Watt

AC wattage calculator

Enter any two of voltage, current, power, or resistance and the calculator returns the rest. Supports DC, single-phase AC (with power factor), and three-phase AC. Useful for converting nameplate values into circuit current, sizing wire, and verifying meter readings.

The watt formulas

Power expressed in watts is the product of voltage and current. Ohm\'s Law substitution gives three equivalent volt ohm watt forms; the right one to use depends on which two of voltage, current, and resistance you have measured.

A 1 kW heater running for 1 hour uses 1 kWh = 3.6 MJ of energy. Utility bills measure energy in kWh, never in watts — confusion between power (instantaneous) and energy (cumulative) is the #1 source of household electricity-bill puzzlement.

The watt in the SI system

The watt has been an SI base-derived unit since the SI was first formalised at the 11th General Conference on Weights and Measures (CGPM) in 1960. It was adopted into the metric system at the 14th CGPM in 1971 to honour James Watt. Since 2019 (the SI redefinition), the watt is defined entirely from fundamental constants:

• The kilogram is fixed by the Planck constant h = 6.626 070 15 × 10⁻³⁴ J·s.
• The second is fixed by the caesium-133 hyperfine transition: 9 192 631 770 Hz.
• The metre is fixed by the speed of light: 299 792 458 m/s.
• The ampere is fixed by the elementary charge e = 1.602 176 634 × 10⁻¹⁹ C.

From these definitions, 1 watt = 1 kg·m²/s³ = 1 J/s = 1 V·A. Standards bodies that maintain the watt definition: BIPM (Bureau International des Poids et Mesures) at the international level; NIST (USA), NPL (UK), PTB (Germany), BIPM-CCEM at national-laboratory level. Calibration of high-precision wattmeters traces through Kibble (watt) balances — the same instruments that realise the kilogram from h.

Reference: typical wattages of common loads

Approximate steady-state power draw of household and commercial loads. Use for energy-budget estimates; check the nameplate for the actual wattage of any specific appliance.

How to compute watts from your measurements

1. Identify what you have measured. For DC or for resistive AC loads: voltage (V) and current (A). For AC with a reactive load: voltage, current, and the power factor (cos φ). For three-phase: line-to-line voltage, line current, and PF. For appliances: nameplate kW or HP.
2. Apply the right formula for the system. DC or resistive AC: P = V × I. Reactive single-phase AC: P = V × I × cos φ. Three-phase AC: P = √3 × V_LL × I × cos φ. Mechanical from rotational: P = T × ω. Solar / battery: P = V × I (always DC).
3. Convert kilowatts to watts and back. 1 kW = 1 000 W. A 1.5 kW heater consumes 1 500 W. Always include units explicitly — a "1.5 power" claim is meaningless without the W or kW.
4. Verify against the appliance nameplate. Most appliances list rated power in W or kW directly. For motors, the nameplate gives mechanical HP — multiply by 745.7 to get electrical input watts (or by 746 / efficiency for the actual electrical draw). The wattmeter on a clamp meter or smart plug returns true RMS power that already includes power-factor correction.
5. Sum across loads and over time. Adding watts across loads gives the total instantaneous load on the supply. Multiplying watts by hours gives watt-hours (Wh) — the energy consumed, what your utility bill measures in kWh. A 100 W light bulb on for 10 hours uses 1 000 Wh = 1 kWh.
6. Cross-check with the calculator. The embedded power calculator below accepts any two of voltage, current, power, and resistance, and returns the rest — useful for sanity checks against the nameplate.

Worked example: kettle and EV charger

Kettle: pure resistive load, 230 V, draws 9.5 A. EV charger: continuous load, 240 V single-phase, 40 A at PF 1.0 (modern PFC chargers).

The kettle pulls 2.2 kW briefly; the EV charger pulls 9.6 kW for hours. Same 240 V circuit, very different sizing requirements. The kettle plugs into a standard 13 A outlet (UK) or 15 A 120 V (US split-phase, would need higher voltage); the EV charger needs its own 50 A branch with a continuous-duty conductor.

Watt vs kilowatt vs kilowatt-hour

Three units that get mixed up daily, especially on utility bills.

Bill-shock checklist: your utility charges for energy (kWh), not power (kW). A 1 kW heater that runs continuously for a 30-day month uses 1 × 24 × 30 = 720 kWh. At UK average 2024 rate £0.27/kWh that\'s about £195 for one heater for one month — illustrates why heat is the dominant winter electricity cost.

Edge case: volt-amperes (VA) is not the same as watts

For DC and pure-resistive AC loads, V × A = W. For reactive AC loads (motors, transformers, switching power supplies without PFC), the two diverge: volt-amperes (VA) is apparent power; watts is real power; the ratio is the power factor: PF = W / VA.

A small uninterruptible power supply (UPS) might be rated 1 500 VA but only 900 W. The VA rating sets the sustained current the UPS can deliver; the W rating sets the actual load it can supply continuously. Always size by both — the limit that bites first depends on the connected load\'s power factor.

Three-phase apparent power uses kVA (kilovolt-amperes) for the same reason. A 100 kVA transformer delivers 100 kW only if the load PF is 1.0 (rare in industrial settings). At PF 0.85, the same transformer delivers only 85 kW of real work, even though it\'s carrying its full 100 kVA of apparent power. Power-factor correction (capacitor banks) shifts the ratio back toward unity and recovers usable kW.

The watt in the SI 9th Edition

The watt, symbol W, is the SI derived unit of power. It is defined as one joule per second (1 W = 1 J·s⁻¹), which is equivalent to one kilogram metre squared per second cubed (1 kg·m²·s⁻³) and to one volt-ampere (1 V·A). The unit was named in honour of the Scottish engineer James Watt (1736–1819) and was adopted into the SI by the 14th General Conference on Weights and Measures in 1971.

Related concepts on this site

Frequently asked questions

What is watt in physics?

In physics, the watt is the SI unit of power — the rate at which energy is transferred or work is done. One watt equals one joule of energy per second (1 W = 1 J/s). Power can be mechanical, electrical, thermal, or radiant; in every case the unit is the watt. Named in honour of James Watt, the Scottish inventor whose improvements to the steam engine in the 1770s made the Industrial Revolution possible.

What is the difference between a watt and a kilowatt?

A kilowatt (kW) is one thousand watts (1 000 W). The two units are identical in dimension — both measure power — and differ only by the SI multiplier prefix "kilo" (k = 10³). A 1 500 W kettle is the same as a 1.5 kW kettle. Engineering convention uses kW for industrial / utility scale (motors, heaters, building loads) and W for small electronics (LEDs, sensors, USB chargers). Energy is multiplication: a 1 kW load over 1 hour consumes 1 kWh of energy.

What is a watt in physics?

A watt is one joule of energy per second. In mechanical terms, lifting a 1 kg mass 0.102 m in one second against gravity (9.81 m/s²) requires 1 W. In electrical terms, pushing 1 A of current through a 1 V potential difference dissipates 1 W. The unit is named for James Watt, who quantified the rate of work done by his steam engines.

What is the watt unit equal to in SI base units?

The watt unit equals 1 kg·m²/s³ in SI base units (mass × distance squared / time cubed). Equivalently: 1 W = 1 J/s = 1 V·A = 1 N·m/s. The 2019 SI redefinition fixed the watt to the new exact values of Planck's constant h, the elementary charge e, and the Avogadro constant. Before 2019 the watt depended on the prototype kilogram in Sèvres; today it is defined entirely from atomic and quantum constants.

How do I convert watts to kilowatts (watt to kw)?

Divide by 1 000. 1 500 W ÷ 1 000 = 1.5 kW. Going the other way: 2.4 kW × 1 000 = 2 400 W. The same factor relates kWh ↔ Wh for energy. Larger SI multipliers: 1 MW (megawatt) = 1 000 kW = 10⁶ W; 1 GW (gigawatt) = 10⁹ W (utility power-station scale).

What is the relationship between volt, ohm, and watt?

These three units interlock through Ohm's Law: V = I × R. Substituting into the power equation P = V × I gives three equivalent forms: P = V²/R = I²·R = V·I. So if you know any two of {V, I, R, P}, you can compute the others — that is exactly what the AC wattage calculator embedded above does. Example: a 12 V battery driving 5 A through a load dissipates 60 W; the load resistance is 12 / 5 = 2.4 Ω.

How do I use an AC wattage calculator?

Enter the AC voltage (typically 120 V or 230 V single-phase, 400 V or 480 V three-phase), the current, and the power factor (1.0 for resistive, 0.85 for typical motor, 0.95 for modern PFC electronics). The calculator returns real power in watts (or kW). For three-phase systems, multiply by √3. The embedded power calculator on this page handles DC, single-phase, and three-phase in one form.

How many watts does a typical household appliance use?

LED bulb 5–15 W; laptop 30–60 W; refrigerator 100–400 W (cycling); microwave 1 000–1 500 W; clothes dryer 3 000–5 000 W; central air-conditioning 2 000–5 000 W per ton; electric tankless water heater 18 000–28 000 W (18–28 kW). See the reference table below for a fuller list.

Sources and further reading

1. BIPM. The International System of Units (SI), 9th Edition, 2019. Official definition of the watt and other SI units.
2. NIST. NIST Special Publication 330 — The International System of Units (SI), 2019.
3. IEEE. IEEE Std 100 — The Authoritative Dictionary of IEEE Standards Terms, 7th Edition.
4. NFPA. National Electrical Code (NEC) NFPA 70, 2023 Edition. Articles 220 (loads), 625 (EV charging).
5. Williams, R. James Watt: Master of Steam, Phoenix, 2009. Biography of James Watt and the steam engine\'s impact on units of power.
6. Halliday, D., Resnick, R., Walker, J. Fundamentals of Physics, 11th Edition. Wiley, 2018. Power in mechanical and electrical systems.
7. UK Department for Energy Security and Net Zero. Quarterly Energy Prices, latest issue. Average residential electricity prices in £/kWh.