Watts Volts Amps
Last reviewed: January 2026
An electrical power calculator computes watts, volts, amps, or resistance using the relationships P = VI, P = I squared times R, and P = V squared / R. It helps electricians size circuits, engineers design systems, and students solve physics problems.
Electrical power (measured in watts) is the rate at which electrical energy is consumed or generated. The fundamental relationship P = V × I (power = voltage × current) means that a standard U.S. outlet at 120V supplying 15 amps can deliver up to 1,800 watts.[1] Three equivalent formulas — P = VI, P = I²R, and P = V²/R — let you calculate power from any combination of two known electrical quantities (voltage, current, resistance). These derive from Ohm Law (V = IR) combined with the power equation.[2] In AC circuits, real power (watts) differs from apparent power (volt-amps) by the power factor — motors and other inductive loads draw more current than the wattage suggests, which is why electrical panels are rated in amps rather than watts.[3] Use the Ohm's Law Calculator for voltage, current, and resistance calculations.
| Known Values | Formula | Example |
|---|---|---|
| Voltage & Current | P = V × I | 120V × 10A = 1,200W |
| Current & Resistance | P = I² × R | 10A² × 12Ω = 1,200W |
| Voltage & Resistance | P = V² / R | 120V² / 12Ω = 1,200W |
Power is the rate at which energy is transferred, converted, or used — measured in watts (W), where one watt equals one joule per second. Power quantifies not just how much work is done but how quickly it happens. A person climbing stairs and an elevator carrying the same person to the same floor do the same amount of work (same mass × same height × gravity), but the elevator delivers more power because it does the work faster. This distinction between energy (total work) and power (rate of work) is fundamental across physics, engineering, and everyday applications from sizing electrical circuits to choosing exercise equipment. For related calculations, see our Speed Distance Time Calculator.
| Domain | Formula | Variables | Units |
|---|---|---|---|
| Mechanical | P = W/t = F×v | Work, time, force, velocity | Watts |
| Electrical (DC) | P = V×I = I²R = V²/R | Voltage, current, resistance | Watts |
| Electrical (AC) | P = V×I×cos(φ) | +Power factor cos(φ) | Watts |
| Rotational | P = τ×ω | Torque, angular velocity | Watts |
| Thermal | P = mcΔT/t | Mass, specific heat, temp change | Watts |
| Hydraulic | P = p×Q | Pressure, flow rate | Watts |
In electrical systems, power determines wire sizing, circuit breaker ratings, heat dissipation, and energy costs. The three equivalent formulas for DC circuits — P = V × I (watts = volts × amps), P = I²R, and P = V²/R — let you calculate power when you know any two of voltage, current, and resistance. A standard US household outlet provides 120V at up to 15A, delivering a maximum of 1,800 watts per circuit. Heavy appliances (ovens, dryers, AC units) use 240V circuits at 30–50A, providing 7,200–12,000 watts. Understanding power ratings prevents circuit overloads — plugging too many devices into one circuit exceeds the breaker rating and trips the breaker, or in worst cases causes overheating and fire.
AC power adds complexity because voltage and current are sinusoidal waves that may not peak simultaneously. Real power (watts) is the actual energy consumed; apparent power (volt-amps, VA) is the product of RMS voltage and RMS current; and reactive power (volt-amps reactive, VAR) represents energy that oscillates between source and load without being consumed. The power factor (ratio of real to apparent power) indicates efficiency — a power factor of 1.0 means all delivered energy is consumed; lower values mean the system draws more current than necessary. Motors, transformers, and fluorescent lighting typically have power factors of 0.7–0.9, which is why large commercial facilities install power factor correction equipment. For home energy costs, see our Electricity Cost Calculator and Appliance Energy Cost Calculator.
Mechanical power measures the rate of physical work — how quickly a machine lifts, pushes, or rotates. The unit "horsepower" was defined by James Watt (1 HP = 745.7 watts) to help sell steam engines by comparing their output to horses. Modern applications use horsepower for engines and motors: a 200 HP car engine produces 149,140 watts of peak mechanical power. The relationship P = F × v shows that power depends on both force and speed — a tractor produces high force at low speed, while a sports car produces lower force at high speed, but both might have similar power ratings. This explains why gear ratios are essential: they trade speed for force (or vice versa) while the power remains constant (minus friction losses).
In rotational systems, power equals torque times angular velocity: P = τ × ω. Torque (rotational force) is measured in newton-meters (Nm) or foot-pounds (ft-lb), and angular velocity in radians per second (or RPM converted). A motor producing 100 Nm of torque at 3,000 RPM generates P = 100 × (3000 × 2π/60) = 31,416 watts ≈ 42 HP. Engine and motor manufacturers specify both peak power and peak torque, which typically occur at different RPM — diesel engines produce peak torque at lower RPM than gasoline engines, which is why diesels feel stronger during acceleration from low speeds. Calculate torque applications with our Torque Calculator.
Power is expressed in various units across different industries and regions. The standard SI unit is the watt (W), with kilowatts (kW = 1,000W), megawatts (MW = 1,000,000W), and gigawatts (GW = 1,000,000,000W) for larger quantities. The kilowatt-hour (kWh) is not a power unit — it is an energy unit representing one kilowatt sustained for one hour (= 3,600,000 joules). Your electricity meter measures energy in kWh, not power. Other power units include BTU per hour (1 BTU/h = 0.293W, used for heating/cooling), calories per hour (1 kcal/h = 1.163W, used in metabolism), and refrigeration tons (1 ton = 3,517W = 12,000 BTU/h, used for air conditioning). Understanding these conversions is essential when comparing equipment across industries — a 3-ton air conditioner removes 10,551 watts of heat, consuming approximately 3,000–4,000 watts of electrical power (with the difference representing the efficiency gain from the refrigeration cycle). For BTU and HVAC calculations, see our AC BTU Calculator and HVAC Load Calculator.
The human body generates power through metabolic processes, converting food energy into mechanical work and heat. A healthy adult produces approximately 75–100 watts of sustained mechanical power (cycling, walking uphill, or manual labor) and can briefly generate 300–2,000+ watts during explosive activities like sprinting or weightlifting. Elite cyclists sustain 300–400 watts for extended periods, with professional Tour de France riders producing 6–7 watts per kilogram of body weight during mountain stages. For context, a 75-watt sustained effort roughly equals powering a standard incandescent light bulb — demonstrating why human-powered generators are impractical for household electricity needs. Basal metabolic rate (the power consumed at rest to maintain body functions) is approximately 80–100 watts, with total daily energy expenditure of 100–250 watts averaged over 24 hours depending on activity level. Track your exercise output with our Calorie Calculator and One Rep Max Calculator.
Understanding power consumption helps manage electricity costs and prevent circuit overloads. A standard US household circuit provides 15 or 20 amps at 120 volts (1,800 or 2,400 watts maximum). Space heaters, hair dryers, and microwaves each draw 1,200–1,800 watts — running two on the same circuit can trip the breaker. Large appliances (dryers, ovens, AC compressors) use 240-volt circuits providing 4,800–7,200 watts. The difference between watts and watt-hours matters for billing: a 100-watt bulb running 10 hours uses 1,000 watt-hours (1 kWh). At $0.15/kWh, that costs 15 cents. Use our Electricity Cost Calculator to estimate your appliance-level energy expenses and Ohm's Law Calculator for circuit analysis.
→ Check your units. Science calculations are unit-sensitive. Make sure all inputs use the same system (SI or Imperial) to avoid conversion errors.
→ Understand the assumptions. Most science calculators use idealized models. Real-world conditions (friction, air resistance, impurities) may cause your results to differ from calculated values.
→ Use for estimation and verification. These tools are excellent for quick estimates and checking homework — but always show your work for academic submissions.
→ Explore the relationships. Change one variable at a time to see how it affects the result. This builds physical intuition about the underlying principles.
See also: Wire Gauge Calculator · Energy Converter · Torque Calculator · Ohm's Law Calculator