Watts to Amps Calculator

About the Watts to Amps Calculator

A watts to amps calculator converts power in watts to electrical current in amperes at a specified voltage — the essential reverse of the amps to watts conversion. This calculation is critical any time you know how much power a device consumes and need to determine what circuit, wiring, or breaker is required to support it safely. The relationship between watts, amps, and volts is fundamental to every electrical installation, and getting it wrong can mean tripped breakers, overheated wires, or serious safety hazards. Consider the most common scenarios where this conversion matters. A homeowner wants to install a 5,500-watt tankless electric water heater: how large does the circuit breaker need to be? An electrician is adding a 240V outlet for a new electric dryer rated at 4,800 watts: what wire gauge is required? A solar installer is sizing the cable between a 3,000-watt inverter and the battery bank at 48V DC: what current will the cable carry? In each case, converting watts to amps is the essential first step before any sizing decision can be made. The calculation differs depending on whether the circuit is DC, single-phase AC, or three-phase AC. For DC and resistive single-phase AC loads, the formula is straightforward: amps equal watts divided by volts. For three-phase AC, the square root of 3 (approximately 1.732) appears in the denominator because three-phase power is distributed across three conductors. Power factor — the ratio of real power to apparent power — must also be considered for inductive loads such as motors, compressors, and transformers, which draw more current than a purely resistive load of the same wattage. In the United States and Canada, the National Electrical Code (NEC) adds an important sizing rule for continuous loads — any load that operates for three or more consecutive hours. For these loads, the circuit breaker and wiring must be sized at 125% of the calculated current draw. A continuously running 1,920-watt load at 120V draws 16 amps, requiring a 20-amp breaker and 12 AWG wire. Without the 125% rule applied, a 15-amp circuit might appear adequate — but would be a code violation and a potential fire hazard for a device that runs all day. The calculator is used by a broad range of people: electricians sizing new circuits for kitchens, garages, and workshops; EV charger installers determining the dedicated circuit requirements for Level 2 charging stations; RV and boat owners designing 12V and 24V electrical systems where lower voltages mean much higher currents for the same power; solar system designers sizing charge controllers, battery cables, and inverter connections; and homeowners planning generator backup systems who need to know what amperage loads their generator must handle. International contexts matter here too. UK homes operate at 230V single-phase while US homes use 120/240V split-phase. A 2,300-watt electric kettle in the UK draws 10 amps at 230V — the same kettle would need to be designed differently for US voltage. Australian 240V systems follow similar principles to the UK. Understanding these differences prevents dangerous mistakes when working with imported equipment or relocating appliances between countries.

How It Works

The core formulas are: I = P divided by V for DC and single-phase AC, and I = P divided by (V times the square root of 3 times power factor) for three-phase AC. Power factor ranges from 1.0 for purely resistive loads down to 0.7 or lower for heavily inductive loads. Practical examples illustrate the range of applications. A 2,400-watt toaster oven at 120V draws 2,400 divided by 120 equals 20 amps — requiring a dedicated 20-amp circuit at minimum, or a 25-amp circuit if the load is considered continuous. A 4,500-watt electric water heater at 240V draws 4,500 divided by 240 equals 18.75 amps, rounding up to a 20-amp circuit with 10 AWG copper wire. A Level 2 EV charger rated at 11,520 watts at 240V draws 48 amps continuously, requiring a 60-amp circuit (48 times 1.25 equals 60 amps per NEC continuous load rule) and 6 AWG copper conductors. For three-phase systems: a 30 kW industrial motor at 480V three-phase with a power factor of 0.90 draws 30,000 divided by (480 times 1.732 times 0.90) equals 30,000 divided by 748 equals 40.1 amps. The circuit breaker would be sized at 40.1 times 1.25 equals 50.1 amps, rounded to a standard 50-amp breaker, with conductors rated for at least 40.1 amps continuous.

Tips & Best Practices

• ✓Apply the NEC 125% continuous load rule for any load that operates three or more consecutive hours. A water heater, EV charger, computer server, or refrigerator is a continuous load. Multiply the calculated amps by 1.25 before selecting a breaker size. This single rule prevents more overheated wiring than almost any other code requirement.
• ✓Standard US circuit breaker sizes run at 15, 20, 25, 30, 40, 50, 60, 70, 80, 100, and 125 amps. Always select the next standard size above your calculated requirement — never round down. A 21-amp load requires a 25-amp breaker, not a 20-amp one.
• ✓Wire gauge follows amperage directly from NEC Table 310.15: 15A uses 14 AWG, 20A uses 12 AWG, 30A uses 10 AWG, 40A uses 8 AWG, 60A uses 6 AWG, and 100A uses 2 AWG for copper conductors. Temperature rating of the insulation also affects ampacity — 90 degree C rated THHN allows higher currents than 60 degree C rated wire of the same gauge.
• ✓EV Level 2 charger sizing: a 48-amp EVSE (the most common residential high-speed charger) is a continuous load. Circuit must be rated at 48 times 1.25 equals 60 amps minimum. Wire with 6 AWG copper at minimum. The 240V outlet must be a NEMA 14-50 or hardwired connection rated for 50-60 amps.
• ✓Low-voltage DC systems carry much higher current than equivalent 120V or 240V AC circuits. A 1,200-watt inverter at 12V DC draws 1,200 divided by 12 equals 100 amps. Battery cables for 12V systems must be very large — often 2 AWG or larger — to handle the high current safely without excessive voltage drop and heat.
• ✓Power factor correction matters for motor loads. A 5 kW motor at 240V with a power factor of 0.80 draws 5,000 divided by (240 times 0.80) equals 26 amps, not 5,000 divided by 240 equals 20.8 amps. Wiring must handle the actual 26 amps, not the lower resistive equivalent. Failing to account for power factor undersizes the circuit for real-world operation.
• ✓Three-phase calculations use line-to-line voltage in the denominator — not line-to-neutral. A common mistake is using 120V (line-to-neutral) in a 208V three-phase formula, which gives a result that is 1.73 times too high. Always confirm which voltage measurement applies to the formula you are using.
• ✓Inverter sizing for off-grid solar: add up all AC loads in watts, convert to amps at your AC voltage, then account for inverter efficiency (typically 85 to 95 percent). DC battery current equals AC watts divided by inverter efficiency divided by battery voltage. A 3,000-watt AC load at 90% efficiency from a 48V battery draws 3,000 divided by 0.90 divided by 48 equals 69.4 amps DC — requiring battery cables rated for at least 70 amps continuously.

Who Uses This Calculator

Licensed electricians use this calculation constantly when sizing branch circuits and service entrances for new construction and renovation projects. EV charger installers rely on it to specify the correct dedicated circuit for Level 1, Level 2, and DC fast charger installations. RV and marine electricians design complete 12V and 24V DC electrical systems, where the high currents resulting from low-voltage operation demand careful attention to wire gauge and connection quality. Solar designers size all the DC components of photovoltaic systems — strings, combiners, charge controllers, and battery cables — based on the currents derived from system power levels and operating voltages. IT managers planning data center power distribution calculate how many amps each rack draw imposes on its power circuit. Homeowners planning whole-home backup generators need to know the total amperage of their loads to select an appropriately sized transfer switch.