The raw battery capacity equation is simple: Capacity (Wh) = Voltage (V) × Amp-Hours (Ah). But if you use that raw number to plan a circuit, you will strand your load in the dark. In real-world off-grid and backup power planning, the governing rule is the 80% Depth of Discharge (DoD) limit combined with inverter efficiency losses. This is the DC equivalent of the NEC 80% continuous load rule for AC breakers.
The Direct Answer: For a standard 12V 100Ah LiFePO4 battery (1280Wh nominal), applying an 80% DoD limit and a 90% inverter efficiency yields 921 usable AC watt-hours. This exact capacity will run a continuous 250W circuit load for 3.6 hours, or a 500W load for 1.8 hours. If you attempt to pull 1200W (a microwave) from this single battery, you will draw over 100A DC, instantly tripping the battery's internal BMS overcurrent protection, regardless of how full the battery is.
The Real-World Battery Capacity Equation (Load Tally & Sizing)
To properly size a battery bank, we must adjust the nominal capacity for chemistry limits and conversion losses. The working equation for usable AC energy is:
Usable AC Wh = (Nominal V × Ah) × DoD Limit × Inverter Efficiency
For lithium iron phosphate (LiFePO4), a safe continuous DoD is 80% to 90%. For AGM or flooded lead-acid, the DoD must be restricted to 50% to prevent rapid degradation. Inverter efficiency typically sits between 85% and 92% depending on the load curve; we use 90% as a conservative baseline for pure sine wave units like the Victron MultiPlus or Renogy 2000W.
Below is a data-dense load tally for a typical 12V cabin or van subpanel. Notice how the DC amp draw is calculated using the working voltage (11.5V under load), not the nominal 12V, and accounts for the 90% inverter efficiency penalty.
| Device / Circuit | AC Watts (Running) | DC Amps @ 11.5V (w/ 90% eff) | Inrush Multiplier | Peak DC Inrush (Amps) | Daily Wh (Est.) |
|---|---|---|---|---|---|
| LED Lighting (6 fixtures) | 54W | 5.2A | 1.0x (Resistive) | 5.2A | 270 Wh |
| Starlink Standard Router | 65W | 6.3A | 1.2x (Capacitive) | 7.5A | 325 Wh |
| 120V AC Fridge (Compressor) | 150W | 14.5A | 5.0x (Inductive) | 72.5A | 900 Wh |
| CPAP Machine (w/ Heater) | 90W | 8.7A | 1.1x | 9.5A | 720 Wh |
| Laptop Charging (USB-C PD) | 100W | 9.7A | 1.5x (SMPS) | 14.5A | 400 Wh |
The Tally: The continuous DC draw of these devices running simultaneously is roughly 44.4A. The total daily energy requirement is 2,615 Wh. A single 12V 100Ah battery (921 usable Wh) will only cover about 35% of this daily load. To run this panel for 24 hours without solar input, you need a minimum of three 12V 100Ah LiFePO4 batteries wired in parallel, yielding 2,763 usable Wh.
What Trips the System Before the Math Runs Out?
In an AC panel, a thermal breaker trips when heat from overcurrent bends a bimetallic strip. In a DC battery system, the Battery Management System (BMS) acts as the breaker. Even if your math says you have 50% capacity left, three physical phenomena will trip the BMS and cut your power prematurely.
1. Voltage Sag and Low-Voltage Cutoff
Every battery has internal resistance, and every cable has resistance. According to Ohm's Law (V = I × R), high current draws cause voltage to drop. A 12V LiFePO4 battery might read 12.8V at rest, but under an 80A load (like the fridge compressor starting), the voltage at the BMS terminals can sag to 10.2V. Most BMS units are programmed with a low-voltage disconnect at 10.0V or 10.5V to protect the cells. If your DC cabling is undersized (e.g., using 6 AWG instead of 2/0 AWG for a 100A inverter), the voltage drop across the wire alone will trick the BMS into thinking the battery is dead, tripping the system mid-cycle.
2. The Peukert Effect (Lead-Acid Only)
If you are using AGM or Gel batteries, you must factor in Peukert's Law, which dictates that battery capacity shrinks exponentially as the discharge rate increases. A 100Ah AGM battery rated at a 20-hour discharge (5A draw) will only deliver about 60Ah of usable capacity if you pull 50A continuously. LiFePO4 chemistry is largely immune to the Peukert effect, which is why it dominates modern U.S. Department of Energy solar storage guidelines.
3. Inrush Current and BMS Overcurrent
Look at the fridge compressor in Table 1. It runs at 14.5A DC, but the inductive inrush spikes to 72.5A for a fraction of a second. If you add a microwave (100A DC) and the fridge kicks on simultaneously, the combined transient spike can exceed the 100A continuous / 150A peak rating of a standard 100A BMS, triggering an immediate hard shutdown.
Decision Tree: When to Add a Parallel Bank or Dedicated Circuit
Knowing when to scale your battery bank or restructure your loads prevents nuisance trips and voltage collapse. Use this decision matrix based on NEC Article 480 and 690 principles adapted for DC storage:
| System Condition / Symptom | Underlying Cause | Required Action |
|---|---|---|
| Continuous DC draw exceeds 50A on a single 100Ah battery. | Discharge rate > 0.5C; generates excess internal heat and severe voltage sag. | Add a second battery in parallel to split the current, or upgrade to a 200Ah single cell. |
| Total continuous DC load exceeds 120A. | 12V architecture requires massive, expensive cabling (e.g., 4/0 AWG) to prevent voltage drop. | Restructure to a 24V or 48V battery bank and use a corresponding high-voltage inverter. |
| High-surge loads (well pump, AC unit) trip BMS on startup. | Transient inrush exceeds BMS peak amp rating. | Add a dedicated 'soft-start' device on the AC circuit, or install a dedicated parallel battery just for surge buffering. |
| Daily Wh requirement exceeds 1.5x usable bank capacity. | Insufficient headroom for cloudy days or generator failure. | Expand parallel bank capacity or implement automated load-shedding via a smart shunt/relay. |
Headroom, Future Loads, and System Margins
When planning circuit capacity, never size a battery bank to exactly match your calculated load tally. In 2026, the standard practice for off-grid and backup systems is to build in a 20% to 30% headroom buffer above your worst-case daily Wh calculation. This accounts for inverter idle draw (which consumes 15W to 30W just being turned on), phantom loads from device power bricks, and capacity degradation over the battery's 10-year lifespan.
Furthermore, modern LiFePO4 batteries feature Bluetooth-enabled smart BMS units. Instead of guessing why a circuit tripped, you can monitor individual cell voltages and exact DC amperage in real-time via your phone. If you notice your 12V fridge circuit is consistently pulling 16A instead of the spec-sheet 14.5A, you know the compressor is working harder than expected—perhaps due to poor ventilation—and you can adjust your load planning before the BMS forces a shutdown.
Ultimately, the battery capacity equation is just the starting line. True circuit capacity planning requires respecting the physics of DC voltage drop, the reality of inductive inrush, and the hard limits of solid-state BMS protection. Size your wires for the peak spike, size your batteries for the daily Wh, and always respect the 80% DoD rule.
