9. Energy Storage & Batteries

Learning objectives

State the reasons to add storage and how they shape sizing.
Compare lithium chemistries (LFP vs NMC) on safety, density, and cost.
Define the battery parameters that drive sizing (usable capacity, DoD, C-rate, round-trip efficiency, cycle life).
Distinguish AC- vs DC-coupled architectures.
Identify the storage safety standards an installer must respect.

Batteries are added for one or more of: backup during outages (resilience), self-consumption of solar that would otherwise export, time-of-use arbitrage (store cheap/solar energy, use it during expensive peak periods), and grid services. Which of these drives a project determines how you size it (Chapter 17). Backup sizes to critical-load duration; arbitrage sizes to the daily peak window.

9.2 Chemistry: LFP vs NMC

Modern solar storage is overwhelmingly lithium-ion, in two main flavors:

Thermal runaway: a self-reinforcing cycle in which heat from one cell accelerates chemical reactions that generate more heat, potentially cascading to adjacent cells. Chemistry, temperature, and BMS design all affect how easily it starts and how far it spreads.

LFP (lithium iron phosphate, LiFePO₄): the dominant residential/commercial choice. Lower energy density but markedly safer (thermal runaway onset ~270–300 °C; the cathode doesn’t shed oxygen, so fires are less energetic and more containable), longer cycle life, and lower cost (BloombergNEF placed 2025 average LFP pack prices around $81/kWh). Preferred for indoor, garage, and warm-climate installs.
NMC (nickel manganese cobalt): higher energy density and better cold-weather performance, but higher fire risk (lower thermal-runaway threshold, oxygen-releasing cathode → faster cell-to-cell propagation, hotter fires, more toxic gas) and higher cost (~$128/kWh). More common where density/footprint dominates.
Lead-acid: legacy, low cost, still seen in some off-grid systems, but heavy, shallow usable depth, and short cycle life.

9.3 Battery parameters that drive sizing

Nameplate vs usable capacity (kWh): you can’t use 100% of a battery; usable = nameplate × allowable depth.
Depth of Discharge (DoD): the fraction you can draw; LFP tolerates deep DoD (often 90–100% usable).
C-rate: charge/discharge rate relative to capacity; bounds how fast you can pull power (a 10 kWh battery at 0.5C delivers ~5 kW).
Continuous vs surge power (kW): the inverter/battery power rating sets what loads you can run; surge handles motor startups.
Round-trip efficiency: energy out ÷ energy in (~90%+ for lithium); the rest is loss.
Cycle life / warranty: cycles or throughput (MWh) and years guaranteed.

9.4 AC- vs DC-coupling

DC-coupled: the battery shares the PV DC bus through a charge controller/hybrid inverter; slightly higher efficiency for solar charging and better for new installs.
AC-coupled: the battery has its own inverter and connects on the AC side; simpler to retrofit onto an existing PV system. The tradeoff is an extra conversion step.

A Battery Management System (BMS) governs every pack, balancing cells, enforcing voltage/temperature/current limits, and protecting against the abuse that triggers thermal runaway.

9.5 Storage safety standards

Storage carries fire risk, so its standards are strict and an installer must know them:

AHJ (Authority Having Jurisdiction): the government agency, office, or individual responsible for enforcing a code or standard in a given jurisdiction. For solar and storage installs, the AHJ is typically the local building or fire department that issues permits and conducts inspections.

UL 9540: the system-level safety certification for an energy storage system (battery + BMS + inverter + enclosure evaluated together).
UL 9540A: a fire-propagation test method (not a certification) measuring whether thermal runaway in one cell cascades to others. Its report feeds UL 9540 and AHJ approvals.
NFPA 855: the installation code for lithium-ion systems. Above ~20 kWh it expects UL 9540 listing and UL 9540A data, and it dictates separation distances (commonly a 3 ft minimum between units unless 9540A data justifies less), ventilation, signage, and suppression. LFP’s better fire behavior often eases these requirements versus NMC.

⚠️ Battery and fire safety get a full treatment in Chapter 29; this is the component-level orientation.

9.6 AC- vs DC-coupling and chemistry

 DC-COUPLED (best for NEW builds)         AC-COUPLED (best for RETROFIT)
 [PV]─DC─[charge ctrl/hybrid inv]─┬─AC    [PV]─[existing inverter]─AC─┬─grid
                  [battery]───────┘                  [battery+own inverter]─┘
 fewer conversions; higher solar-     adds the battery on the AC side without
 charging efficiency                  replacing the original PV inverter

	LFP (LiFePO₄)	NMC
Safety	higher (runaway ~270–300 °C, no O₂ release)	lower (releases O₂, faster propagation)
Energy density	lower	higher
Cost (2025)	~$81/kWh pack	~$128/kWh
Typical use	residential/indoor/warm climates	density-constrained sites

Chapter 9 summary

Storage is added for backup, self-consumption, arbitrage, or grid services. The reason sets the sizing logic. LFP is the safer, cheaper, mainstream chemistry; NMC trades safety for density. Size around usable capacity (nameplate × DoD), C-rate, continuous/surge power, and round-trip efficiency. DC-coupling suits new builds, AC-coupling suits retrofits, and a BMS protects every pack. Compliant storage is UL 9540 listed with UL 9540A data, installed to NFPA 855.

LFP (lithium iron phosphate): the dominant, safer, lower-cost lithium chemistry for solar storage; thermal runaway onset ~270–300 °C.
NMC (nickel manganese cobalt): higher-density lithium chemistry with greater fire risk; common where space is constrained.
Thermal runaway: self-reinforcing heat cycle that can cascade across battery cells; the core safety hazard in lithium systems.
DoD (Depth of Discharge): the fraction of nameplate capacity you can draw; LFP typically allows 90–100%.
C-rate: charge/discharge rate relative to capacity; sets the maximum power a battery can deliver.
BMS (Battery Management System): the electronics that balance cells, enforce limits, and protect the pack.
DC-coupled: battery connects to the PV DC bus via a hybrid inverter; fewer conversion steps.
AC-coupled: battery connects on the AC side with its own inverter; preferred for retrofits.
UL 9540: system-level safety certification for energy storage systems.
UL 9540A: fire-propagation test method that determines whether thermal runaway cascades cell-to-cell.
NFPA 855: installation code governing separation distances, ventilation, and suppression for battery storage.
AHJ (Authority Having Jurisdiction): the local authority that enforces codes and issues permits.

Full definitions: Appendix A (glossary).

Practice Problems: Chapter 9

Which coupling (AC or DC) is generally preferred for retrofitting a battery onto an existing PV system, and why?
Which chemistry is safer and cheaper, and which is denser?
What does the BMS do?
Define usable capacity in terms of nameplate and DoD.
Why might a project choose NMC despite its higher fire risk?

Solutions: Chapter 9

AC-coupling: it adds the battery (with its own inverter) on the AC side without replacing the existing PV inverter.
LFP is safer and cheaper; NMC is denser.
The Battery Management System balances cells and enforces voltage/temperature/current limits, protecting against the abuse that triggers thermal runaway.
Usable = nameplate × DoD (depth of discharge).
When energy density / footprint is the binding constraint (limited space), NMC’s higher density wins despite the safety trade-off.

Three architectures cover nearly every PV system. The choice among them is set by one question: what happens at the grid connection? It cascades into every later design and code decision.

The Solar Primer