Watt Calculator
Free online watt calculator to compute electric power in watts (W), voltage (V), current (A), and resistance (Ω) instantly.
Calculate Watts Instantly
Enter any two known values to calculate the remaining.
Enter any two known values to calculate the third:
- Voltage (V) — electrical potential difference, measured in volts
- Current (I) — electrical flow, measured in amps
- Resistance (R) — opposition to current flow, measured in ohms
- Power factor (PF) — required for AC circuits where voltage and current are out of phase
This calculator computes DC power and pure resistive single-phase AC power (P = V × I). For AC loads with power factor or three-phase systems, use our specialized Power Factor and 3-Phase calculators.
How Do You Calculate Watts?
To calculate watts, multiply voltage (V) in volts by current (I) in amperes (A) using the power equation P = V × I. One watt (W) is the electric power produced when current of one ampere flows through an electric potential difference of one volt. This watt calculator uses Watt's law to find watts from two known electrical values in any direct current (DC) or alternating current (AC) circuit.
Where:
- P = power in watts (W)
- V = voltage in volts (V)
- I = current in amperes (A)
Scenario: Voltage = 120 V, Current = 5 A
P = 120 × 5
P = 600 W
The electric power in this circuit is 600 watts.
Watt Calculator Formula
The watt calculator formula has 3 variations to find watts from different known values. All 3 formulas derive from combining Watt's law (P = V × I) with Ohm's law (V = I × R). Each formula requires two known electrical values to calculate power in watts.
Multiply voltage by current. A 120 V circuit with 10 A produces:P = 120 × 10 = 1,200 W
Square the voltage, then divide by resistance. A 240 V circuit with 48 Ω:P = 240² ÷ 48 = 1,200 W
Square the current, then multiply by resistance. A circuit with 5 A and 48 Ω:P = 5² × 48 = 1,200 W
| Quantity | Symbol | SI Unit | Unit Symbol |
|---|---|---|---|
| Power | P |
watt | W |
| Voltage | V |
volt | V |
| Current | I |
ampere | A |
| Resistance | R |
ohm | Ω |
Ohm's Law: Volts, Amps, and Ohms
Ohm's law states that voltage (V) equals current (I) multiplied by resistance (R): V = I × R. Georg Ohm established this relationship in 1827 for electrical circuits. One volt (V) is the electric potential that drives one ampere (A) of current through one ohm (Ω) of resistance in a conductor.
Ohm's law has 3 rearrangements:
Combining Ohm's law with Watt's law produces the full set of 12 formulas for power, voltage, current, and resistance in electrical circuits.
Power, Voltage, Resistance, Current
Power (P), voltage (V), resistance (R), and current (I) are 4 measurable quantities in electrical circuits. Each quantity has a specific SI unit and connects to the others through Ohm's law and Watt's law.
Power (P)
Unit: watt (W)
Power is the rate of electrical energy transfer per unit of time. One watt equals one joule (J) per second. Power measures how fast a device converts electrical energy into work, heat, or light.
P = V × I
P = V² ÷ R
P = I² × R
Voltage (V)
Unit: volt (V)
Voltage is the electric potential difference between two points in a circuit. One volt drives one ampere of current through one ohm of resistance. Voltage is the "pressure" that pushes current through a conductor.
V = P ÷ I
V = I × R
V = √(P × R)
Resistance (R)
Unit: ohm (Ω)
Resistance is the opposition to current flow in a circuit. One ohm is the resistance that allows one ampere to flow when one volt is applied. A resistor limits the flow of electric current through a conductor.
R = V ÷ I
R = V² ÷ P
R = P ÷ I²
Current (I)
Unit: ampere (A)
Current is the flow of electric charge through a conductor. One ampere equals one coulomb of charge per second. Current flows from high electric potential to low electric potential in a circuit.
I = P ÷ V
I = V ÷ R
I = √(P ÷ R)
Watt Calculator For Room
A watt calculator for room estimates total power consumption by summing the wattage of each electrical appliance in the room. To calculate room wattage, list all appliances, note each power rating in watts, and add the running watts together.
A standard bedroom with 3 LED bulbs (10 W each), one ceiling fan (75 W), one laptop (65 W), and one phone charger (5 W) consumes 175 watts total. Add a television (150 W) and the room total becomes 325 watts. Use this interactive room watt calculator to estimate power consumption for any room configuration.
Watt Calculation of Electrical Appliances
Watt calculation of electrical appliances uses the power rating printed on the nameplate or calculated from voltage and current measurements using P = V × I. There are 8 common appliance categories with typical power ranges:
To calculate watts for any appliance, read the nameplate voltage and current values, then multiply: P = V × I. A 1500 watt heater operating at 120 V draws 12.5 amps of current (1,500 ÷ 120 = 12.5 A). These power ratings are approximate and vary by model, size, and operating conditions — check the manufacturer's nameplate for exact figures.
Watt Calculator With Volts And Amps
A watt calculator with volts and amps applies the formula P = V × I to convert voltage and current into power. To find watts from volts and amps, multiply the two values. To calculate amps from watts and volts, divide power by voltage: I = P ÷ V.
| Voltage | Current | Power |
|---|---|---|
| 12 V | 1 A | 12 W |
| 12 V | 5 A | 60 W |
| 120 V | 5 A | 600 W |
| 120 V | 10 A | 1,200 W |
| 120 V | 15 A | 1,800 W |
| 240 V | 10 A | 2,400 W |
| 240 V | 20 A | 4,800 W |
To calculate amps from watts and volts, divide: I = P ÷ V. A 1,000 W appliance at 120 V draws 8.33 A of current (1,000 ÷ 120 = 8.33). For AC circuits, the actual current can be higher when the power factor is less than 1, because apparent power (VA) exceeds real power (W).
Examples of Conversion Between Volts, Amps, Watts, and Ohms
These 5 worked examples show how to convert between volts, amps, watts, and ohms using the watt calculator formulas.
Scenario: A 1,500 W appliance operating at 120 V — find the current in amps.
I = 1,500 ÷ 120
I = 12.5 A
The appliance draws 12.5 amperes of current.
Scenario: A light bulb operates at 120 V and draws 0.5 A — find the power in watts.
P = 120 × 0.5
P = 60 W
The light bulb consumes 60 watts of electric power.
Scenario: A resistor in a 120 V circuit has 24 Ω of resistance — find the power in watts.
P = 120² ÷ 24
P = 14,400 ÷ 24
P = 600 W
The resistor dissipates 600 watts of power.
Scenario: A 12 V, 50 Ah battery (12 volts, 50 amp-hours) — find the stored energy in watt-hours (Wh).
Wh = 12 × 50
Wh = 600 Wh
The battery stores 600 watt-hours (0.6 kilowatt-hours) of electrical energy.
Scenario: A 1500 watt heater — find the heat output in British thermal units per hour (BTU/h).
BTU/h = 1,500 × 3.412
BTU/h = 5,118
The 1500 watt heater produces 5,118 BTU/h of thermal output.
| Conversion | Formula |
|---|---|
| Watts from volts and amps | P = V × I |
| Watts from voltage and resistance | P = V² ÷ R |
| Watts from current and resistance | P = I² × R |
| Amps from watts and volts | I = P ÷ V |
| Volts from watts and amps | V = P ÷ I |
| Resistance from voltage and power | R = V² ÷ P |
| Watt-hours from watts and time | Wh = W × h |
| Kilowatt-hours | kWh = W × h ÷ 1,000
|
| Watts to BTU/h | BTU/h = W × 3.412 |
Specialized Watt Calculators
Every tool features its own custom-engineered calculator, dynamic real-time outputs, formula derivations, and deep-dive technical references.
3-Phase Watt Calculator
Calculate active power (W), apparent power (VA), reactive power (VAR), and line current in 3-phase Wye and Delta AC systems.
Watts to kVA & VA Calculator
Convert real power in watts (W) to apparent power in kilovolt-amperes (kVA) and volt-amperes (VA) using power factor.
Power Factor Calculator
Calculate power factor, reactive power (VAR), apparent power (VA), and phase angle from real power and any one additional known value.
Wire Gauge Wattage Calculator
Find the correct AWG wire gauge for your wattage, voltage, and wire run distance. Calculates voltage drop and suggests conductor sizes based on standard ampacity tables.
Electricity Cost Calculator
Calculate exactly how much any device or appliance costs to run per hour, day, month, and year based on wattage and your electricity rate.
Appliance Wattage Calculator
Add all your home appliances to calculate total household power consumption in watts. Includes preset wattages for common devices.
Heater Wattage Calculator
Calculate the ideal heater wattage for any room based on square footage, ceiling height, insulation quality, and climate zone.
LED Watt to Lumens Calculator
Convert incandescent, halogen, or CFL bulb wattage to LED equivalent. See exact lumens output, energy savings, and annual cost reduction.
Watt to Ton Converter
Convert between watts, tons of refrigeration, and BTU/hr for air conditioning sizing. Factor in SEER/EER ratings for actual electrical consumption.
Solar Panel Wattage Calculator
Calculate the total solar panel wattage needed to meet your daily energy consumption based on location, peak sun hours, and system efficiency.
Battery Watt-Hour Calculator
Calculate battery energy in watt-hours (Wh), convert between Ah and Wh, and estimate runtime under any load with depth of discharge.
Generator Wattage Calculator
Calculate total running and starting (surge) wattage to determine the right generator size for your home, jobsite, or emergency backup.
EV Charging Watt Calculator
Calculate EV charging power (kW), charging time, miles of range added per hour, and electricity cost for Level 1, Level 2, and DC fast charging.
PC Power Supply Calculator
Calculate total PC power consumption from individual components (CPU TDP, GPU TGP, RAM, storage, fans) to find the ideal PSU wattage for your build.
Motor Power Calculator
Calculate motor power from torque and RPM, convert between horsepower and watts, and find electrical input power using motor efficiency.
Speaker & Amplifier Watt Calculator
Calculate the amplifier wattage needed to achieve your target sound pressure level (dB SPL) based on speaker sensitivity and listening distance.
Watts to dBm Calculator
Convert between watts, milliwatts, and dBm (decibel-milliwatts) for RF, telecom, and audio power level calculations.
Rowing Watt to Split Calculator
Convert between rowing ergometer watts and pace (split time per 500m) using the official Concept2 formula. Includes training pace zones and comparison tables.
Cycling FTP & Power Zone Calculator
Calculate your Functional Threshold Power (FTP), power-to-weight ratio (W/kg), and all 7 Coggan power training zones from a 20-minute FTP test.
FAQs
What does a Watt measure?
A watt (W) measures electric power, which is the rate of electrical energy transfer per unit of time. One watt equals one joule per second (1 W = 1 J/s). In electrical circuits, one watt is the power produced when current of one ampere flows through a voltage of one volt. The watt is the SI unit of power, named after James Watt.
How many volts are in a Watt?
Watts and volts measure different electrical properties. Power in watts is the product of voltage and current: P = V × I. Therefore, at a current flow of 1 ampere, 1 volt produces 1 watt of power. To convert watts to volts, divide power by current: V = P ÷ I. For example, a 60 W device drawing 0.5 A operates at 120 V (60 ÷ 0.5 = 120 V).
How many kilowatts are in a Watt?
One watt equals 0.001 kilowatts (kW). To convert watts to kilowatts, divide by 1,000. For example, 1,500 W = 1.5 kW, and 2,000 W = 2.0 kW. One kilowatt equals 1,000 watts.
How many BTUs are in a Watt?
One watt equals approximately 3.412 BTU/h (British thermal units per hour). To convert watts to BTU/h, multiply the power in watts by 3.412. For example, 100 W = 341.2 BTU/h, and 500 W = 1,706 BTU/h. The British thermal unit (BTU) measures thermal energy, and BTU/h measures thermal power output.
How many BTUs are in a 1500 Watt heater?
A 1500 watt heater produces approximately 5,118 BTU/h (1,500 × 3.412 = 5,118). This calculation converts electrical power in watts to thermal output in British thermal units per hour. A 1500 watt heater at 120 V draws 12.5 amps of current (1,500 ÷ 120 = 12.5 A).
How do I calculate the Watt hours of a battery?
To calculate watt-hours (Wh) of a battery, multiply the battery's voltage (V) by its amp-hour (Ah) rating: Wh = V × Ah. A 12 V, 50 Ah battery (12 volts, 50 amp-hours) stores 600 watt-hours (12 × 50 = 600 Wh). A 24 V battery with 100 Ah stores 2,400 Wh (2.4 kWh). Battery energy capacity (Wh) is different from instantaneous power (W) — watt-hours measure stored electrical energy, and watts measure the rate of energy transfer.
What Is 1 Watt Equal To?
One watt is equal to one joule per second (1 W = 1 J/s), one volt times one ampere (1 W = 1 V × 1 A), and approximately 3.412 BTU/h. The watt is the SI unit of power. One watt is the rate of energy transfer when current of one ampere flows through an electric potential of one volt. One watt also equals 0.001 kilowatts (kW) and approximately 0.00134 mechanical horsepower.
How Much Is 5 Watts?
5 watts is 0.005 kilowatts (kW) and approximately 17.06 BTU/h. 5 watts is a low-power load typical of a USB phone charger, a small LED bulb, or a standby indicator light. A 5 W device running for 10 hours consumes 50 watt-hours (Wh) of electrical energy.
How Much is 7.5 Watts?
7.5 watts is 0.0075 kilowatts (kW) and approximately 25.59 BTU/h. 7.5 watts is typical for a small LED night light, a low-power wireless charger, or a mini desk fan. A 7.5 W device running for 24 hours consumes 180 Wh (0.18 kWh) of electrical energy.
What is 2.5 Watts?
2.5 watts is 0.0025 kilowatts (kW) and approximately 8.53 BTU/h. 2.5 watts is a very low power level, typical of a standby LED indicator, a small sensor, or a USB-powered device in low-power mode. A 2.5 W device running for 24 hours uses 60 Wh (0.06 kWh).
What is 4 Watts Equivalent to?
4 watts is equivalent to 0.004 kilowatts (kW) and approximately 13.65 BTU/h. 4 watts powers a small LED candelabra bulb, an E-reader, or a low-power Bluetooth speaker. 4 watts equals 4 joules of energy per second.
What is 8 Watts Equivalent to?
8 watts is equivalent to 0.008 kilowatts (kW) and approximately 27.30 BTU/h. 8 watts is typical for an LED light bulb that replaces a traditional 60 W incandescent bulb (due to higher efficiency), a tablet charger, or a small powered speaker. 8 watts equals 8 joules of energy per second.
Is 9 Watts a Lot?
No, 9 watts is not a lot of power. 9 watts is 0.009 kilowatts (kW) and approximately 30.71 BTU/h. A 9 W LED bulb produces about 800 lumens of light, equivalent to a 60 W incandescent bulb. A 9 W device running for 24 hours consumes 216 Wh (0.216 kWh), which costs less than $0.03 at $0.12/kWh electricity rates.
Which Watts is Better?
"Better" watts depends on the application and efficiency requirements. Lower wattage is better for energy savings — a 9 W LED produces the same 800 lumens as a 60 W incandescent bulb while using 85% less power. Higher wattage is necessary for heavy loads, including space heaters (1,500 W), electric ovens (2,000–5,000 W), and industrial motors. The best wattage matches the power requirement of the load without excessive energy waste.
How do you convert watts to kilowatts?
To convert watts to kilowatts, divide the wattage by 1,000: kW = W ÷ 1,000. For example: 500 W = 0.5 kW, 1,500 W = 1.5 kW, 2,500 W = 2.5 kW, and 10,000 W = 10 kW. One kilowatt equals 1,000 watts.
What is a watt-hour (Wh)?
A watt-hour (Wh) is a unit of electrical energy equal to one watt of power sustained for one hour. Wh = W × hours. A 100 W device running for 5 hours uses 500 Wh (0.5 kWh). Watts measure power (rate of energy use), and watt-hours measure total energy consumed. One kilowatt-hour (kWh) = 1,000 Wh.
How do you calculate electricity consumption from watts?
To calculate electricity consumption from watts, multiply the device wattage by hours of use, then divide by 1,000 to get kilowatt-hours (kWh): kWh = (W × hours) ÷ 1,000. A 200 W television running for 5 hours per day uses 1 kWh daily (200 × 5 ÷ 1,000 = 1). Multiply daily kWh by your electricity rate to find the cost.
How many watts does a typical appliance use?
Typical appliance wattage ranges from 5 W to 5,000 W, depending on the appliance type. There are 6 common ranges: LED bulbs use 5–15 W, laptops use 30–100 W, televisions use 50–200 W, microwaves use 600–1,500 W, space heaters use 1,000–1,500 W, and air conditioners use 900–5,000 W. Check the nameplate on each appliance for the exact power rating in watts.
How much electricity does a 100-watt appliance use?
A 100-watt appliance uses 100 watt-hours (Wh) per hour of operation, which equals 0.1 kilowatt-hours (kWh). Running a 100 W appliance for 10 hours consumes 1 kWh. At an average electricity rate of $0.12 per kWh, running a 100 W device for 10 hours costs $0.12.
How much electricity does 1,000 watts use?
A 1,000 W appliance operating continuously for 1 hour consumes 1 kilowatt-hour (kWh) of electricity. Running a 1,000 W device for 8 hours consumes 8 kWh. At an average electricity rate of $0.12 per kWh, running 1,000 watts for one hour costs $0.12. Running the same 1,000 W load continuously for 24 hours costs $2.88 (24 × $0.12 = $2.88).
Calculation Methodology & Standards Compliance
✓ Verified for Technical Accuracy (Sept 2026)Calculations on Watt Calculator adhere strictly to standard electrical engineering formulations, including Watt's Law (P = V × I) and Ohm's Law (V = I × R), computed with 64-bit IEEE 754 floating-point mathematical precision.
Strict Joule/Watt equivalence with zero phase displacement.
Apparent power (VA) scaled by Power Factor per IEEE Std 1459.
Balanced Wye and Delta formulations per IEC 60038 and IEEE 100.
Responsible Use Disclaimer: This calculator serves as an educational and engineering estimation resource. Complex real-world installations, conductor sizing, and electrical panels should always be reviewed and installed by a licensed electrician in compliance with your regional electrical code (NEC NFPA 70, CE Code, or IEC).