Sizing a Battery Bank AND OR Inverter for Off-Grid Power

Every reliable off-grid or backup power system follows a strict DC-to-AC pipeline: the generation source (solar array or generator) feeds a charge controller, which regulates voltage into the battery bank for storage, which then feeds an inverter to convert DC to AC for your main panel and loads. When planning this architecture, correctly sizing your battery bank and or inverter is the difference between a resilient setup and a tripped breaker at 2 AM. You cannot simply match the inverter's wattage to the battery's amp-hour rating and call it done. You must account for DC current limits, voltage drop, surge requirements, and electrochemical discharge curves.

This guide breaks down the exact sizing math, wiring topologies, and safety protocols required to build a 12V, 24V, or 48V energy storage system that actually performs under load.

The Core Sizing Matrix: Load, Inverter, and Battery Bank

Before running any complex math, you need a baseline. The table below maps common continuous AC loads to the required inverter size, the resulting DC current draw, the minimum recommended copper wire gauge (assuming a 3-foot run with standard THHN in free air), and the minimum LiFePO4 battery bank capacity required to sustain that load for 4 hours without violating depth-of-discharge (DoD) limits.

How to read this table: Notice how the 12V system pulling 1,200W requires the exact same 1/0 AWG wire and 111A current handling as the 24V system pulling 2,400W. This is why 12V systems are generally abandoned once your continuous load exceeds 1,500W; the DC current becomes unmanageable, causing massive voltage drop and requiring expensive, stiff copper cabling. For any modern home or cabin, 48V is the undisputed standard.

Series vs. Parallel: Voltage, Amp-Hours, and C-Rate Limits

How you wire your cells or monoblocks fundamentally changes the system's electrical characteristics. The consequences of series vs parallel wiring dictate your charge controller compatibility and your physical safety limits.

Series Wiring (Voltage Adds, Ah Stays Same)

When you wire batteries in series, you connect the positive terminal of one battery to the negative terminal of the next. The Amp-hour (Ah) capacity remains identical to a single battery, but the nominal voltage multiplies.
Example: Four 12V 100Ah LiFePO4 batteries in series yield a 48V 100Ah bank. The total energy is 5,120Wh (48V × 100Ah). Series strings are preferred because they keep DC currents low and ensure equal current flow through all cells, making balancing easier for the Battery Management System (BMS).

Parallel Wiring (Ah Adds, Voltage Stays Same)

When you wire batteries in parallel, you connect positive to positive and negative to negative. The voltage remains the same, but the Ah capacity multiplies.
Example: Four 12V 100Ah batteries in parallel yield a 12V 400Ah bank. Total energy is still 5,120Wh (12V × 400Ah).

Charge and Discharge Limits: C-Rate and DoD

Your battery chemistry dictates how fast you can safely pull energy (discharge C-rate) and how deep you can drain it (Depth of Discharge).

• Flooded Lead-Acid (FLA): Max continuous discharge of 0.2C (a 100Ah battery can safely output 20A). Usable DoD is 50%. Discharging below 50% causes irreversible sulfation.
• AGM / Gel: Max continuous discharge of 0.25C to 0.3C. Usable DoD is 50% to 60%.
• LiFePO4 (Lithium Iron Phosphate): Max continuous discharge of 0.5C to 1C (a 100Ah battery can output 50A to 100A continuously, depending on the BMS rating). Usable DoD is 80% to 95%. Most quality BMS units will hard-cutoff at 10% SoC to protect the cells from copper dissolution.

Sizing Math: Peukert's Law, Inverter Efficiency, and Real-World Losses

Theoretical math assumes a perfect world. Bench math accounts for heat, chemical limitations, and conversion losses. When sizing a battery bank and or inverter, you must apply two major derating factors.

1. Inverter Efficiency Losses

Inverters are not 100% efficient. High-frequency inverters typically operate at 88-92% efficiency, while low-frequency (transformer-based) inverters like the Victron MultiPlus-II operate around 93-95% at optimal load.
Worked Example: If your AC load is 3,000W and your inverter is 90% efficient, the DC power required is 3,000W / 0.90 = 3,333W. On a 48V nominal system (which actually sits around 51.2V under load), your DC current draw is 3,333W / 51.2V = 65.1 Amps. You must size your battery cables and BMS to handle this 65A continuous draw, not the 58.5A you'd get if you ignored efficiency.

2. Peukert's Law (The Lead-Acid Penalty)

If you are using lead-acid batteries, a 100Ah rating is only valid if you draw the current over 20 hours (a 5A draw). If you pull current faster, the usable capacity plummets due to internal resistance and chemical depletion at the plates. This is calculated using Peukert's Law:

t = H × (C / (I × H))^k
Where: t = time, H = rated hour (usually 20), C = rated capacity (100Ah), I = actual draw, k = Peukert exponent (typically 1.3 for FLA).

Worked Example: You have a 12V 100Ah FLA battery and your inverter pulls 50A DC.
t = 20 × (100 / (50 × 20))^1.3
t = 20 × (0.1)^1.3
t = 20 × 0.0501 = 1.00 hour.
Even though 100Ah / 50A = 2 hours on paper, Peukert's law proves you will only get 1 hour of runtime before the battery is dead. This is precisely why high-draw off-grid systems must use lithium, which has a Peukert exponent of nearly 1.05 (effectively zero penalty).

Inverter/Charger Sizing and Lithium Safety Protocols

Selecting the right inverter/charger requires looking beyond continuous wattage. You must account for inductive surge loads and proper battery charging algorithms.

Sizing the Inverter/Charger

Resistive loads (heaters, incandescent lights) draw exactly what their label says. Inductive loads (well pumps, refrigerator compressors, table saws) require a massive spike of current to overcome initial inertia—often 3x to 5x their running wattage for a few milliseconds.
The Rule of Thumb: Size your inverter's surge rating to cover your largest inductive motor's starting wattage, and size the continuous rating to cover your total simultaneous running loads plus a 20% safety buffer. For a standard cabin with a 1/2 HP well pump (1,000W running, 3,500W surge) and a 1,500W microwave, a 3,000W continuous / 6,000W surge inverter (like the Growatt 5000ES or Victron MultiPlus 48/3000) is the minimum viable hardware.

Charger Sizing: If your unit includes an AC-to-DC charger (for generator or grid integration), the charge current should be 10% to 20% of your battery bank's total Ah capacity. A 48V 200Ah LiFePO4 bank requires a charger capable of 20A to 40A to reach full SoC in a reasonable timeframe without sitting in the absorption phase for hours.

By mapping your true AC loads back through inverter efficiency, respecting the electrochemical limits of your chosen battery chemistry, and wiring for the lowest possible DC current via a 48V topology, you build a system that doesn't just work on the bench—it survives the realities of daily off-grid life.