If your specific query is how to convert 120 volts and 15 amps to watts, the direct answer is 1,800 watts. The foundational formula used here is W = V × A, which substitutes as 120V × 15A = 1,800W. This assumes a direct current (DC) circuit or an alternating current (AC) circuit with a purely resistive load (Power Factor = 1.0). However, treating this single calculation as a universal rule will lead to undersized breakers, tripped GFCIs, and melted terminal lugs when you step into the real world of inductive loads and multi-phase panels.
Reference Tables: Volts and Amps to Watts
Before we break down the physics, here are the quick-reference conversion tables. The first table shows how a standard 15A load behaves across a ±20% voltage variance, which is critical for sizing wire on long feeder runs where voltage drop occurs. The second table maps real-world bench and household appliances to their actual wattage draws.
Neighboring Values: 15A Constant Across ±20% Voltage Variance
| Voltage (V) | Current (A) | Calculated Watts (W) | Context / Scenario |
|---|---|---|---|
| 96V (-20%) | 15A | 1,440W | Severe voltage drop on long 120V branch circuit |
| 108V (-10%) | 15A | 1,620W | Brownout condition or heavy grid sag |
| 120V (Nominal) | 15A | 1,800W | Standard US 120V receptacle at max continuous load |
| 132V (+10%) | 15A | 1,980W | High-line voltage condition (transformer tap high) |
| 144V (+20%) | 15A | 2,160W | Dangerous overvoltage; risk of equipment failure |
Real-World Appliance Power Draws (Factoring in Power Factor)
| Equipment | Voltage | Measured Amps | Power Factor (PF) | Real Power (Watts) |
|---|---|---|---|---|
| 1500W Space Heater | 120V | 12.5A | 1.00 (Resistive) | 1,500W |
| 1HP Air Compressor | 120V | 10.0A | 0.85 (Inductive) | 1,020W |
| EV Level 2 Charger | 240V | 32.0A | 0.98 (Corrected) | 7,526W |
| Industrial 5HP Motor | 208V (3-Phase) | 15.0A | 0.88 (Inductive) | 4,738W |
The Assumptions That Fix (or Break) Your Calculation
The W = V × A formula is absolute for DC circuits. According to Georgia State University's HyperPhysics, electrical power in a DC system is strictly the product of potential difference (volts) and charge flow rate (amps). But in AC systems, three assumptions fix your answer: voltage stability, phase count, and power factor (PF).
For purely resistive loads like incandescent bulbs, toasters, and electric baseboard heaters, the voltage and current waveforms peak at the exact same time. The PF is 1.0, and Watts = VA. For inductive loads (motors, transformers, compressors) or capacitive loads (large LED driver banks), the current waveform lags or leads the voltage. You must multiply by the PF decimal to find the true wattage.
System Shifts: 120V vs 230V vs 3-Phase Power
The math shifts dramatically when you move from a standard US 120V branch circuit to European 230V single-phase, or up to commercial 3-phase power. In a 3-phase system, power is delivered across three overlapping sine waves, meaning you must introduce the square root of 3 (approximately 1.732) into your formula.
| System Type | Formula | Example (at 20A) | Resulting Watts (Assuming PF=1.0) |
|---|---|---|---|
| 120V Single-Phase (US) | W = V × A | 120 × 20 | 2,400W |
| 230V Single-Phase (EU/UK) | W = V × A | 230 × 20 | 4,600W |
| 208V 3-Phase (US Commercial) | W = V × A × √3 | 208 × 20 × 1.732 | 7,205W |
| 480V 3-Phase (US Industrial) | W = V × A × √3 | 480 × 20 × 1.732 | 16,627W |
Notice how a 20A draw on a 480V 3-phase system yields nearly seven times the wattage of a 120V single-phase system. This is why industrial facilities use high-voltage 3-phase power: it delivers massive wattage while keeping the amperage (and therefore the required copper wire gauge and conduit size) manageable. For a deeper breakdown of the √3 derivation, Electronics Tutorials provides excellent phasor diagrams showing how the 120-degree phase offsets create this multiplier.
Quick Conversion FAQ
How many watts is 5 volts and 2 amps?
Using the DC formula (W = V × A), 5V × 2A equals exactly 10 watts. This is the standard output for a basic USB-A phone charger (5V/2A). If you are using a USB-C PD charger that steps up to 20V at 5A, the wattage jumps to 100W, even though the connector is the same.
Can I convert watts back to amps if I only know the voltage?
Yes. The rearranged formula is A = W / V. For example, if you have a 1,500W microwave running on a 120V circuit, the current draw is 1,500 / 120 = 12.5 amps. This tells you immediately that you cannot run this microwave and a 10A toaster on the same 15A breaker without tripping it (12.5A + 10A = 22.5A).
Why does my 12V RV fridge say it draws 10 amps, but the inverter says it pulls 80 watts?
Inverter inefficiency and DC-to-AC conversion losses. A 12V compressor drawing 10A theoretically uses 120W (12 × 10). However, if the compressor's duty cycle averages out, or if the 80W figure represents the AC side output of a specific subsystem, the numbers will diverge. Always measure with a DC clamp meter on the battery side of the BMS to get the true wattage draw of 12V/24V RV and marine systems.
