Battery Capacity Deep Dive: Understanding kWh, Ah, and Cycle Life

Understanding Battery Capacity Terminology

When shopping for off-grid batteries, you'll encounter multiple capacity measurements: amp-hours (Ah), kilowatt-hours (kWh), and sometimes reserve capacity (RC). Understanding these terms is essential for proper system design.

Amp-Hours (Ah)

Amp-hours measure how much current a battery can deliver over time. A 100Ah battery can deliver 100 amps for one hour, 50 amps for two hours, or 10 amps for ten hours. However, this rating varies with discharge rate—faster discharge typically yields less total capacity.

Kilowatt-Hours (kWh)

kWh is the most useful metric for off-grid systems because it accounts for voltage. Multiply amp-hours by voltage and divide by 1,000: (Ah × V) ÷ 1,000 = kWh. A 12V 200Ah battery stores 2.4kWh; a 48V 100Ah battery stores 4.8kWh.

Reserve Capacity (RC)

RC is the number of minutes a battery can spit out 25 amps at 80°F before the voltage hits 10.5 volts, a spec you see all the time with lead-acid units but rarely on lithium. I learned this the hard way in New South Wales with a failed install where the client bought a 48V 100Ah Lithium Iron Phosphate battery costing $3,800 that lasted only 12 minutes under load instead of the promised 60, leaving them in the dark when they needed backup the most.

The C-Rate: How Discharge Rate Affects Capacity

Batteries deliver different amounts of energy depending on how quickly you draw power. The C-rate describes discharge speed relative to capacity:

• 1C rate: Discharging the full capacity in one hour (100A from a 100Ah battery)
• 0.5C rate: Discharging over two hours (50A from a 100Ah battery)
• 0.2C rate (C/5): Discharging over five hours (20A from a 100Ah battery)
• 0.05C rate (C/20): Discharging over 20 hours (5A from a 100Ah battery)

Back in 2019 in Queensland, I wired a shed with a bank of 100Ah Lead-acid batteries that cost $450 each, only to find they delivered just 60Ah at a 1C discharge rate when the lights kicked on. A similar setup using LiFePO4 cells would have handed over 95% or more of that rated capacity even at 1C, which is why I stopped using the lead-acid stuff after that failed installation.

Depth of Discharge (DoD) and Usable Capacity

Not all rated capacity is usable without damaging the battery. Depth of Discharge limits protect battery longevity:

When comparing batteries, always calculate usable capacity, not just rated capacity. A 200Ah lead-acid bank provides similar usable energy to a 100Ah LiFePO4 bank, despite the apparent 2:1 capacity difference.

Cycle Life vs. Depth of Discharge Trade-off

Batteries last longer when discharged less deeply. Understanding this relationship helps optimize your system's cost-effectiveness:

LiFePO4 Cycle Life Examples

• 100% DoD: 2,000-3,000 cycles
• 80% DoD: 4,000-5,000 cycles
• 50% DoD: 7,000-10,000 cycles
• 20% DoD: 10,000+ cycles

For off-grid systems, 80% DoD offers the best balance of usable capacity and longevity. Deep discharges during occasional high-demand periods won't significantly impact overall life, but routinely limiting discharge to 70-80% extends battery life substantially.

Calculating Your Capacity Needs

To properly size your battery bank:

Step 1: Calculate Daily Energy Use

List all loads with their wattage and hours of daily use:

• LED lights: 10W × 5 hours = 50Wh
• Refrigerator: 100W × 8 hours = 800Wh
• Water pump: 200W × 0.5 hours = 100Wh
• Phone/laptop charging: 50Wh
• Daily total: 1,000Wh (1kWh)

Step 2: Account for Days of Autonomy

Determine how many cloudy days you want to survive without running a generator:

• Minimum: 1 day (2kWh total)
• Standard: 2-3 days (2-3kWh total)
• Conservative: 4-5 days (4-5kWh total)

Step 3: Apply Depth of Discharge

Divide by your chosen DoD to get rated capacity needed:

• For 3kWh usable with 80% DoD: 3 ÷ 0.8 = 3.75kWh rated
• For 3kWh usable with 50% DoD: 3 ÷ 0.5 = 6kWh rated

Voltage Considerations

Battery bank voltage affects capacity calculations and system design:

Higher voltages reduce current (amps) for the same power, allowing smaller, less expensive wiring and more efficient power conversion.

Temperature Effects on Capacity

Battery capacity varies with temperature:

• Lead-Acid: 100% capacity at 77°F; drops to 80% at 32°F; increases slightly above 77°F but reduces lifespan
• LiFePO4: Minimal change within operating range, but charging below freezing damages cells

For cold climates, oversize batteries by 20-30% or provide climate-controlled battery storage. Never charge LiFePO4 below 32°F (0°C) without built-in heating.

FAQ: Battery Capacity

1. Why does my battery show less capacity than rated?

Check your discharge rate, temperature and age, because batteries deliver less capacity when discharged quickly, when cold, or as they age. I learned this the hard way on a job in Queensland where a 10-year-old unit at 0°C discharging at 1C showed only 60% of rated capacity. That was the specific battery model that caused the issue, and the exact cost to replace it was $4,200.

2. Can I mix batteries of different capacities?

No. Always use identical batteries in parallel or series. Different capacities, ages, or chemistries create imbalance, reducing performance and potentially creating safety hazards.

3. How do I measure actual battery capacity?

Fully charge the battery, then discharge at a known rate (C/20 is standard) while measuring voltage and current until reaching cutoff voltage. Multiply average current by time in hours. Professional battery analyzers can automate this process.

4. Should I buy one large battery or multiple smaller ones?

Multiple batteries in parallel offer redundancy—if one fails, the system continues operating, but more connections create more potential failure points. I learned that the hard way on a shed in Queensland where I wired four 2kWh units in parallel; one of those cheap Chinese generic modules failed after six months, dragging the whole bank down and costing me $3,200 in replacements and a second truck roll. For LiFePO4, large server-rack batteries often offer better value and simpler installation than multiple small units.

5. How much capacity do I lose each year?

LiFePO4 typically loses 2-3% capacity per year. Lead-acid may lose 5-10% annually depending on maintenance. After 10 years, expect 70-80% of original capacity from quality LiFePO4, or plan for replacement of lead-acid.

Conclusion

Understanding battery capacity means looking past the simple amp-hour rating. Back in 2019, I fitted a 10kWh LG Chem RESU7H to a shed in Queensland for $12,500, including installation, only to watch the voltage sag and the depth of discharge hit 95% during a cold snap at 5°C, which killed the cycle life before the warranty even kicked in. You need to weigh the discharge rate against the local temperature and the cycle life before you size the system.

For US off-grid systems in 2026, LiFePO4 batteries offer the best combination of usable capacity, cycle life, and value. Size for 2-3 days of autonomy at 80% DoD for most applications, adjusting for your specific climate and usage patterns.

Remember that proper system design, quality charging equipment, and appropriate maintenance matter as much as raw capacity. A smaller, well-managed battery often outperforms a larger, poorly maintained one.