To calculate battery capacity for an off-grid or backup system, divide your total daily watt-hours (Wh) by the nominal system voltage, then divide by the allowable Depth of Discharge (DoD). For a 12V LiFePO4 system powering a 1,500Wh daily load, you need a minimum 156Ah battery (1,500Wh ÷ 12V = 125Ah; 125Ah ÷ 0.80 DoD = 156Ah). This 80% DoD limit is the battery equivalent of the NEC 80% continuous load rule for AC breakers. If you are using flooded lead-acid (FLA), that DoD limit drops to 50%, instantly doubling your required Ah rating.
Sizing a battery bank is not just about total energy; it is about instantaneous current delivery. A 100Ah battery might hold enough energy for your daily load, but if your inverter pulls 150A to start a well pump, the Battery Management System (BMS) will trip before the fuse ever blows. Below is the exact framework for tallying loads, respecting BMS limits, and picking a concrete battery configuration.
The Governing Rules: DoD, Inrush, and BMS Limits
When sizing a DC circuit, you are constrained by three hard limits. Ignoring any one of them results in a system that shuts down unpredictably under load.
- Depth of Discharge (DoD): According to Battery University, regularly discharging lithium iron phosphate (LiFePO4) below 20% State of Charge (SoC) accelerates degradation. We use an 80% DoD as the maximum continuous draw. For lead-acid, the limit is 50% DoD to prevent sulfation.
- Inrush Current: Inductive loads like fridge compressors, well pumps, and power tool motors draw 3x to 7x their running wattage for the first 500 milliseconds. Your battery's BMS must support this surge, or it will interpret it as a short circuit and disconnect.
- BMS Continuous Discharge Rating: A "12V 100Ah" LiFePO4 battery typically features a 100A BMS. This means the absolute maximum continuous power you can pull at 12V is 1,200W (12V × 100A). If your inverter is rated for 2,000W, a single 100Ah battery cannot safely support it at full capacity.
Load Tally: Sizing a 12V Cabin Bank
Let us calculate the exact capacity needed for a small off-grid cabin running a 12V system. We must account for both daily energy consumption (Wh) and peak instantaneous current (A) including inrush.
| Device | Running Watts | Qty | Daily Hours | Daily Wh | Inrush Multiplier | Peak Surge (W) |
|---|---|---|---|---|---|---|
| 12V Compressor Fridge (e.g., Dometic CFX3) | 45W | 1 | 24 (duty cycle 25%) | 270 Wh | 3x | 135W |
| LED Lighting (Total) | 30W | 1 | 6 | 180 Wh | 1x | 30W |
| Laptop Charger | 65W | 1 | 4 | 260 Wh | 1.2x | 78W |
| 1/2 HP Well Pump (via Inverter) | 800W | 1 | 1 (intermittent) | 800 Wh | 5x | 4,000W |
| Totals | - | - | - | 1,510 Wh | - | 4,243W |
The Math:
1,510 Wh ÷ 12V = 125.8 Ah required daily.
125.8 Ah ÷ 0.80 (LiFePO4 DoD) = 157.25 Ah minimum battery capacity.
However, look at the peak surge: 4,243W. At 12V, delivering 4,243W requires 353 Amps of instantaneous current. No single standard 12V LiFePO4 BMS will pass 353A. You must either parallel multiple batteries to stack the BMS current limits, or change the system voltage.
What Trips the System Before the Fuse Blows?
In AC panels, a thermal-magnetic breaker trips when current exceeds the rating. In DC battery systems, the BMS and the inverter's internal protections act much faster than the physical Class T or ANL fuse on the positive terminal. Here is what will shut your system down before the fuse melts:
A 12V LiFePO4 battery resting at 13.2V will experience voltage sag under heavy loads due to internal resistance. If you pull 150A, the terminal voltage might drop to 11.8V. Most 12V inverters have a hardcoded Low Voltage Disconnect (LVD) at 11.0V or 11.5V to protect themselves. If your wire gauge is too small (e.g., using 4 AWG instead of 2/0 AWG for a 2000W inverter), the voltage drop across the copper will push the inverter's input terminals below the LVD threshold, causing an immediate shutdown even if the battery is at 80% SoC.
- BMS Overcurrent Cutoff: If your load demands 120A but the BMS is rated for 100A continuous, the MOSFETs inside the BMS will open the circuit in milliseconds. The physical 150A fuse will never see the fault.
- BMS Thermal Shutdown: Pushing a 100A BMS to 95A continuously generates significant heat inside the battery casing. If the internal thermistor reads >65°C (149°F), the BMS will disconnect to prevent thermal runaway.
- Busbar Heat: According to Victron Energy's Wiring Unlimited guide, loose connections at the busbar create high resistance. A 100A load across a 0.01-ohm loose lug generates 100W of localized heat, melting the insulation and tripping thermal sensors long before the main fuse clears.
Headroom, Future Loads, and Adding Parallel Strings
When do you add a "dedicated circuit" in a DC system? In battery terms, this means adding a parallel string or upgrading to a higher voltage architecture. You must add parallel batteries or increase voltage when:
- Continuous DC current exceeds a single BMS limit: If your inverter draws 120A continuous, and your battery BMS is 100A, you must parallel a second identical battery to share the load (yielding a 200A combined BMS limit).
- Voltage drop exceeds 3%: If you cannot physically route thick enough copper (e.g., 4/0 AWG) to keep voltage drop under 3% on a 12V high-current run, you must switch to a 24V or 48V system to cut the amperage in half or quarter.
- Future Load Expansion: If you plan to add a 1,500W space heater or an induction cooktop later, your current 12V 200Ah bank will be entirely inadequate. Design for a 20% headroom buffer on your calculated Ah to accommodate future phantom loads and seasonal inefficiencies.
For any system exceeding 2,000W of continuous inverter load, abandon 12V. A 2,000W load on a 12V system pulls 166A, requiring massive 2/0 AWG copper cables. That same 2,000W load on a 48V system pulls only 41A, allowing you to use standard 8 AWG or 6 AWG wire, drastically reducing copper costs and voltage drop.
Decision Path: Picking Your Exact Battery Configuration
Use this decision tree to terminate your planning phase and select a concrete battery architecture. This aligns with NEC Article 480 guidelines for safe storage battery installations regarding disconnects and overcurrent protection.
| Total Daily Load (Wh) | Max Continuous Inverter Load | Max Surge (Inrush) | System Voltage | Required Ah (at 80% DoD) | Concrete Pick / Configuration |
|---|---|---|---|---|---|
| < 800 Wh | < 600W | < 1,200W | 12V | 83 Ah | 1x 12V 100Ah LiFePO4 (Drop-in replacement) |
| 800 - 1,500 Wh | 600W - 1,500W | < 3,000W | 12V | 156 Ah | 2x 12V 100Ah LiFePO4 in parallel (200Ah total, 200A BMS limit) |
| 1,500 - 3,000 Wh | 1,500W - 3,000W | < 6,000W | 24V | 156 Ah @ 24V | 2x 12V 100Ah in series (24V 100Ah) OR 1x 24V 100Ah Server Rack |
| > 3,000 Wh | > 3,000W | > 6,000W | 48V | 78 Ah @ 48V | 1x 48V 100Ah Server Rack LiFePO4 (e.g., SOK or EG4) |
The Default Recommendation: If your load tally from Table 1 reflects a modern off-grid cabin with a well pump and standard appliances, do not build a massive 12V parallel bank. The default, most reliable pick is a 48V 100Ah Server Rack LiFePO4 battery (such as the SOK 48V 100Ah or EG4 48V100, typically priced around $1,300 - $1,500) paired with a 48V 3,000W hybrid inverter. This configuration natively supports the 4,243W well pump surge, keeps continuous current under 65A (eliminating heavy copper requirements), and provides 5,120Wh of total storage—yielding 4,096Wh of usable capacity at an 80% DoD, which easily covers your 1,510Wh daily tally with massive headroom for future expansion.
