By Teresa Zhang, VP of operations and Paul Grana, co-founder, Folsom Labs
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. As the team behind HelioScope, we are frequently asked, “What’s a good value for kWh/kWp?” The answer is, as with many solar design questions, it depends.
In this article, we discuss the factors that drive specific yield up or down and present typical kWh/kWp values for a variety of locations, weather data sources and representative designs.
Factors that impact specific yield
Specific yield (or simply “yield”) refers to how much energy (kWh) is produced for every kWp of module capacity over the course of a typical or actual year. While typical values can range from 1,000 kWh/kWp to over 2,000 kWh/kWp, the actual value is driven by many factors, including:
- Location. A project’s location determines the amount of sunlight, or irradiance, it will receive. Irradiance is usually the biggest driver of specific yield because irradiance can vary so much. For example, El Paso, Texas, gets twice the annual global horizontal irradiance (GHI) as mountainous areas of Washington or Montana. Irradiance depends directly on the location of an array, as well as on other factors related to location, such as the array’s exposure to shade, soiling and snow cover.
- Weather file. There are a number of different ways of constructing a weather file, including ground-based measurements, satellite-based models and hybrid approaches. These different methodologies can result in weather files that differ by 10% or more for the same location and the same period of time.
- Module orientation. The module tilt and azimuth impact how much of the available sunlight (GHI) will actually hit the surface of the array (also known as plane-of-array or POA irradiance). The POA gain from tilting modules from horizontal to a tilt of 18° can, for example, range from 8% in Miami, Florida, to over 20% in Anchorage, Alaska.
- Module selection. Modules are often described by a single number—module wattage. But in terms of energy per watt, not all modules are created equal. All modules lose efficiency in hot and low-light conditions, but they lose efficiency at different rates. Temperature performance in particular can make a real difference; a module with a better temperature coefficient can improve the yield of a project by as much as 2-4%.
- Balance of system efficiency. BOS efficiency includes inverter efficiency, inverter clipping, MPP tracking losses, DC and AC wire losses, mismatch losses and more. Many energy production model tools simply assume a fixed value for system losses, but HelioScope rigorously models each system loss for every hour of the year. See Figure 1 below for a sample graph of system losses from HelioScope.
Typical kWh/kWp values
Now that we’ve defined the drivers of yield, let’s drill down to yield values for a couple of real examples to see how location, weather file and design individually impact yield, starting with location. Figure 2 shows GHI for four representative locations, as reported by NREL Solar Prospector.
For any given location, you may have access to a number of weather files. In Figure 3, you can see that different sources of weather data report different irradiance values for the same location, and that the ranking of values reported by different weather file sources is not consistent from location to location.
Finally, design is usually the biggest driver of yield at your control. While there are infinite design options, we’ll stick to five representative designs, as shown in Table 1.
Table 2 shows yield values for these designs in four representative locations:
In each location, there’s a difference of 20% or more between the lowest yield design and the highest yield design (that is, between the high-shade residential design and the utility design). All of these designs have different impacts of shading, temperature and even plane-of-array orientation adjustments.
Many solar developers have a sense for the kWh/kWp yield they expect to see based on their own history. But not many appreciate how those will vary based on the location, the system type or even the weather file used. As with everything in solar, the devil is in the details!