To convert a 12V system's power requirement to amperes, divide the load's wattage by the system voltage (12V), then divide by the inverter or conversion efficiency (typically 0.85 to 0.95). For example, a 1200W load on a 12V system with 90% efficiency draws roughly 111 amperes from the battery bank. Getting this number right dictates your wire gauge, fuse size, and whether your battery BMS will trip under load.
The Source-to-Load System Block
Before running the math, you need to visualize where the amperage actually flows. In a standard off-grid or mobile power setup, the highest current exists at the lowest voltage point. Here is the physical system block from source to load:
12V Source (Battery Bank) → Overcurrent Protection (Class T or ANL Fuse) → DC Disconnect Switch → Inverter / DC-DC Converter → AC/DC Load
When you convert 12V to 120V AC, the voltage steps up by a factor of 10, but the amperage steps down by a factor of 10 (minus efficiency losses). A 10-ampere draw on the 120V AC side translates to over 100 amperes on the 12V DC side. This is why your battery interconnects and inverter feed cables must be sized for massive DC current, while your AC branch circuits can use standard 12 AWG or 14 AWG NM-B wire.
The Math: Converting 12V to Amperes with Real-World Losses
The high-school physics formula is I = P / V (Current = Power / Voltage). But on the workbench, ignoring conversion efficiency and battery chemistry will leave you with a tripped BMS or melted terminal lugs.
The Real-World Formula:
DC Amps = AC Watts / (System Voltage × Inverter Efficiency)
If you are using lead-acid (AGM, Gel, or Flooded), you must also account for Peukert's Law. Peukert's effect dictates that the faster you discharge a lead-acid battery, the lower its effective capacity. A 100Ah AGM battery rated at a 20-hour discharge (5A) will only deliver about 60Ah if you pull 100A from it. Lithium Iron Phosphate (LiFePO4) cells have a Peukert exponent very close to 1.0, meaning they deliver nearly their full rated capacity even at high discharge rates.
| AC Load (Watts) | Ideal DC Amps | Real DC Amps (90% Eff.) | Min. Copper Wire (AWG)* | Max Fuse Size (Amps) |
|---|---|---|---|---|
| 400W | 33.3A | 37.0A | 8 AWG | 50A (ANL) |
| 1000W | 83.3A | 92.5A | 2 AWG | 125A (Class T) |
| 2000W | 166.6A | 185.1A | 2/0 AWG | 225A (Class T) |
| 3000W | 250.0A | 277.7A | 4/0 AWG | 350A (Class T) |
*Wire sizing assumes 75°C column, 30°C ambient, and a maximum 3% voltage drop over a 5-foot one-way run. Always verify against NEC Table 310.16 or local equivalent.
Battery Configurations: Series, Parallel, and Discharge Limits
To safely deliver the amperes you just calculated, you need to configure your battery bank correctly and respect the chemistry's physical limits.
Series vs. Parallel Consequences
- Series: Voltages add, Amp-hours (Ah) remain the same. Wiring two 12V 100Ah batteries in series yields 24V at 100Ah. This halves your DC amperage draw for the same wattage, allowing for smaller wires.
- Parallel: Amp-hours add, Voltage remains the same. Wiring two 12V 100Ah batteries in parallel yields 12V at 200Ah. This increases your total energy capacity and splits the amperage draw across both batteries, reducing strain on individual cells.
Never wire batteries in parallel if they have different chemistries, different ages, or different internal resistances. The battery with the higher voltage will force current into the lower voltage battery, potentially causing uncontrolled heating, venting, or fire. If paralleling 12V lithium batteries, ensure they are the exact same model, purchased at the same time, and ideally feature internal BMS communication (like CAN bus) to balance charge/discharge loads evenly.
Charge and Discharge Limits (C-Rates and DoD)
Your calculated amperes must not exceed the battery's maximum C-rate (discharge rate relative to capacity).
- Lead-Acid (AGM/Flooded): Maximum recommended continuous discharge is 0.2C (20A for a 100Ah battery). Depth of Discharge (DoD) should be limited to 50% to prevent rapid sulfation and capacity loss.
- LiFePO4: Maximum continuous discharge is typically 1C (100A for a 100Ah battery), though high-discharge models can handle 2C or 3C. DoD can safely be pushed to 80%–100% without degrading cycle life.
LiFePO4 is the safest lithium chemistry available, but it is not immune to thermal runaway if abused. Never charge lithium cells below freezing (0°C / 32°F) without a BMS that explicitly features low-temperature charge cutoff. A 12V LiFePO4 battery must have an integrated Battery Management System (BMS) rated for your maximum calculated amperes. If your inverter pulls 150A and your BMS is rated for 100A, the BMS will hard-cutoff, killing power to your loads instantly. Always size your BMS continuous discharge rating 20% higher than your maximum calculated real-world DC amperes.
Inverter and Charger Sizing for Your Calculated Amperes
Once you know your DC amperage, you can properly size the inverter and the AC-to-DC battery charger.
| Component | Sizing Rule | Real-World Example (1200W Load, 12V System) |
|---|---|---|
| Inverter Continuous | 125% of maximum continuous AC load wattage. | 1200W × 1.25 = 1500W minimum continuous rating. |
| Inverter Surge | Must handle 2x to 3x continuous load for <5 seconds (for motor compressors/pumps). | If running a 1200W fridge compressor, inverter needs a 3000W+ surge rating. |
| Battery Charger | 10% to 20% of total battery bank Ah capacity (in DC amps). | For a 200Ah LiFePO4 bank, use a 20A to 40A smart charger (LiFePO4 profile). |
When sizing the charger, remember that the charger's AC input amperes will add to your generator or shore-power load. A 40A DC output charger operating at 14.4V draws roughly 576W from the AC side. At 120V AC, that is about 5 amperes of AC draw. Factor this into your upstream breaker sizing.
Frequently Asked Questions
How do I convert 12V to amperes for a specific wattage load?
Divide the wattage of your load by 12, then divide that result by your inverter's efficiency (usually 0.85 to 0.95). For instance, if you are running a 600W microwave on a 12V system with a 90% efficient inverter: 600 / 12 = 50A. Then, 50 / 0.90 = 55.5 amperes. This is the actual current your battery and cables must handle.
Does wiring 12V batteries in parallel increase the amperes available?
Wiring in parallel increases the Amp-hour (Ah) capacity and allows the bank to deliver higher total amperes by splitting the load across multiple batteries. However, it does not change the system voltage (it stays 12V). If you need to reduce the amperes flowing through your wires for a high-wattage load, you should wire batteries in series to create a 24V or 48V system, which cuts the DC amperage draw in half or quarter, respectively.
What size fuse do I need for a 12V to 120V inverter drawing 100 amperes?
Following NEC-style overcurrent protection guidelines, you must size the fuse at 125% of the continuous maximum current draw. For a 100-ampere continuous draw, 100 × 1.25 = 125 amperes. You should install a 125A or 150A Class T fuse (preferred for high short-circuit let-through current protection) or an ANL fuse as close to the battery positive terminal as possible. Ensure the wire connecting the battery to the fuse is rated for the fuse's amperage (e.g., 2 AWG or 1/0 AWG copper).
For deeper reading on DC system architecture and voltage drop calculations, refer to the Victron Energy Wiring Unlimited guide, which provides excellent bench-tested diagrams for marine and mobile 12V/24V systems.
