17-0102 v3.0/Lick Wilmerding High School/Battery Size Required for PV-Interconnected System


We would like to request reconsideration of your response to our request for 3 days of resilience (in lieu of 7) for a PV-interconnected battery system, on the grounds that the most likely disasters to hit our neighborhood will be (1) an earthquake, or (2) fire, neither of which is correlated (except weakly, in the case of fire) with a storm event that would generate more than three days of overcast sky which might reduce the PV system’s generation capacity. (This may not be true for other areas of the country where the likely disaster will be weather-related, as in the case of hurricanes.) As for the resilience of the PV system itself, it and the renovation will be built to the latest seismic and fire safety codes; it is much more likely to survive an earthquake than the surrounding neighborhood that it serves. 
As you know, the virtue of a PV-interconnected system is that in most likely scenarios, it will provide far more than 7 days of power; it can power our systems indefinitely, and as such, represents a significant improvement over non-PV-interconnected systems.
Finally, in order to make the significant expense of this battery system viable for the school, we would like to ask whether this battery system can be used for demand management. 
Please let us know what you think of these arguments, or whether you would like us to submit data showing the probability of weather events greater than 3 days in San Francisco, or any other supporting documentation.


In an effort to align our energy storage requirements with industry standards, ILFI has updated its methodology for calculating a project's battery back-up capacity. Teams must still have some storage for resilience to aid in maintaining critical loads for 7 days, with a minimum battery capacity of 24 hours. However, regeneration from on-site renewables is now allowed. Project teams pursuing the Energy Petal must use the following methodology to calculate the minimum battery storage capacity required to supply their refrigeration and 10% of lighting loads for at least one week. 

Battery Storage Calculation Methodology:

  • Calculation must use daylight hours for project location on Winter Solstice (December 21st) 
  • Battery back-up system must have AC-coupled configuration to enable regeneration from on-site renewables
  • Calculate Estimated Regeneration:
    • On-site Renewable System Capacity (kW-DC) x 0.8 (Derate Factor) x Hours of Daylight x (7 days) = Estimated Regeneration 
  • Calculate Critical Load Estimate (10% of lighting and refrigeration for 24 hours)
  • Calculate Total Battery Storage Capacity (the greater of):  
    • Critical Load Estimate for 1 day OR
    • Total Battery Storage Capacity (kWh) = 1 week Critical Load Estimate (kWh) - 1 week Estimated Regeneration (kWh) 

The following documentation must be included to support this methodology:

I06-1 Energy Narrative

The energy storage system section of the Energy Narrative must include: 

  • Regional environmental threats to on-site renewables
  • A description of the system's location, installation technique, and/or standard test conditions of the generation and storage technologies, showing the system has been designed to limit potential system failures
  • Any non-standard maintenance that is required to bring the on-site renewables back online after a system failure

I06-5 Resilient Energy Storage Documentation

In addition to the summary and refrigeration information: 

  • Battery specifications
  • Battery storage calculations

Post ID 6336

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