Battery Presets
Kilowatt Hours (kWh) measures total energy storage - it's the universal unit for comparing batteries regardless of voltage. Amp Hours (Ah) measures charge capacity at a specific voltage, which is useful for sizing wiring and charge controllers.
When to Use Each Unit
- Use kWh to compare batteries of different voltages or estimate how long your loads can run
- Use Ah to size battery cables, fuses, and charge controllers for your specific voltage system
Common Battery System Voltages
- 12V systems - RVs, boats, small off-grid setups (higher current)
- 24V systems - Larger RVs, medium solar systems
- 48V systems - Home battery backup, large solar (lower current, thinner wires)
Depth of Discharge (DoD)
DoD determines how much of your battery capacity you can safely use:
- Lead-acid batteries - 50% DoD recommended to maximize lifespan
- LiFePO4 batteries - 80-90% DoD with minimal impact on cycle life
Sizing Batteries for 3 Days of Autonomy
An off-grid cabin uses 4 kWh/day. For 3 cloudy days: 12 kWh needed. At 48V with LiFePO4 (90% DoD): 12,000Wh ÷ 48V ÷ 0.9 = 278Ah. At 24V with lead-acid (50% DoD): 12,000 ÷ 24 ÷ 0.5 = 1,000Ah — showing why 48V lithium uses dramatically less Ah.
EV Charging from Home Battery
A homeowner wants to charge their EV (10 kWh nightly) from stored solar. At 48V: 208Ah. With 90% DoD: 232Ah. Plus 5 kWh for home overnight: 116Ah more. Total: 348Ah at 48V (~16.7 kWh) — equivalent to about 1.2 Tesla Powerwalls.
Critical Load Backup Only
Backing up just the refrigerator (1.5 kWh/day) and internet equipment (0.5 kWh/day) = 2 kWh for 1 day. At 48V with 90% DoD: 46.3Ah. A single 48V 50Ah LiFePO4 battery ($400–600) covers this modest backup — a fraction of a full Powerwall.
- Designing a complete off-grid system — Start with energy audit (kWh/day), then convert to Ah to select specific batteries. A family using 8 kWh/day wanting 2 days of autonomy: 16,000 ÷ 48 ÷ 0.9 = 370Ah at 48V.
- Upgrading an existing battery bank — An RV owner with 2 × 12V 100Ah lead-acid (1.2 kWh usable) wants 4 kWh usable. Converting: 4,000 ÷ 12 ÷ 0.9 = 370Ah at 12V. Two 12V 200Ah LiFePO4 batteries (400Ah = 4.6 kWh usable) meet the goal.
- Commercial solar + storage bids — A project requiring 50 kWh: at 48V = 1,042Ah. Using 48V 100Ah rack batteries = 11 units. Using 48V 200Ah = 6 units. The kWh-to-Ah conversion bridges customer energy needs to the installer's bill of materials.
