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What’s a good value for kWh/kWp? An overview of specific yield

By Teresa Zhang | August 14, 2017

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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:

  1. 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.
  2. 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.
  3. 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.
  4. 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%.
  5. 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.

Figure 1: Typical system losses for a residential project in Columbia, South Carolina.

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.

Figure 2: GHI values for four representative locations (Image courtesy of NREL).

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.

Figure 3: GHI values from three source of weather data.

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 1: Four typical system designs

Table 2 shows yield values for these designs in four representative locations:

Table 2: Yield values for four representative 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!

About The Author

Teresa Zhang

Teresa Zhang is VP of operations at Folsom Labs. She is an engineer, environmentalist, and expert witness with a decade of experience in solar, spanning module manufacturing, project development, technical due diligence, and software. She holds BS and PhD degrees in mechanical engineering from MIT and U.C. Berkeley.

Comments

  1. aquinasd says

    May 16, 2021 at 6:16 am

    I think when you mention specific yield it’s also important to state duration because it is the energy yielded per unit installed capacity over time. So saying 2000kWh/kWp or even 120% is a bit ambiguous.

    Reply
    • mohammed says

      October 21, 2021 at 5:11 pm

      yearly

      Reply
  2. Tarikua says

    March 28, 2020 at 8:02 am

    for my solar design which data should be taken out of global irradiance, direct normal irradiance or specific yield?
    Is specific yield ,which is found on data site , associates for a particular type of pv panel ?

    Reply
  3. Kyle @ Green Coast says

    December 11, 2019 at 3:30 pm

    Awesome writeup and detail on yield. Thank you! This is the type of transparency we need in solar and clean energy.

    Reply
  4. David says

    October 9, 2019 at 2:46 am

    Although not stated, I assume the formula is:
    specific yield = energy produced in one year [kWh] / rated power of system [kW]
    Therefore, the unit of specific yield is hour [h].
    Furthermore, it seems easier to me to think of it as the number of hours per year at which the system effectively operates at its rated power.

    Reply
    • Erwin says

      December 20, 2020 at 7:54 pm

      Very good idea. Hours of rated system output seems a simple and effective measure.
      It could be refined by using cloudless sky days for the four seasons as bench marks.
      A cloudless day would be defined by the ideal shape of the PV generation curve

      Reply
  5. KM Lee says

    February 5, 2019 at 10:22 pm

    Ya, I agree with Daniel. KWH/m2 is also called Peak Sun Hour of a PV location
    Specific yield = PSH * various de-rating factors of PV system

    Reply
  6. joel says

    October 19, 2018 at 3:08 am

    is 2000 kwh/kwp good pv out put

    Reply
    • Mike says

      February 16, 2021 at 12:15 am

      Yes. Think of the kwh/kwp (or just kw) as the power produced per nameplate. So 1050 is like getting 105% of the nameplate rating of the panel. If you get 950, you’re at 95%.

      In my area, NJ, 105-115 is a welcome sight, now and then get close to 120% (1200), and ground mounts in a field I can hit 125-130%. So if you’re getting 2000 I’d like to hear about it! Unless you have dual-axis trackers, I was involved with a few of them and we were getting 200% (your 2000).

      Reply
  7. Abdul Wassay says

    August 25, 2018 at 7:04 am

    I needed to ask how can we calculate the mean value ratio for a specific inverter?
    And how to set the mean value threshold for each inverter?

    Reply
  8. Francesco says

    March 23, 2018 at 3:32 pm

    One question: why is kWh/m2 equivalent to kWh/kWp?

    Reply
    • Daniel says

      June 22, 2018 at 7:47 am

      It’s not equivalent, Francesco. kWh/m2 is irradiance and kWh/kWp is specific yield

      Reply

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