A battery is strictly a Direct Current (DC) device. The electrochemical reactions inside a cell only push electrons in one direction, from the anode to the cathode. There is no such thing as an "AC battery" at the cell level. When you see an AC output on a portable power station or a home backup system, you are looking at a DC battery bank paired with an internal inverter that rapidly switches the DC polarity to simulate an Alternating Current (AC) sine wave.
Understanding this DC-to-AC boundary is the foundation of sizing off-grid solar arrays, RV power systems, and home battery backups. You cannot simply match your AC appliance wattage to a battery's Amp-hour (Ah) rating without accounting for inverter inefficiencies, chemistry limits, and voltage conversion. Here is exactly how to map your AC loads to a DC battery bank and size the hardware in between.
The DC-to-AC Power Path: Source to Load Block Diagram
Every hybrid or off-grid power system follows a strict unidirectional block path from the energy source to the AC load panel. Understanding this signal flow prevents catastrophic wiring mistakes.
- Source (DC/AC): Solar panels (DC) or the utility grid (AC).
- Charge Controller / Charger (DC): An MPPT controller steps solar DC down to battery charging voltage, or an AC-to-DC charger rectifies grid power to charge the bank.
- Battery Bank (DC): Stores energy chemically at a nominal DC voltage (12V, 24V, or 48V).
- Inverter (DC to AC): Draws DC from the bank and chops it into a 120V/240V 60Hz AC sine wave.
- Load Panel (AC): Distributes AC power to standard household outlets and appliances.
Inverter and Charger Sizing for the Stated Load
If your calculated continuous AC load is 2,000W, you do not buy a 2,000W inverter. You must apply a 1.25x safety factor for continuous duty, and account for Locked Rotor Amps (LRA) if you are running compressor motors (fridges, AC units, well pumps). A 2,000W continuous load requires a minimum 2,500W inverter, but a 3,000W pure sine wave inverter (like the Victron MultiPlus-II 12/3000, roughly $1,450) is the bench standard here. It provides a 5,500W peak surge capacity to start motors without tripping the internal low-voltage cutoff.
Sizing the DC Bank: From AC Watts to DC Amp-Hours
The most common mistake DIYers make is ignoring inverter loss and battery chemistry when converting AC Watt-hours (Wh) to DC Amp-hours (Ah). Inverters are not 100% efficient; a high-quality pure sine wave inverter operates at about 85% to 90% efficiency under typical loads. Furthermore, lead-acid batteries suffer from Peukert's Law, while lithium batteries do not.
Below is a real-world sizing matrix for a daily cabin load profile, converting AC requirements into the actual DC capacity needed at the battery terminals.
| Appliance | AC Watts | Daily Hours | AC Watt-Hours | Inverter Loss (15%) | DC Wh Required |
|---|---|---|---|---|---|
| Energy Star Fridge | 150W (avg) | 8.0 (run time) | 1,200 Wh | +180 Wh | 1,380 Wh |
| LED Lighting (10 bulbs) | 90W | 5.0 | 450 Wh | +67 Wh | 517 Wh |
| Laptop & Router | 110W | 10.0 | 1,100 Wh | +165 Wh | 1,265 Wh |
| Microwave (Intermittent) | 1,000W | 0.25 | 250 Wh | +37 Wh | 287 Wh |
| Totals | — | — | 3,000 Wh | +449 Wh | 3,449 Wh |
Applying Chemistry Limits: Peukert vs. C-Rate
To supply 3,449 DC Wh per day, your required battery capacity depends entirely on the chemistry. According to the U.S. Department of Energy, understanding depth of discharge (DoD) and voltage sag is critical for system longevity.
- Lead-Acid (AGM/Gel): You cannot discharge below 50% DoD without destroying the plates. Furthermore, Peukert’s Law dictates that as your discharge current increases, your usable capacity drops exponentially. If you pull 100A from a 100Ah AGM battery, a Peukert exponent of 1.3 means you will only get about 40 minutes of runtime, not one hour. To get 3,449 Wh (287 Ah at 12V) with a 50% DoD and Peukert penalty, you need a massive 700Ah 12V lead-acid bank.
- Lithium Iron Phosphate (LiFePO4): LiFePO4 has a Peukert exponent near 1.0, meaning you get nearly the full rated capacity regardless of the draw rate. With a safe 80% DoD, you only need a 360Ah 12V LiFePO4 bank (e.g., three 12V 120Ah batteries in parallel) to deliver the exact same daily usable energy.
Series vs. Parallel: Configuring Voltage and Capacity
Once you know your total required Ah, you must decide how to wire the physical cells. The configuration dictates your DC system voltage, which directly determines the wire gauge (AWG) required between the battery and the inverter.
The Math: Series vs. Parallel Consequences
- Series Wiring (Voltage Adds, Ah Stays Same): Wiring four 12V 100Ah batteries in series yields 48V at 100Ah (4,800 Wh total). This is the preferred method for inverters over 2,000W. A 3,000W inverter on a 48V bank draws roughly 62.5 Amps of DC current, which can safely be carried by 2 AWG copper wire.
- Parallel Wiring (Ah Adds, Voltage Stays Same): Wiring four 12V 100Ah batteries in parallel yields 12V at 400Ah (4,800 Wh total). A 3,000W inverter on a 12V bank will attempt to pull 250 Amps of continuous DC current. This requires massive, expensive 4/0 AWG welding cable and poses severe thermal risks at the terminal lugs.
Never parallel mismatched lithium cells, different battery brands, or batteries of different ages. If one cell in a parallel bank has a higher internal resistance or a lower state of charge, the other batteries will force massive equalization currents into it during charging, potentially triggering thermal runaway and a lithium fire. If you must parallel LiFePO4 batteries, ensure they are the exact same model, purchase date, and are top-balanced to the exact same voltage before connecting. Always rely on a high-quality Battery Management System (BMS) to enforce cell-level limits, and follow NFPA 855 guidelines for energy storage system spacing.
Charge and Discharge Limits: Protecting Your DC Investment
A battery's rated capacity (e.g., 100Ah) is not a free-for-all limit. You must respect the manufacturer's C-rate specifications. The "C-rate" is a measure of the rate at which a battery is charged or discharged relative to its maximum capacity. A 1C rate means a 100Ah battery can safely deliver 100 Amps for one hour.
Discharge Limits (Inverter Sizing Check)
Most consumer LiFePO4 batteries (like the popular SOK 12V 106Ah or Redodo 12V 100Ah) feature a BMS rated for 1C discharge (100A continuous). If your 3,000W inverter pulls 250A from a single 12V 100Ah battery, the BMS will instantly trip its short-circuit/over-current protection, killing power to your AC panel. This is why high-wattage inverters require parallel batteries (to share the current load) or a higher voltage series bank (to reduce the amperage draw).
Charge Limits (Solar and Grid Charging)
Charging limits are typically stricter than discharge limits to prevent lithium plating on the anode.
- LiFePO4 Charge C-Rate: Typically limited to 0.5C. A 100Ah battery should not be charged at more than 50 Amps. If your MPPT solar charge controller is capable of outputting 80A, you must either limit it via software or add more batteries in parallel to absorb the current safely.
- Lead-Acid Charge C-Rate: Typically limited to 0.2C. A 100Ah AGM battery should not see more than 20 Amps of charge current, or the internal electrolyte will boil and vent.
- Voltage Cutoffs: Your inverter/charger must be programmed with the correct Low Voltage Disconnect (LVD) and High Voltage Disconnect (HVD). For LiFePO4, set the LVD to 11.5V (to prevent the BMS from doing the heavy lifting of low-voltage cutoff, which can sometimes leave the battery in a "sleep" state that standard solar controllers cannot wake up from).
By treating the battery strictly as a DC source and mathematically bridging the gap to your AC loads with proper efficiency and C-rate derating, you build a system that delivers reliable power without tripping breakers or degrading cells prematurely.
