Amps to Watts: Complete Conversion Guide
Amps to Watts Formulas
The conversion depends on circuit type and power factor:
- DC: P (W) = V (V) × I (A)
- Single-phase AC: P (W) = V (V) × I (A) × PF
- Three-phase AC: P (W) = VLL (V) × I (A) × PF × √3
Quick Conversion at Common Voltages
| Amps | 120V (DC/PF=1) | 120V (PF=0.8) | 240V (DC/PF=1) | 240V (PF=0.8) |
|---|---|---|---|---|
| 1A | 120W | 96W | 240W | 192W |
| 5A | 600W | 480W | 1,200W | 960W |
| 10A | 1,200W | 960W | 2,400W | 1,920W |
| 15A | 1,800W | 1,440W | 3,600W | 2,880W |
| 20A | 2,400W | 1,920W | 4,800W | 3,840W |
| 30A | 3,600W | 2,880W | 7,200W | 5,760W |
| 50A | 6,000W | 4,800W | 12,000W | 9,600W |
| 100A | 12,000W | 9,600W | 24,000W | 19,200W |
| 200A | 24,000W | 19,200W | 48,000W | 38,400W |
Why Power Factor Matters
For AC circuits, power factor (PF) accounts for the phase difference between voltage and current. Resistive loads (heaters, bulbs) have PF = 1.0. Inductive loads (motors, transformers) have PF = 0.6–0.9.
Without PF: You calculate VA (apparent power). With PF: You calculate watts (real power).
Example: A motor drawing 10A at 120V with PF 0.8: P = 120 × 10 × 0.8 = 960W. Not 1,200W.
Common Appliance Current Draw
| Appliance | Voltage | Amps | Watts | Circuit |
|---|---|---|---|---|
| LED bulb (100W equiv) | 120V | 0.09A | 10W | Any |
| Laptop charger | 120V | 0.5A | 60W | Any |
| Window AC (10,000 BTU) | 120V | 10A | 1,200W | 20A dedicated |
| Microwave | 120V | 12A | 1,440W | 20A dedicated |
| Space heater | 120V | 12.5A | 1,500W | 20A dedicated |
| Hair dryer | 120V | 12.5A | 1,500W | 20A |
| Electric dryer | 240V | 25A | 6,000W | 30A dedicated |
| Electric range | 240V | 40A | 9,600W | 40-50A dedicated |
| EV charger (Level 2) | 240V | 32A | 7,680W | 40A dedicated |
| Central AC (3-ton) | 240V | 15A | 3,600W | 30A dedicated |
Common Mistakes
- Ignoring power factor: Motors draw more amps than their wattage suggests. A 960W motor at 120V, PF 0.8 draws 10A, not 8A.
- Confusing VA with watts: VA = V × I (apparent). Watts = V × I × PF (real). For sizing wires and breakers, use amps (from VA).
- Using 240V for 120V loads: Check your equipment nameplate. A 1,500W heater at 240V draws 6.25A; at 120V it draws 12.5A.
Standards Reference
- NEC Article 220 — Branch Circuit Calculations
- IEEE 1459 — Power Definitions
- IEC 60038 — Standard Voltages
Frequently Asked Questions
How many watts is 15 amps?
At 120V: 15A × 120V = 1,800W (DC or PF=1). At 240V: 15A × 240V = 3,600W. For AC with power factor 0.8 at 120V: 15 × 120 × 0.8 = 1,440W. Always specify voltage and power factor when converting amps to watts.
How many amps does 1000 watts draw?
At 120V: 1,000W / 120V = 8.33A. At 240V: 1,000W / 240V = 4.17A. For AC motors with PF 0.8 at 120V: 1,000 / (120 × 0.8) = 10.4A. The lower the voltage, the more amps for the same wattage.
What's the formula for amps to watts?
DC: Watts = Volts × Amps. Single-phase AC: Watts = Volts × Amps × Power Factor. Three-phase AC: Watts = Volts × Amps × PF × √3. Power factor is 1.0 for resistive loads and 0.6–0.9 for motors and inductive loads.
Can I convert amps to watts without voltage?
No. Amps (current) and watts (power) are related through voltage: P = V × I. Without knowing the voltage, you cannot determine power. A 10A load at 12V is 120W, but at 120V it's 1,200W — a 10× difference.
How many watts can a 20-amp circuit handle?
At 120V: 20A × 120V = 2,400W maximum. For continuous loads (>3 hours): 2,400 × 0.80 = 1,920W (NEC 210.20). At 240V: 20A × 240V = 4,800W, or 3,840W continuous.