The fundamental formula for electrical power is Watts = Amps × Volts (for DC circuits) or Watts = Amps × Volts × Power Factor (for AC circuits). While the math is simple, looking up exact values under pressure on the jobsite or at the workbench wastes time. Below is a comprehensive amp to watt conversion chart covering the most common residential, RV, and solar DC voltages.
The Master Amp to Watt Conversion Chart
Bookmark Quick-Jumps for Common Queries:
- 15 Amps at 120V (Standard US Outlet): 1,800W (Resistive) / 1,440W (Inductive)
- 20 Amps at 240V (Dryer/Welder Circuit): 4,800W (Resistive) / 3,840W (Inductive)
- 30 Amps at 12V (RV/Solar DC): 360W
- 50 Amps at 120V (Shore Power/Subpanel): 6,000W (Resistive) / 4,800W (Inductive)
| Amps (I) | 12V DC (W) | 120V AC @ 1.0 PF (W) | 120V AC @ 0.8 PF (W) | 240V AC @ 1.0 PF (W) | 240V AC @ 0.8 PF (W) |
|---|---|---|---|---|---|
| 1 | 12 | 120 | 96 | 240 | 192 |
| 5 | 60 | 600 | 480 | 1200 | 960 |
| 10 | 120 | 1200 | 960 | 2400 | 1920 |
| 15 | 180 | 1800 | 1440 | 3600 | 2880 |
| 20 | 240 | 2400 | 1920 | 4800 | 3840 |
| 30 | 360 | 3600 | 2880 | 7200 | 5760 |
| 40 | 480 | 4800 | 3840 | 9600 | 7680 |
| 50 | 600 | 6000 | 4800 | 12000 | 9600 |
How to Read This Table
The columns are split by system voltage and Power Factor (PF). The 12V DC column applies to automotive, marine, and off-grid solar battery banks where power factor does not exist. The 1.0 PF columns apply to purely resistive AC loads like space heaters, incandescent lighting, and standard toaster ovens. The 0.8 PF columns apply to inductive AC loads with electric motors, compressors, or magnetic ballasts (like refrigerators, AC units, and workshop machinery). If you are sizing a generator or UPS for inductive loads, you must use the 0.8 PF column, or the equipment will stall or trip the breaker despite the "wattage" appearing to match.
Applying NEC Derating to Your Wattage Calculations
A raw conversion chart assumes ideal, non-continuous conditions. In real-world electrical installations, you must apply derating factors that modify the base usable wattage of a circuit. There are two primary derating scenarios that dictate how much actual wattage you can safely pull through a given amperage limit.
If a load is expected to run for 3 hours or more (e.g., baseboard heaters, EV chargers, commercial lighting), the National Electrical Code requires the circuit to be derated to 80% of its breaker rating. Therefore, a 20A breaker at 120V yields a theoretical 2,400W, but your continuous usable wattage is strictly limited to 1,920W.
Furthermore, wire bundling derating (NEC 310.15) modifies the base ampacity before you even calculate watts. If you pull four current-carrying THHN conductors through a single conduit, the ampacity of a 12 AWG wire drops from 25A (90°C column) to 20A (80% adjustment factor). If that conduit runs to a 240V circuit, your maximum theoretical wattage drops from 6,000W down to 4,800W, and your continuous allowable wattage drops to 3,840W. Always calculate the derated ampacity first, then use the chart above to find your true wattage ceiling.
| Breaker Size | Theoretical Max Watts | Continuous Max Watts (80%) | Typical Application |
|---|---|---|---|
| 15 Amp | 1,800W | 1,440W | Bedroom outlets, lighting circuits |
| 20 Amp | 2,400W | 1,920W | Kitchen small appliances, bathroom GFCI |
| 30 Amp | 3,600W | 2,880W | RV receptacles, small window AC units |
| 50 Amp | 6,000W | 4,800W | EV Level 2 chargers, subpanel feeders |
What This Chart Cannot Tell You
While an amp to watt conversion chart is essential for load balancing and basic sizing, it omits three critical variables that will cause a circuit to fail if ignored.
1. Voltage Drop Over Distance
Watts are calculated using nominal voltage. If you run 12 AWG wire 150 feet to a 120V, 15A space heater (1,800W), the resistance of the wire will cause the voltage at the receptacle to drop to roughly 112V. The heater will now draw more current to compensate for the lower voltage (if it's a constant-power device) or produce less heat (if it's a simple resistive coil). The chart assumes you are measuring voltage at the load, not at the breaker panel.
2. Inverter Efficiency Losses (DC to AC)
If you are using the 12V DC column to size battery cables for an AC inverter, you must account for conversion inefficiency. A typical pure sine wave inverter operates at 85% to 92% efficiency. To supply 1,200W of AC power (10A at 120V), the inverter will actually pull roughly 115A to 125A from the 12V battery bank, not the 100A listed in the DC column. Always multiply your DC amp requirement by 1.15 when sizing battery-to-inverter cabling.
3. Wire Ampacity and Insulation Ratings
This chart tells you how much power a circuit delivers, but it does not tell you what size wire to use. Wire sizing is based strictly on amps and the temperature rating of the terminal lugs (usually 60°C or 75°C per NEC 110.14(C)), completely independent of the system voltage. A 10 AWG copper wire is rated for 30A whether it is carrying 12V DC (360W) or 240V AC (7,200W). Never use wattage to select wire gauge; always convert back to amps and consult NEC Table 310.16.
