10 amps at 12 volts DC equals exactly 120 watts. The foundational DC power formula is P = I × V. Substituting your exact values: 10A × 12V = 120W. This direct conversion assumes a purely resistive DC load (like a 12V heating element, LED array, or compressor) connected to a stable 12V source, where the power factor is a perfect 1.0 and phase angle is irrelevant.
The 12V DC Conversion Table and Core Formula
In bench and field work, current rarely sits at a perfect integer. If you are sizing a solar charge controller or a DC-DC converter, you need to see the neighboring values to account for startup surges or voltage sag. Below is the ±20% current range for a nominal 12V DC system.
| Current (Amps) | Nominal Voltage | Real Power (Watts) | Typical Application |
|---|---|---|---|
| 8A | 12V DC | 96W | Small 12V ventilation fans |
| 9A | 12V DC | 108W | Portable 12V fridge compressors (running) |
| 10A | 12V DC | 120W | Standard 12V LED lighting arrays |
| 11A | 12V DC | 132W | Marine water pumps |
| 12A | 12V DC | 144W | 12V portable air compressors |
How the Math Shifts for 120V, 230V, and 3-Phase AC
The 120W answer is strictly for DC (or single-phase AC with a unity power factor). When you move to mains AC or 3-phase power, the formula expands to account for phase angles and multiple legs. The single-voltage DC answer is never universal across AC systems.
Here is how a 10-amp draw translates to watts across different global power architectures, assuming a standard 0.9 power factor for inductive AC loads (like motors or transformers) and 1.0 for resistive loads.
| System Architecture | Nominal Voltage | Phase / PF | Formula Used | Real Power (Watts) |
|---|---|---|---|---|
| 12V DC Battery | 12V | N/A / 1.0 | P = V × I | 120W |
| US Residential (Resistive) | 120V | 1-Phase / 1.0 | P = V × I × PF | 1200W |
| US Residential (Inductive) | 120V | 1-Phase / 0.8 | P = V × I × PF | 960W |
| EU/UK Residential | 230V | 1-Phase / 1.0 | P = V × I × PF | 2300W |
| US Commercial HVAC | 208V | 3-Phase / 0.9 | P = √3 × V × I × PF | 3274W |
Notice the 3-phase calculation. The inclusion of the square root of 3 (≈1.732) accounts for the 120-degree phase shift between the three voltage legs. If you measure 10A per leg on a 208V 3-phase motor, you are moving over 3.2 kilowatts of real power, not the 2080W you might guess by just multiplying voltage and current.
When the Conversion is Meaningless: The Power Factor Trap
Multiplying amps by volts to get watts becomes mathematically meaningless in AC circuits when the Power Factor (PF) is unknown.
In AC systems with inductive loads (compressors, induction motors, fluorescent ballasts), the current waveform lags behind the voltage waveform. This creates a gap between Apparent Power (measured in Volt-Amps, or VA) and Real Power (measured in Watts).
- Apparent Power (VA): V × I. This is what your wiring, breakers, and inverters must physically handle. 10A at 120V is always 1200VA.
- Real Power (W): V × I × PF. This is the actual work being done (heat, light, mechanical torque). If the PF is 0.7, 1200VA becomes just 840W.
If you clamp a multimeter around the feed wire of an AC motor and read 10A, you cannot state the wattage without knowing the motor's power factor. Sizing a backup UPS based on the 1200VA apparent power is correct for the inverter's MOSFETs, but sizing your solar battery bank based on 1200W will result in an oversized, unnecessarily expensive battery array.
Frequently Asked Questions: Sizing and Inverter Math
What size wire and fuse do I need for a 12V 10A (120W) load?
According to NEC 310.16 ampacity tables, 14 AWG copper wire is rated for 15A in the 60°C column, which technically covers a 10A load. However, at 12V, voltage drop is the governing constraint. A standard 3% allowable drop on a 12V system is only 0.36V. Using the Southwire Voltage Drop Calculator, pushing 10A through 15 feet of 14 AWG wire yields a 0.38V drop (3.1%), which fails the 3% rule. For a 15-foot run, step up to 12 AWG or 10 AWG copper. For overcurrent protection, use a 15A blade fuse or breaker placed within 7 inches of the battery positive terminal, as recommended in Victron Energy's Wiring Unlimited guide.
If I run a 120W AC device from a 12V battery via an inverter, how many DC amps will it pull?
More than 10A. Inverters are not 100% efficient; typical high-frequency pure sine wave inverters operate at 85% to 92% efficiency. To supply 120W of AC power at 90% efficiency, the inverter must pull roughly 133W from the DC side. At a resting battery voltage of 12.0V, that equates to 11.1 DC amps (133W ÷ 12V). Always size your DC-side wiring and fuses for the inverter's maximum continuous DC draw, not the AC load label.
Does the 10A to 120W conversion change if the battery is cold?
The wattage consumed by a purely resistive 120W load remains 120W regardless of battery temperature. However, a cold battery suffers from increased internal resistance, causing its terminal voltage to sag under load. If the battery sags to 11.2V under a 10A load, the load will only receive 112W of power (assuming the load's resistance remains constant and it isn't a switched-mode power supply that draws more current to compensate). This is why cold-cranking amps (CCA) and voltage sag curves matter in 12V system design.
