2024 solar eclipse sheds light on behind-the-meter solar
March 4, 2024
Dr. Anil Jampala

Anil Jampala & David Thomas

This April, a total solar eclipse will cross the United States from Texas to Maine. While the eclipse will only last for several minutes, the predicted drop in temperature and solar radiation will be an ideal opportunity for utilities in two distinct ways.

First, utilities can test their assumptions about how much behind-the-meter (BTM) solar (typically residential rooftop solar) is currently not connected to net metering and Advanced Metering Infrastructure (AMI).

Second, the temperature drop caused by the eclipse can be a test case to validate the readiness for the Federal Electricity Regulatory Commission’s (FERC) Order 881. Starting in July 2025, FERC order 881 will mandate hourly Adjusted Ambient Limit Ratings (AARs). The eclipse will therefore serve as an early opportunity to examine and better understand the implications of FERC order 881 on utilities’ infrastructure.

Residential solar

As the eclipse moves through the path of totality and through utilities’ operating areas, it will offer a unique field test for utilities to gain full visibility into midday PV production from BTM rooftop solar in their service area.

During the last total solar eclipse in 2017, the estimated drop in rooftop solar capacity was 4-6.5 GW. While utilities were well-prepared for that event, the impact of this year’s eclipse is likely to be larger because there is significantly more rooftop solar generation capacity today than in 2017.

A total solar eclipse would (obviously) cause a dramatic decrease in solar radiation and hence reduce rooftop solar generation. According to an earlier DOE estimate, it will affect solar generation capacity by as much as 18%.

This is especially important for utilities in areas where many residential rooftop solar modules are not connected to net energy metering or AMI. AMI meters measure and record electricity usage in real time and provide data to the utility. This allows utilities to manage the grid better, implement demand response programs, and manage load more effectively.

About 73% of residential electric meters in the US were AMI meters in 2021. This means that more than a quarter of residential utility customers do not provide real-time energy usage data to their utilities. Utilities, therefore, need to make a best guess based on utility program registrations.

By measuring the overall change in energy demand during the eclipse, and comparing that difference with immediate past data, utilities can better understand the total level of BTM solar generation in their service area.

FERC order 881

US utilities are diligently preparing for FERC order 881’s July 2025 compliance deadline. These preparations are still in very early stages for most. Realistically, none will be FERC 881-compliant by the April 8 solar eclipse. For the few companies who may be ahead of the curve, the eclipse will allow a test of one specific aspect of FERC order 881.

FERC order 881 mandates transmission operators to calculate hourly AARs and forecasted limits for the next ten days. Furthermore, the rule requires transmission line limits to be recalculated when the ambient temperature changes by more than 5°F. The AAR values will then be sent to the relevant ISOs/RTOs.

This year’s solar eclipse offers an early opportunity to gather and analyze weather data vital to compliance with FERC order 881. The temperature drop during the eclipse will likely be around 10°F, exceeding the 5°F threshold for recalculating transmission line limits. Utilities can use this rare event to collect and preserve the telemetry and AARs for later analysis and verification of their FERC order 881-compliant installations.

Utilities and ISOs/RTOs are not the only ones still in the early stages of implementing FERC Order 881 support. Only a few EMS vendors have publicly announced their solutions for FERC order 881. Protracted product development timelines will obviously make implementation plans even tighter. Utilities will need to be assertive in putting their compliance plans into place.

How PSC can help

Whether you’re preparing for FERC order 881 implementation or working towards grid modernization, PSC can help.

Our PSC engineering team can guide you cost-effectively through the FERC 881 planning process, pilot phase and deployment. You can refer to our white paper on how to craft a FERC 881 roadmap or contact us to talk about your project.

PSC offers deep expertise across the power industry, going beyond product support to solving real-world problems in a thoughtful, pragmatic way. PSC helps customers address an evolving grid as more and more distributed energy resources (DERs) continue to impact the larger system.

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