Battery Presets
Amp Hours (Ah) measures a battery's charge capacity - how much current it can deliver over time. Kilowatt Hours (kWh) measures total energy storage, which is more useful for comparing batteries at different voltages.
Why Voltage Matters
A 100Ah 12V battery and a 50Ah 24V battery both store the same energy (1.2 kWh), but they're suited for different applications:
- 12V systems - RVs, boats, small off-grid setups
- 24V systems - Larger RVs, medium solar systems
- 48V systems - Home battery backup, large solar installations
Depth of Discharge (DoD)
Not all battery capacity is usable. DoD tells you how much you can safely use:
- Lead-acid batteries - 50% DoD recommended (using more reduces lifespan)
- LiFePO4 batteries - 80-90% DoD safely usable (longer lifespan at any DoD)
Lead-Acid vs Lithium Battery Comparison
Lead-acid bank: 4 × 6V 225Ah batteries in series for 24V = 5.4 kWh total, but only 2.7 kWh usable (50% DoD). LiFePO4: 1 × 48V 100Ah = 4.8 kWh total, 4.32 kWh usable (90% DoD). Despite lower raw capacity, the lithium battery delivers 60% more usable energy.
DIY LiFePO4 vs Tesla Powerwall
Tesla Powerwall 2: 13.5 kWh at ~50V = ~270Ah. DIY build with 4 × 3.2V 280Ah EVE cells in series = 12.8V per module. Four modules in series = 51.2V × 280Ah = 14.34 kWh (12.9 kWh usable) — comparable to a Powerwall at roughly half the cost.
Sizing Overnight Backup
Average US home uses ~30 kWh/day. Overnight (6pm–6am) is roughly 12 kWh. At 48V: 250Ah needed. With lithium (90% DoD): 278Ah. With lead-acid (50% DoD): 500Ah — showing why lithium needs roughly half the Ah capacity for the same usable energy.
- Comparing batteries at different voltages — A 12V 200Ah battery and a 24V 100Ah battery both store 2.4 kWh. Without converting, the 200Ah battery appears to have "twice the capacity" — a common misconception.
- Calculating battery ROI — A 48V 200Ah LiFePO4 battery ($2,400) stores 9.6 kWh (8.64 kWh usable at 90% DoD). Cost per usable kWh: $278. Over 4,000 cycles: $0.069/kWh/cycle.
- Matching battery to solar production — A solar array producing 6 kWh on a typical day needs 125Ah at 48V for nighttime storage. With 90% DoD, the bank needs at least 139Ah at 48V.
