U.S. Department of Energy Energy Efficiency and Renewable Energy

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February 14, 2013
Q.

We are trying to determine what the maximum expected output might be on our solar install.  We have 120 255W LG panels ground mounted facing due South with little obstruction.  There are 4 x 6,000W inverters. By our math the panels could produce 30.6kW. But the powerONE inverters will throttle us at 24kW.  What is the maximum output we could expect from this setup? -Andre

SunShot Solar Outreach Partnership

The SunShot Solar Outreach Partnership has been tasked with helping accelerate adoption of photovoltaic (PV) solar at the local level by providing timely and actionable information to local governments.

A.

You are quite right that the amount of useful solar electricity produced by your system will be limited by your inverter capacity. However, the impact of an undersized inverter on system output is not as dramatic as the figures you cited above suggest; indeed, it is fairly common practice to pair PV arrays with comparatively smaller inverters. This is done for a number of reasons. First off, it is important to understand that solar modules receive their DC capacity ratings (i.e., the 255W of your LG panels) by evaluating their performance at standard test conditions (STC). The module temperature parameter for STC is 25°C (77°F), deviations from which have an inverse impact on system output (i.e., output falls as module temperature rises above 25°C, and vice versa). For your LG panels, power output decreases by 0.459% for each degree of temperature increase. Because module temperatures often exceed STC under real-world conditions, a system’s actual power output can generally expected to be lower than its nameplate capacity rating. In addition, the electrical components of a PV array aren’t 100% efficient which, combined with other “derate factors” such as shading and soiling, serve to further reduce system output.

For these reasons, PV array DC capacity ratings can be 110% to 130% of the inverter's AC output power rating (as per page 41 of the North American Board of Certified Energy Practitioners' Photovoltaic Installer Resource Guide). However, a PV array sized above, or even in the upper limits of, this range can sometimes produce power in excess of the inverter’s capacity ratings. While most grid interactive inverters can handle such “overloading” without being damaged, the surplus power is sacrificed as the inverter "throttles" the output down to its capacity rating.

If you're looking for an estimate of system energy production (as opposed to power output), you’ll need to consider your system’s capacity rating and the temperature effects and derate factors mentioned above, along with a number of other elements, such as available solar resource, degradation factors, system orientation, and module tilt. The National Renewable Energy Laboratory's System Advisor Model ("SAM"; free download) provides a powerful tool for estimating solar energy system output. Using the parameters provided in your request, we were able to estimate your system's annual energy production. As you can see in the chart below, your system should produce around 44,282 kWh in its first full year of operation. The model notes that the difference in size between the capacity of your inverter and PV array causes the inverter to "throttle" production down for 215 hours each year. Running the model again with adequate inverter capacity reveals that this translates into an annual energy loss of 300 kWh. Valued at the latest average commercial retail rate for New Jersey ($0.129/kWh, according to the U.S. Energy Information Administration), this extra production is worth just over $900 over a 25-year time horizon. If you own the Solar Renewable Energy Credits (SRECs) the system generates, this foregone generation will cost you approximately 4 SRECs over the system’s 15 year eligibility period. At the latest NJ SREC prices ($110), this amounts to another $440 in lost value.

Given that inverters from the manufacturer you have chosen cost between $1,000 and $2,500 each and may require replacement every 15 years, you may find it to be more cost effective to simply forego the meager increase in energy production you would gain from additional inverter capacity.

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