To properly size a battery in circuits, you must calculate your daily watt-hours, divide by the inverter efficiency, divide by the chemistry's usable depth of discharge (DoD), and verify the peak C-rate. For a standard 2,000W continuous off-grid or backup circuit, the default pick is a 48V 100Ah LiFePO4 server rack battery (such as the SOK 48V 100Ah) paired with a 3,000W hybrid inverter. This configuration prevents high-amperage voltage sag, minimizes copper costs, and provides 9.6kWh of nameplate capacity.
The Source-to-Load System Block
A battery does not exist in isolation; it is the source node in a DC-to-AC power chain. Before calculating capacity, you must define the physical circuit path. A code-compliant, low-loss system block for a 48V architecture looks like this:
- Source: 48V LiFePO4 Battery Bank (e.g., two 48V 100Ah units in parallel).
- Overcurrent Protection: Class T Fuse (150A) mounted within 7 inches of the battery positive terminal, per NEC Article 480 and marine ABYC standards.
- Disconnect: 150A DC rated disconnect switch or heavy-duty contactor.
- Conductors: 2/0 AWG pure copper welding cable (THHN in conduit if run over 5 feet), keeping voltage drop under 1% at peak current.
- Inverter/Charger: 48V DC to 120/240V AC split-phase hybrid inverter.
- Load: AC subpanel feeding branch circuits.
Series vs. Parallel: Voltage, Capacity, and the Mismatch Trap
When building a battery bank, how you wire the cells or modules dictates your system voltage and amp-hour (Ah) capacity.
- Series Wiring: Voltages add, Ah remains constant. Four 12V 100Ah batteries in series yield 48V at 100Ah (4,800Wh). This is ideal for keeping DC current low.
- Parallel Wiring: Ah adds, voltage remains constant. Two 48V 100Ah batteries in parallel yield 48V at 200Ah (9,600Wh). This increases total energy capacity while maintaining the 48V architecture.
Sizing Math: Peukert’s Law, DoD, and Inverter Efficiency
Sizing a battery in circuits requires moving past simple 'Watts = Volts x Amps' math. You must account for inverter losses and chemical limitations. Let us size a bank for a cabin running a 1,500W continuous load (refrigerator, lights, laptop, well pump) for 4 hours.
Step 1: Calculate DC Watt-Hours Required
AC Load: 1,500W × 4 hours = 6,000Wh.
Inverters are not 100% efficient. A high-quality low-frequency inverter operates at roughly 92% efficiency under this load.
DC Energy Required = 6,000Wh / 0.92 = 6,521Wh.
Step 2: Apply Chemistry Constraints (Peukert and DoD)
According to Battery University, Peukert's Law describes how a battery's usable capacity shrinks as the discharge rate increases. Lead-acid batteries suffer heavily from this; lithium-ion does not.
| Chemistry | Peukert Exponent (k) | Max Safe DoD | Usable Factor | Required Nameplate |
|---|---|---|---|---|
| Flooded Lead Acid (FLA) | 1.25 (Heavy penalty) | 50% | ~0.40 | 6,521 / 0.40 = 16,302Wh |
| LiFePO4 (Lithium Iron Phosphate) | ~1.05 (Negligible) | 80% to 100% | 0.80 | 6,521 / 0.80 = 8,151Wh |
To get 6,521Wh of usable energy, a lead-acid bank requires over 16kWh of nameplate capacity (massive weight and space). A LiFePO4 bank requires 8,151Wh. At a 48V nominal system voltage, 8,151Wh / 48V = 169.8Ah. Therefore, we specify two 48V 100Ah batteries in parallel, giving us 200Ah (9,600Wh nameplate), which safely covers the load with a 15% buffer for cloudy days or aging.
Charge/Discharge Limits and Inverter/Charger Matching
Capacity is only half the equation; the rate of energy transfer (C-rate) dictates your inverter and charger sizing.
Discharge Limits (Inverter Sizing)
A 1C discharge rate means drawing the battery's Ah rating in amps over one hour. For a 100Ah LiFePO4 battery, 1C = 100A. Most quality LiFePO4 BMS units limit continuous discharge to 100A and peak surge to 150A for 30 seconds.
At 48V, 100A yields 4,800W of continuous power. If you attempt to pull 3,000W from a 12V 100Ah battery, you are demanding 250A (a 2.5C rate). The BMS will trip, or the cables will melt. This is exactly why 12V systems are restricted to loads under 1,000W, and 48V is mandatory for 2,000W+ circuits.
Inverter Pick: For a 1,500W continuous load with motor surges (well pumps), size the inverter at 2x continuous. A Victron MultiPlus 48/3000 (3,000VA continuous, 5,500W peak) perfectly matches the 48V 200Ah bank's discharge capabilities.
Charge Limits (Charger Sizing)
Lithium batteries can accept high charge currents, but pushing them too hard degrades the cells and generates excess heat. The National Renewable Energy Laboratory (NREL) notes that keeping charge rates between 0.2C and 0.5C maximizes cycle life.
For our 200Ah bank, 0.2C is 40A; 0.5C is 100A. The Victron MultiPlus 48/3000 features a built-in 35A AC charger. When paired with a 60A MPPT solar charge controller, your total max charge current is 95A (0.47C), sitting perfectly in the optimal longevity sweet spot.
Decision Tree: Picking the Exact Battery for Your Circuit
Stop guessing. Use this decision matrix to select your system voltage and exact battery model based on your maximum continuous AC load.
| Max Continuous AC Load | System Voltage | Required Bank Capacity | Concrete Pick (2026 Market) | Approx. Cost |
|---|---|---|---|---|
| < 800W (RV, Marine, Small Cabin) | 12V | 100Ah - 200Ah | Dakota Lithium 12V 100Ah (Group 31) | $699 |
| 800W - 1,500W (Tiny Home, Teardrop) | 24V | 100Ah | 2x 12V 100Ah LiFePO4 in Series | $1,400 |
| > 1,500W (Off-Grid Home, Heavy Backup) | 48V | 100Ah per 3kW of inverter | SOK 48V 100Ah Server Rack LiFePO4 | $1,299 |
By respecting Peukert's law, matching your inverter to the DC C-rate limits, and enforcing strict parallel-matching rules, your battery in circuits will deliver reliable, safe power for over a decade without unexpected BMS trips or voltage sag.
