NABCEP Energy Storage 5 — Questions and Answers
Question 1: An ESS is required to provide backup power for a 120V, 20A critical load circuit. What minimum continuous output current rating must the battery inverter provide?
- 16A
- 20A (Correct answer)
- 25A
- 30A
Correct answer: 20A
The inverter must be rated to continuously supply at least the full 20A load; the 80% continuous load rule applies to overcurrent protection, not to inverter output ratings.
Question 2: Which IFC/NFPA standard provides fire safety and installation requirements specifically for stationary storage battery systems?
- NFPA 70 (NEC)
- NFPA 855 (Correct answer)
- UL 1741
- IFC Section 605
Correct answer: NFPA 855
NFPA 855 is the Standard for the Installation of Stationary Energy Storage Systems, covering fire protection, separation distances, and ventilation requirements.
Question 3: A battery-based system uses 'self-consumption' mode. Which operational strategy does this describe?
- Charging the battery from the grid during off-peak hours and discharging during on-peak hours
- Using solar energy to charge the battery first, then powering loads from stored energy instead of exporting (Correct answer)
- Operating entirely off-grid and never importing from the utility
- Maximizing solar export to the grid by discharging the battery to serve all loads
Correct answer: Using solar energy to charge the battery first, then powering loads from stored energy instead of exporting
Self-consumption mode prioritizes using solar energy on-site by storing excess generation in the battery and discharging it to serve loads, minimizing grid import and export.
Question 4: What is the purpose of a 'state of health' (SoH) measurement in a battery management system?
- To indicate the current percentage of charge remaining in the battery
- To express the battery's present maximum capacity as a percentage of its original rated capacity (Correct answer)
- To measure the voltage difference between cells in a string
- To track the number of remaining warranted cycles
Correct answer: To express the battery's present maximum capacity as a percentage of its original rated capacity
State of health compares the battery's current maximum capacity to its original rated capacity, expressing overall degradation as a percentage.
Question 5: For a grid-tied battery storage system to provide backup power during a utility outage, it must include which critical feature?
- A transfer switch or intentional islanding capability (Correct answer)
- A higher DC voltage than the grid-tied solar inverter
- A second utility meter for net metering
- UL 1741 grid-tied certification only
Correct answer: A transfer switch or intentional islanding capability
A transfer switch or anti-islanding override capability is required to safely disconnect from the grid and form an island to power critical loads during an outage.
Question 6: Which wiring method is typically required when installing battery cables in an ESS with lithium-ion batteries indoors?
- Open wiring on insulators is preferred for ease of inspection
- Listed and labeled cables or conduit-wired conductors rated for the voltage and current (Correct answer)
- Standard NM-B cable is acceptable for battery interconnections under 50V
- Any insulated copper wire is acceptable as long as the ampacity is sufficient
Correct answer: Listed and labeled cables or conduit-wired conductors rated for the voltage and current
NEC 706 and manufacturer installation requirements specify that battery interconnections use listed cables or conductors in appropriate conduit rated for the system voltage and current.
Question 7: What does a battery inverter's 'transfer time' specification indicate in a backup power application?
- The time required to fully charge the battery from empty
- The time elapsed between a grid outage and when the inverter begins supplying backup power (Correct answer)
- The duration the battery can sustain the rated load at full discharge
- The switching frequency of the inverter's PWM control circuit
Correct answer: The time elapsed between a grid outage and when the inverter begins supplying backup power
Transfer time is the delay between detection of a grid outage and the battery inverter supplying power to the backup loads, typically measured in milliseconds to cycles.
NABCEP Energy Storage 5
An ESS is required to provide backup power for a 120V, 20A critical load circuit.
What minimum continuous output current rating must the battery inverter provide?