Watts to Amps 12V Conversion: Formula, Chart, and Solar Battery Sizing

To convert watts to amps at 12V, divide the wattage by 12. For a standard 100-watt load on a 12-volt DC battery system, the current draw is exactly 8.33 amps (100W ÷ 12V = 8.33A). This baseline calculation assumes a pure DC circuit, a power factor (PF) of 1.0, and a true 12.0V terminal voltage at the battery posts.

The 12V DC Conversion Formula and Core Assumptions

The fundamental relationship between power, voltage, and current in a DC circuit is defined by Watt's Law. The formula to find current (Amps) when you know power (Watts) and voltage (Volts) is:

While the math is simple, the assumptions behind it dictate whether your wire sizing and breaker selection will actually work on the bench. This calculation fixes the answer based on three strict assumptions:

1. Voltage is exactly 12.0V: In reality, a '12V' battery is rarely at 12.0V. A fully charged lead-acid battery rests at 12.6V, while a 12V LiFePO4 lithium battery (like a Victron or Ampere Time model) rests around 13.4V. Under a heavy load, a lead-acid battery might sag to 11.5V, which forces the amperage higher to deliver the same wattage.
2. Power Factor (PF) is 1.0: In pure DC circuits, power factor is always 1.0. All the power supplied by the battery is consumed as real work (watts). There is no reactive power bouncing back and forth.
3. Single-Phase DC: There is no phase angle or alternating waveform to account for, meaning RMS calculations are unnecessary.

For a deeper dive into the physics of DC power calculations, All About Circuits provides an excellent breakdown of how Watt's Law applies to resistive DC loads.

12V Watt-to-Amp Reference Chart (±20% Range)

When sizing fuses, breakers, and wire for 12V solar or battery banks, you rarely deal with a single static number. Below is a reference chart centered around a 100W baseline, showing a ±20% range. I have included the theoretical draw at exactly 12.0V, alongside the real-world draw at 11.5V (representing voltage sag in a depleted lead-acid battery), and the minimum recommended copper wire gauge (AWG) for short runs under 5 feet to prevent excessive voltage drop.

Note: Wire sizing assumes THHN/THWN-2 copper in a 30°C ambient environment. For longer runs (over 10 feet), you must calculate voltage drop and typically step up two AWG sizes.

How the Math Shifts: 120V, 230V, and 3-Phase AC

The 12V DC formula falls apart the moment you introduce alternating current (AC). If you are sizing the AC output side of an inverter or wiring a household branch circuit, the voltage and phase geometry drastically change the amperage.

• 120V AC (Single-Phase): At 120V, a 100W load draws just 0.83A (100 ÷ 120). This is why high-voltage AC is used for power transmission; higher voltage slashes current, allowing for thinner, cheaper wires.
• 230V AC (Single-Phase): Common in Europe and the UK, a 100W load at 230V draws only 0.43A (100 ÷ 230).
• 3-Phase AC: Industrial 3-phase power introduces the square root of 3 (≈1.732) into the denominator. The formula becomes I = P / (√3 × V × PF). For a 100W load on a 208V 3-phase system with a PF of 1.0, the draw is just 0.28A.

When the Conversion Becomes Meaningless

Converting watts to amps using simple division is meaningless in AC circuits when the Power Factor (PF) is unknown. If you are wiring an AC motor or a compressor, the load is inductive. A motor rated for 100W of real power (Watts) might have a PF of 0.6. The actual apparent power (VA) is 166 VA, meaning the wiring and breakers must be sized for 1.38A at 120V, not the 0.83A the raw wattage suggests. Sizing wire purely on the wattage rating of an inductive load is a common DIY mistake that leads to tripped breakers and overheated terminals. For more on measuring this in the field, see Fluke's guide on Power Factor.

Real-World 12V Inverter Losses and Voltage Sag

If you are running a 120V AC appliance off a 12V DC battery bank via an inverter, you cannot use the basic formula without accounting for inverter inefficiency. Inverters generate heat and consume power to run their own internal cooling fans and logic boards.

A typical high-frequency 12V inverter operates at about 88% to 92% efficiency under moderate loads. If you plug a 100W laptop charger into the inverter's AC outlet, the inverter must pull more than 100W from the battery to deliver it.

Furthermore, if that 9.26A draw causes your lead-acid battery terminals to sag to 11.5V due to internal resistance (Peukert's effect), the actual amperage spikes to 9.66A (111.1W ÷ 11.5V). Always size your 12V DC fuses and wiring based on the inverter's maximum continuous DC input rating, not just the AC output wattage.