Battery Chemistries & Specs: LFP, NMC, Lead-Acid, kWh vs kW

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Every battery spec sheet is a page of numbers — kWh, kW, DoD, RTE, cycles. Learn to read that page and you'll be able to compare equipment honestly, size systems correctly, and spot a sales claim that doesn't hold up.

Educational content only — not electrical design, fire-protection engineering, or hands-on installation training, and not a substitute for a licensed electrician or a NABCEP-certified professional. Follow the NEC and fire and building codes as adopted by your Authority Having Jurisdiction (AHJ), the equipment manufacturer's installation instructions, and your utility's interconnection rules. Code editions change and jurisdictions adopt different editions — always confirm the one your AHJ enforces.

Energy vs. power: kWh and kW

The single most important distinction in storage:

• Energy (kWh) is how much the battery holds — the size of the tank. A 10 kWh battery can, in theory, deliver 1 kW for 10 hours or 5 kW for 2 hours.
• Power (kW) is how fast it can deliver — the size of the pipe. The continuous power rating is what it can supply indefinitely; the peak or surge rating is what it can supply for seconds to start motors like well pumps and air conditioners.

A battery can have plenty of energy and still fail to start a large motor because its power or surge rating is too low. You need to size both.

Depth of discharge and usable capacity

Depth of discharge (DoD) is the share of the battery's capacity that is actually used. Manufacturers protect cells by not allowing a full 0–100% swing; the spec sheet should list usable capacity separately from nominal or total capacity. Always size with usable kWh. Lithium systems typically allow a high usable fraction; traditional lead-acid batteries are usually limited to a much shallower routine discharge — often around half — to avoid shortening their life.

Round-trip efficiency

Round-trip efficiency (RTE) is the energy you get out divided by the energy you put in. Losses come from the cells, the inverter or converter, and the battery's own electronics and thermal management. Modern lithium systems commonly land in roughly the mid-80s to low-90s percent range, depending on whether the figure is measured DC-to-DC or AC-to-AC. Read which one the sheet quotes — AC round-trip numbers are lower and more realistic for a home.

Cycle life, calendar life, and warranty

A cycle is one full charge and discharge (or the equivalent in partial cycles). Cycle life is how many cycles before capacity fades to a defined end-of-life point, commonly around 70% of original. Batteries also age with time and heat — calendar aging. Warranties typically guarantee a minimum retained capacity after a set number of years or a set energy throughput (total MWh delivered), whichever comes first. A customer cycling daily for TOU savings can hit the throughput cap before the year limit.

The chemistries

• Lithium iron phosphate (LFP or LiFePO4): the dominant chemistry in new residential and many utility-scale systems. Lower energy density than NMC (bigger and heavier per kWh), but greater thermal stability, long cycle life, and no cobalt. Less prone to thermal runaway, though not immune.
• Nickel manganese cobalt (NMC): higher energy density — more storage in a smaller package — and widely used in electric vehicles and some earlier home batteries. More sensitive to heat and abuse, with a lower thermal runaway onset temperature than LFP.
• Lead-acid (flooded and sealed AGM/gel): the traditional off-grid battery. Inexpensive up front and well understood, but heavy, shallow usable depth, shorter cycle life, and — for flooded types — watering, ventilation for hydrogen, and acid handling. Still found in off-grid cabins and older systems.
• Others: flow batteries, sodium-ion, and other chemistries appear mainly in commercial and utility pilots and are evolving quickly.

Going Deeper (Intermediate)

Batteries come as cells, assembled into modules, assembled into a battery system or rack with a battery management system (BMS). The BMS monitors cell voltage, current, and temperature, balances cells, and disconnects the battery when anything goes out of range. A residential "battery" is usually a complete listed product — cells, BMS, enclosure, and often the inverter — and the installer is not opening modules or mixing parts.

Temperature matters. Lithium cells generally should not be charged below freezing without built-in heating, and capacity drops in cold weather. High heat speeds aging. Installation location — a hot attic-adjacent garage wall versus a shaded exterior wall — affects real-world life and is limited by both code and the manufacturer's instructions.

Advanced / Pro-Level

Pros compare systems on cost per usable kWh over warranted throughput, not sticker price per nominal kWh. They check the C-rate (power relative to capacity — a 10 kWh battery delivering 5 kW is running at 0.5C) because higher C-rates mean more heat and faster aging. They read the warranty's end-of-life capacity, throughput cap, temperature conditions, and whether the warranty requires the battery to stay online with monitoring. And they confirm the system is listed to UL 9540 as a complete ESS, with the battery listed to its applicable standard, because the code and fire officials will.

Practice Challenge

Two quotes: Battery A is 15 kWh nominal, 13.5 kWh usable, 5 kW continuous. Battery B is 16 kWh nominal with 12 kWh usable, 8 kW continuous. The customer needs to start a well pump with a high inrush and run about 11 kWh of critical loads overnight. Which fits? (Answer: both have enough usable energy, but the well pump points toward the higher continuous and surge power — Battery B, provided its surge rating covers the pump's starting current. Compare usable, not nominal, kWh, and verify the surge spec against the motor.)

In Practice

Before any sizing, pull the manufacturer's data sheet and highlight four numbers: usable kWh, continuous kW, peak kW (and its duration), and warranted throughput. Those four answer most design questions.

Common Mistakes to Avoid

• Confusing kWh (energy) with kW (power)
• Sizing to nominal capacity instead of usable capacity
• Ignoring surge ratings for motor loads like pumps and compressors
• Comparing DC round-trip efficiency on one sheet with AC on another
• Overlooking throughput caps in the warranty for daily-cycling customers

Takeaway: kWh is the tank, kW is the pipe — size with usable capacity, check surge for motors, and choose chemistry (usually LFP for homes) with an eye on safety, cycle life, and the warranty's real terms.