NABCEP Inverter Systems 3 — Questions and Answers
Question 1: A battery-based inverter/charger operating in grid-tied mode with battery backup will typically prioritize loads in which order during a grid outage?
- All loads simultaneously with no prioritization
- Critical backup loads first, then non-critical loads if battery capacity allows (Correct answer)
- Non-critical loads first to protect battery from deep discharge
- Utility loads first, then PV loads
Correct answer: Critical backup loads first, then non-critical loads if battery capacity allows
Battery-based hybrid inverters use load prioritization to power critical loads (medical equipment, refrigeration) from the battery first, shedding non-critical loads to extend backup runtime.
Question 2: What is the purpose of the 'ride-through' capability required by IEEE 1547-2018 for grid-tied inverters?
- Allow the inverter to operate through brief grid voltage and frequency disturbances without disconnecting (Correct answer)
- Enable the inverter to supply power to the grid during utility outages
- Protect the inverter from lightning surges by disconnecting instantly
- Allow inverter operation at reduced efficiency during high-temperature conditions
Correct answer: Allow the inverter to operate through brief grid voltage and frequency disturbances without disconnecting
IEEE 1547-2018 requires inverters to ride through (remain connected during) defined ranges of voltage and frequency disturbances rather than immediately disconnecting, improving grid stability.
Question 3: A micro-inverter system with 20 modules produces 240VAC from each unit. How are the micro-inverter AC outputs typically combined?
- Series-connected to increase voltage to 480VAC
- Parallel-connected on a shared AC branch circuit (Correct answer)
- Through a DC combiner box before the central inverter
- Through individual step-up transformers to 480V each
Correct answer: Parallel-connected on a shared AC branch circuit
Micro-inverter AC outputs are connected in parallel on shared AC branch circuits (trunk cables), with all units outputting the same AC voltage and currents summing.
Question 4: What does 'unbalanced loading' mean in the context of a split-phase battery inverter system?
- When the AC output frequency differs between the two phases
- When one 120V leg carries significantly more load than the other (Correct answer)
- When the DC battery bank has unequal cell voltages
- When PV production exceeds battery charging capacity
Correct answer: When one 120V leg carries significantly more load than the other
Unbalanced loading occurs when the 120V loads on L1 and L2 of a split-phase (240V) system are not equal, which can stress the inverter and reduce efficiency.
Question 5: Under NEC 705.12, the maximum allowable backfed breaker current for a 200A residential panel with a 200A main breaker is:
- 40A (120% rule: 240A bus capacity minus 200A main) (Correct answer)
- 60A (30% of bus rating)
- 80A (40% of bus rating)
- 100A (50% of bus rating)
Correct answer: 40A (120% rule: 240A bus capacity minus 200A main)
The NEC 705.12 120% rule allows bus loading up to 120% of rating; for a 200A bus: 200×1.2=240A total, minus 200A main breaker = 40A maximum backfed breaker.
Question 6: Which type of inverter is best suited for a system where individual module-level monitoring and optimization is a priority?
- Central inverter with string combiners
- Single large string inverter
- Micro-inverters or DC optimizers paired with a string inverter (Correct answer)
- Multi-string inverter with separate MPPT inputs
Correct answer: Micro-inverters or DC optimizers paired with a string inverter
Micro-inverters and DC optimizers provide module-level power electronics (MLPE) that enable per-module monitoring and maximize production from each panel independently.
Question 7: What is the significance of the inverter's 'weighted efficiency' (such as CEC or EU efficiency) versus its peak efficiency?
- Weighted efficiency reflects real-world performance across a range of power levels, while peak efficiency is only achieved at one operating point (Correct answer)
- Peak efficiency is always lower than weighted efficiency due to startup losses
- Weighted efficiency measures AC-to-DC conversion for battery charging only
- There is no practical difference between weighted and peak efficiency for system sizing
Correct answer: Weighted efficiency reflects real-world performance across a range of power levels, while peak efficiency is only achieved at one operating point
Weighted efficiency (CEC or EU) averages inverter efficiency at multiple power levels weighted by typical solar irradiance distribution, giving a better estimate of annual energy production than peak efficiency alone.
NABCEP Inverter Systems 3
A battery-based inverter/charger operating in grid-tied mode with battery backup will typically prioritize loads in which order during a grid outage?