To calculate the exact capacity of a battery bank for your loads, sum your daily Watt-hours (Wh), divide by your system voltage to get Amp-hours (Ah), and divide by your battery chemistry's maximum Depth of Discharge (DoD)—typically 80% for LiFePO4 or 50% for Lead-Acid. Finally, multiply by 1.2 to account for inverter inefficiency and future headroom. For example, a 2,400Wh daily load on a 12V system requires 200Ah raw capacity. Divided by 0.80 (LiFePO4 DoD) and multiplied by 1.2 (efficiency buffer), you need exactly a 300Ah usable battery bank.
Sizing a battery isn't just about total energy; it is about circuit capacity, continuous discharge rates, and surge tolerance. Just as a 20A breaker requires you to limit continuous loads to 16A (the 80% NEC rule), a battery's Battery Management System (BMS) and internal chemistry have strict continuous and surge limits that will shut the system down long before the cells are actually empty if you ignore the physics of voltage sag.
The Load Tally: Mapping Watts, Amps, and Inrush
Before you buy a single cell, you must build a load tally. The most common mistake DIYers make is sizing a battery for running watts while ignoring inrush current and continuous draw limits. Motors, compressors, and transformer-based power supplies pull 3x to 5x their rated running wattage for milliseconds when starting. While your inverter handles the millisecond surge, your battery must supply the sustained RMS current without excessive voltage drop.
| Device | Running Watts | Inrush / Surge Watts | Daily Run Hours | Daily Energy (Wh) | Continuous Amps (at 12V) |
|---|---|---|---|---|---|
| 12V Compressor Fridge | 45W | 180W (compressor start) | 8h (duty cycle) | 360 Wh | 3.75A |
| Starlink Roam (120V via Inverter) | 60W | 90W | 10h | 600 Wh | 5.0A (6.2A DC with inverter loss) |
| Laptop Charger (65W PSU) | 50W | 90W (capacitive inrush) | 4h | 200 Wh | 4.1A (5.2A DC) |
| LED Lighting (6 fixtures) | 42W | 42W | 5h | 210 Wh | 3.5A |
| 12V Water Pump | 60W | 240W (motor start) | 0.5h | 30 Wh | 5.0A |
| Totals | 257W (Peak Concurrent) | ~400W Surge | - | 1,400 Wh | ~21.5A Max DC Draw |
In this scenario, your total daily energy is 1,400 Wh. However, your peak concurrent continuous draw is roughly 21.5A at 12V (accounting for inverter efficiency losses on the 120V loads). If you are running a microwave (1000W) concurrently with the fridge and Starlink, your DC draw spikes to over 100A. This is where the '80% continuous rule' for batteries comes into play.
Sizing the Bank: Chemistry, DoD, and the 80% Rule
In AC panel planning, the 80% rule prevents breaker thermal fatigue. In DC battery planning, the 80% rule governs your Depth of Discharge (DoD) and your Continuous C-Rate. You should rarely discharge a LiFePO4 battery past 80% DoD if you want to maximize cycle life, and you should never continuously draw more than 80% of the BMS's maximum rated current to prevent internal heat buildup and premature low-voltage cutoffs.
| Chemistry | Max Usable DoD | Peukert Effect | Continuous Draw Limit (Rule of Thumb) | Estimated Cycle Life (to 80% SoH) |
|---|---|---|---|---|
| LiFePO4 (Lithium Iron Phosphate) | 80% - 90% | Negligible | 0.5C to 1.0C (50A-100A per 100Ah) | 4,000 - 6,000 cycles |
| AGM (Absorbent Glass Mat) | 50% | Moderate | 0.2C (20A per 100Ah) | 500 - 1,000 cycles |
| Flooded Lead-Acid (FLA) | 50% | Severe | 0.1C to 0.2C (10A-20A per 100Ah) | 500 - 800 cycles |
Let's calculate the exact bank size for our 1,400 Wh load tally using LiFePO4:
- Base Ah: 1,400 Wh / 12V = 116.6 Ah.
- Apply DoD (80%): 116.6 Ah / 0.80 = 145.7 Ah.
- Apply Inverter/Headroom Buffer (20%): 145.7 Ah * 1.20 = 174.8 Ah.
You need a minimum of 175Ah of LiFePO4 capacity (typically achieved by wiring two 100Ah batteries in parallel, yielding 200Ah total, giving you excellent headroom).
What Trips the Battery Before It's Empty?
Just as a breaker trips from heat before a wire melts, a battery BMS will trip from voltage sag before the cells are empty. According to U.S. Department of Energy guidelines on battery basics, internal resistance causes voltage to drop under heavy load (Ohm's Law: V = I × R).
If you pull 100A from a single 100Ah LiFePO4 battery with an internal resistance of 40 milliohms, the voltage will sag by 4V (100A × 0.04Ω). A fully charged battery sitting at 13.2V will instantly drop to 9.2V at the terminals. The BMS will read 9.2V, assume the battery is dead, and trigger the Low Voltage Disconnect (LVD), killing your inverter—even if the cells are at 90% State of Charge. To prevent this, parallel multiple batteries to divide the current and halve the internal resistance.
When to Scale Up: Parallel Strings and Busbar Limits
In home wiring, you add a dedicated circuit when a single breaker's capacity is exceeded or voltage drop on a long run becomes unacceptable. In a DC battery bank, you add parallel strings (or upgrade to a 24V/48V architecture) when you hit physical busbar limits, BMS current limits, or excessive voltage drop on your DC cables.
As detailed in Victron Energy's Wiring Unlimited guide, managing high DC currents requires strict attention to busbar ratings and cable ampacity. Pushing 200A through a single 2/0 AWG cable and a standard 250A busbar generates significant heat and voltage drop over distance.
| System Symptom / Load Change | Root Cause | Solution / Next Step |
|---|---|---|
| Inverter shuts off instantly when starting a large motor (e.g., table saw, AC compressor). | Voltage sag tripping the BMS LVD or inverter low-voltage cutoff. | Add a second identical battery in parallel to halve internal resistance, or upgrade to a 24V system to halve the DC current. |
| Busbar or battery terminals feel warm to the touch (>110°F) during continuous high loads. | Exceeding 80% of the hardware's continuous thermal rating (e.g., pulling 200A through a 200A rated busbar). | Upgrade to a 500A shunt/busbar, use 4/0 AWG welding cable, and torque lugs to manufacturer specs (typically 10-12 Nm). |
| Daily load increases by >30% (e.g., adding a space heater or server rack). | Exceeding the 0.5C continuous safe discharge rate of the current bank. | Add parallel battery strings of the exact same brand, age, and capacity. Never mix old and new cells in parallel. |
| Cable runs from battery to inverter exceed 5 feet at 12V. | Voltage drop exceeding 1% (loss of >0.12V on a 12V system). | Switch to a 24V or 48V battery architecture. 48V cuts the current (and the voltage drop) by 75% for the same wattage. |
Protecting the Bank: Fusing and Shunts
Never connect a battery bank directly to an inverter without overcurrent protection. For a 200Ah LiFePO4 bank on a 12V system powering a 2000W inverter, your peak continuous draw could reach 180A. Install a 250A Class T fuse on the positive main feeder within 7 inches of the battery terminal. Class T fuses have a high interrupt capacity (AIC) of 20,000A, which is critical because lithium batteries can dump thousands of amps into a dead short before the BMS has time to react.
Finally, install a DC shunt (like the Victron SmartShunt) on the negative main line. This acts as your 'circuit meter', tracking exact Amp-hours in and out, calculating true State of Charge (SoC) via Coulomb counting, and alerting you via Bluetooth if your continuous loads are creeping into the danger zone of your battery's C-rate limits. By respecting the 80% DoD rule, managing inrush, and scaling your busbars appropriately, your battery bank will deliver reliable, safe power for thousands of cycles.
