To convert from battery to AC power for a standard 1000W AC load on a 12V DC system, you must draw 92.6 Amps of continuous DC current, assuming a 90% efficient inverter. This means your battery bank and DC cabling must be rated for at least 115 Amps (applying the 125% NEC continuous load safety margin), requiring a minimum of 2 AWG copper wire and a 150A Class T fuse. The core formula used to find this is I_DC = P_AC / (V_DC × η). Substituting our baseline values: 92.59A = 1000W / (12V × 0.90).
The Core Conversion Formula and Neighboring Values
When sizing an inverter and battery bank, the DC current draw is the metric that dictates your wire gauge, busbar size, and fuse rating. The formula I_DC = P_AC / (V_DC × η) relies on three fixed assumptions: your nominal DC battery voltage, the continuous AC wattage of your load, and the inverter's DC-to-AC conversion efficiency (η), which typically sits between 85% and 93% for modern high-frequency units.
Below is a quick-reference table showing how the DC amp draw shifts across a ±20% range around our 1000W baseline, assuming a 12V system and 90% efficiency. This helps you account for startup surges or slight load variations.
| AC Load (Watts) | Inverter Efficiency | DC Voltage | Calculated DC Amp Draw | Required Wire Size (Cu) |
|---|---|---|---|---|
| 800W (-20%) | 90% | 12V | 74.1 A | 4 AWG |
| 900W (-10%) | 90% | 12V | 83.3 A | 3 AWG |
| 1000W (Base) | 90% | 12V | 92.6 A | 2 AWG |
| 1100W (+10%) | 90% | 12V | 101.9 A | 1 AWG |
| 1200W (+20%) | 90% | 12V | 111.1 A | 1/0 AWG |
How Voltage, Phase, and Power Factor Shift the Math
A common point of confusion is assuming that changing the AC output voltage (120V vs 230V) changes the battery drain. It does not. The DC side of the inverter only cares about total Watts. However, shifting system voltages and phases drastically alters your AC side wiring, breaker sizing, and inverter topology.
- 120V vs 230V Single-Phase: A 2000W load on a 120V inverter pulls 16.6 Amps on the AC side (assuming a Power Factor of 1.0). That same 2000W load on a 230V European/Australian inverter pulls only 8.7 Amps. The DC battery draw remains identical, but the 230V system allows for thinner AC output wiring and smaller AC breakers.
- 3-Phase Power: Running 3-phase from a battery bank requires either a specialized 3-phase inverter or three synchronized single-phase inverters. While the total DC wattage drawn from the battery remains the same, the AC current per leg drops by a factor of √3 (1.732). This is critical for industrial motor loads but entirely unnecessary for residential DIY setups.
- When the Conversion is Meaningless: Calculating AC current from AC power becomes meaningless if you do not know the load's Power Factor (PF). For resistive loads (heaters, incandescent bulbs), PF is 1.0. For inductive loads (compressors, well pumps, AC units), PF can drop to 0.6 or lower. If you try to size an AC breaker for a 1000W motor without knowing the PF, you will undersize the breaker, because the motor is actually drawing closer to 1600 VA (Volt-Amps) of apparent power. Always check the nameplate for FLA (Full Load Amps) rather than calculating from Watts for motors.
Here is how the DC amp draw shifts when you upgrade your battery bank voltage—a mandatory move for high-power systems to prevent melting your DC busbars.
| AC Load (Watts) | 12V System DC Draw | 24V System DC Draw | 48V System DC Draw | Recommended Min Inverter Size |
|---|---|---|---|---|
| 500W | 46.3 A | 23.1 A | 11.6 A | 1000W |
| 1500W | 138.9 A | 69.4 A | 34.7 A | 2000W |
| 3000W | 277.8 A | 138.9 A | 69.4 A | 4000W |
| 5000W | 463.0 A (Impractical) | 231.5 A | 115.7 A | 6000W |
Inverter Efficiency and Real-World Derating
Datasheets lie—or at least, they highlight peak efficiency rather than continuous operating reality. A Victron MultiPlus or a high-end Growatt unit might boast 93% peak efficiency, but that peak only occurs at roughly 30% to 50% of the inverter's rated load. If you run a 3000W inverter at its absolute maximum 3000W capacity, efficiency drops to 85% or lower, and internal heat generation spikes.
Furthermore, battery voltage sags under heavy DC loads. A '12V' LiFePO4 battery resting at 13.4V will drop to 12.2V or lower when pulling 150A. Because I = P / V, as voltage drops, current must increase to deliver the same AC wattage. This creates a thermal runaway loop in undersized cables. Always calculate your wire ampacity based on the inverter's Low Voltage Disconnect (LVD) threshold (usually 10.5V for lead-acid, 11.5V for LiFePO4), not the nominal 12V.
For a comprehensive breakdown of calculating backup times and accounting for these real-world losses, the methodology outlined by Electrical Technology remains a benchmark for factoring in battery aging and depth-of-discharge limits.
Frequently Asked Questions
Can I use a car battery to convert to AC power for a house?
No. Standard automotive lead-acid batteries are designed for high cranking amps (CCA) for 3 seconds, not continuous deep-cycle DC draw. Pulling 90A continuously from a car battery to run a 1000W inverter will warp the lead plates, cause severe voltage sag, and destroy the battery in a matter of minutes. You must use deep-cycle LiFePO4 or AGM batteries.
Does the inverter draw power when the AC load is turned off?
Yes. Inverters have a 'no-load draw' or quiescent current. A large 3000W inverter might pull 1.5A to 2.5A (18W-30W) from a 12V battery just to keep its internal control board and transformers energized. Over 24 hours, this parasitic drain can consume 40-60 Amp-hours. Always install an inverter remote on/off switch or use a unit with an 'eco-mode' search feature if loads are intermittent.
How do I account for motor startup surges?
