before the official launch.