How many 20W DC devices can you run on a 12V 100Ah LiFePO4 battery for 5 hours? Exactly 9 devices. This is dictated by the battery capacity formula and the 80% Depth of Discharge (DoD) governing rule, which restricts your usable capacity to 80Ah to preserve cycle life and prevent cell degradation. Assuming a 95% efficient DC-DC converter, your usable energy is 912Wh. Nine 20W devices running for 5 hours consume 900Wh, leaving a safe 12Wh buffer. Attempting to run a 10th device will trigger the BMS Low Voltage Disconnect (LVD) before the 5-hour mark.
When planning off-grid, marine, or backup power circuits, guessing your amp-hour requirements leads to either overspending on lithium or tripping inverters at 2 AM. Below is the exact framework for calculating load capacity, handling inrush currents, and sizing your busbars.
The Core Battery Capacity Formula & The 80% Rule
The fundamental battery capacity formula for sizing a bank to a specific AC or DC load profile is:
Required Nameplate Ah = (Total Watts × Runtime Hours) / (System Voltage × DoD × Inverter Efficiency)
To use this formula correctly, you must lock in your governing constants based on battery chemistry and ambient temperature (assuming a standard 25°C / 77°F baseline):
- Depth of Discharge (DoD): For LiFePO4 (Lithium Iron Phosphate), the governing rule is 80% DoD. While a BMS might allow 100% discharge, regularly pulling cells below 20% State of Charge (SoC) accelerates capacity fade. For Flooded Lead-Acid (FLA), the rule is strictly 50% DoD.
- Inverter Efficiency: Use 0.90 (90%) for high-frequency pure sine wave inverters under 3000W, and 0.93 (93%) for low-frequency transformer-based units.
- Peukert’s Law vs. C-Rates: If you are using lead-acid, Peukert’s Law dictates that higher discharge rates artificially shrink your usable Ah. LiFePO4 chemistry largely ignores Peukert’s effect up to a 0.5C discharge rate, meaning a 100Ah battery will reliably deliver 50A continuous without significant capacity loss.
Load Tally: Calculating Exact Amp-Hour Requirements
Let’s apply the battery capacity formula to a realistic 48V off-grid cabin circuit. We are targeting a 12-hour overnight runtime without solar input. Notice how inrush currents are isolated from the continuous Ah calculation but flagged for peak-power sizing.
| Device | Qty | Run Watts (AC/DC) | Inrush / Surge (W) | Daily Hours | Total Wh |
|---|---|---|---|---|---|
| Starlink Standard (DC-DC) | 1 | 50W | 60W | 12 | 600 Wh |
| DC LED Lighting Array | 6 | 10W (60W total) | 60W | 6 | 360 Wh |
| 12V Compressor Fridge | 1 | 45W (avg duty) | 180W (startup) | 24 (cycling) | 1080 Wh |
| 12V Demand Water Pump | 1 | 60W | 120W | 1.5 | 90 Wh |
| Laptop / Phone Charging | 2 | 65W (130W total) | 130W | 4 | 520 Wh |
| Totals | 345W Cont. | ~180W Peak | 2650 Wh |
The Math: 2650 Wh total daily load. Assuming a 48V system, 80% DoD, and a 95% efficient DC-DC / 90% efficient DC-AC mix (we'll use 0.92 as a blended efficiency factor for this predominantly DC load profile):
Required Ah = 2650 / (48 × 0.80 × 0.92) = 2650 / 35.32 = 75.02 Ah
You need a minimum nameplate capacity of 75Ah at 48V. Since 48V 75Ah is not a standard off-the-shelf server rack size, you round up to the nearest standard module: a 48V 100Ah LiFePO4 Server Rack Battery (yielding 5120Wh nameplate, 4096Wh usable).
What Trips the System Before the DC Breaker?
When sizing circuits, hobbyists often obsess over the DC breaker (like a Bussmann 187 series or Blue Sea MRBF fuse). But DC breakers are thermal/magnetic and relatively slow to react to microsecond spikes. What actually trips the circuit and kills your power first are the silicon protections inside the battery and inverter.
Here is the exact sequence of what will trip your system under heavy load, ranked from most likely to least likely:
- Voltage Sag & Inverter LVD (Most Common): If you pull 80A continuously from a 100Ah battery with undersized 4 AWG wire, the voltage will sag from 51.2V down to 46V. Your 48V inverter will interpret this as a dead battery and trigger its Low Voltage Disconnect (LVD) to save the cells, shutting off your AC power instantly.
- BMS Over-Current Protection (OCP): Most 100Ah LiFePO4 BMS units have a hard 100A continuous limit. If your compressor fridge and water pump kick on simultaneously,叠加 (stacking) their inrush currents, the BMS MOSFETs will detect >100A and physically sever the circuit in milliseconds.
- Thermal Runaway / Heat Cutoff: Pushing a 1C continuous draw (100A on a 100Ah battery) generates massive heat at the cell busbars and MOSFETs. The BMS thermistor will trip the system offline long before the external DC breaker feels the heat.
- DC Breaker / Fuse (Least Common First Responder): The fuse will only blow if the BMS MOSFETs fail short-circuit and a dead short occurs on the load side, or if a sustained 1.5x overload slowly heats the breaker's thermal bimetallic strip over several minutes.
Decision Path: Sizing the Bank and Busbar
Use this decision tree to select your exact battery architecture and busbar rating based on your continuous amp draw. This assumes standard 1/0 AWG or 2/0 AWG copper welding cable and an ambient temperature of 25°C.
| If Your Continuous Load Is... | And Peak Inrush Is... | Then Specify This Architecture... | Busbar & Breaker Rating |
|---|---|---|---|
| < 30A (e.g., < 1440W @ 48V) | < 60A | Single 48V 100Ah LiFePO4 Server Rack | 250A Busbar / 80A Class T Fuse |
| 30A - 60A (e.g., 1440W - 2880W @ 48V) | < 120A | Single 48V 100Ah High-Discharge (e.g., EG4) | 500A Busbar / 150A Class T Fuse |
| 60A - 100A (e.g., 2880W - 4800W @ 48V) | < 150A | Two 48V 100Ah in Parallel (2P) | Two 500A Busbars / 250A Class T |
| > 100A (e.g., > 4800W @ 48V) | > 150A | Three+ 48V 100Ah in Parallel (3P+) | 1000A Busbar / 400A ANL Fuse |
Concrete Default Pick: For 90% of off-grid cabins, skoolies, and backup UPS systems running under 3000W continuous, the optimal choice is a single SOK 48V 100Ah Server Rack Battery (or equivalent Pylontech US5000) paired with a Victron MultiPlus 48/3000 inverter/charger. This keeps your continuous draw well under the 0.5C (50A) sweet spot, maximizing cycle life to 6000+ cycles while keeping copper wire sizes manageable (2/0 AWG).
Headroom, Future Loads, and Parallel Strings
Battery capacity planning is not just about today's load tally; it is about managing internal cell resistance as the bank ages. As LiFePO4 cells age past 3000 cycles, their internal resistance (IR) rises. A battery that could easily handle a 40A draw with minimal voltage sag in year one might sag enough to trigger the inverter LVD in year five under the exact same load.
When to add a dedicated circuit (parallel string): In battery architecture, adding a 'dedicated circuit' means dropping a second parallel battery string onto the busbar. You must do this when:
- Your calculated continuous draw exceeds 0.5C of your current bank's nameplate capacity (e.g., pulling 60A from a single 100Ah battery).
- You plan to add high-draw resistive loads later (like a 120V AC induction cooktop or electric water heater via inverter).
- Your voltage sag under load consistently drops below 48V (15.5V per cell) on the BMS monitoring screen.
When paralleling strings, never just daisy-chain them on a single terminal lug. Use a symmetrical busbar topology (positive and negative busbars) with identical length 2/0 AWG copper cables for each battery to ensure balanced current sharing. If one battery has a shorter cable, it will carry a disproportionate share of the inrush current, trip its BMS OCP prematurely, and cascade the failure to the remaining batteries.
For deeper architectural guidance on parallel topologies and BMS communication protocols, reference the Victron Smart Lithium manual and the safety protocols outlined by Battery University. Always verify your final wire ampacity against NEC Article 690 and local AHJ requirements before energizing the busbar.
