Watt Calculator
Calculate electrical power (watts), voltage (volts), current (amps), and resistance (ohms) using Ohm's Law. Enter any two values to find the others.
Electrical Power Calculator
Enter any 2 values to calculate the others
Enter at least 2 values to calculate the others
Ohm's Law & Power Formulas
Ohm's Law
V = I × R
I = V ÷ R
R = V ÷ I
Power Equations
P = V × I
P = I² × R
P = V² ÷ R
Variable Definitions
V
Voltage (Volts)
I
Current (Amps)
R
Resistance (Ohms)
P
Power (Watts)
What is a Watt?
A watt (W) is the SI unit of power, named after Scottish inventor James Watt. It measures the rate at which energy is transferred or converted. One watt is defined as one joule per second (J/s), representing the rate of energy flow in an electrical circuit.
In electrical terms, one watt equals the power produced when one ampere of current flows through a circuit with one volt of potential difference:
Watts are used to measure power consumption in everyday appliances. For example:
- A typical LED bulb uses 9-12 watts
- A laptop charger typically uses 45-65 watts
- A microwave oven uses 600-1200 watts
- An electric heater uses 1000-1500 watts
- A hair dryer uses 1000-2000 watts
Understanding Ohm's Law
Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage, current, and resistance in an electrical circuit. Discovered by German physicist Georg Simon Ohm in 1827, it states:
Voltage equals Current times Resistance
The Three Forms of Ohm's Law
Find Voltage
V = I × R
Multiply current by resistance
Find Current
I = V ÷ R
Divide voltage by resistance
Find Resistance
R = V ÷ I
Divide voltage by current
Example Calculation
A circuit has a 12V battery and a resistance of 4Ω. What is the current?
I = V ÷ R
I = 12V ÷ 4Ω
I = 3 Amperes
Electrical Power Formulas
Electrical power can be calculated using several formulas, depending on which values you know. These are derived from Ohm's Law combined with the basic power equation.
The Power Triangle
P = V × I
Power = Voltage × Current
Use when you know V and I
P = I² × R
Power = Current² × Resistance
Use when you know I and R
P = V² ÷ R
Power = Voltage² ÷ Resistance
Use when you know V and R
Solved Examples
Example 1: Find Power from Voltage and Current
A 120V appliance draws 2A of current. What is the power consumption?
P = V × I
P = 120V × 2A
P = 240 Watts
Example 2: Find Power from Current and Resistance
A circuit has 3A of current flowing through a 10Ω resistor. What power is dissipated?
P = I² × R
P = 3² × 10
P = 9 × 10
P = 90 Watts
Example 3: Find Power from Voltage and Resistance
A 24V source is connected to a 6Ω resistor. What is the power?
P = V² ÷ R
P = 24² ÷ 6
P = 576 ÷ 6
P = 96 Watts
Power Unit Conversions
| Unit | Symbol | Equivalent in Watts | Scientific Notation |
|---|---|---|---|
| Picowatt | pW | 0.000000000001 W | 10⁻¹² W |
| Nanowatt | nW | 0.000000001 W | 10⁻⁹ W |
| Microwatt | μW | 0.000001 W | 10⁻⁶ W |
| Milliwatt | mW | 0.001 W | 10⁻³ W |
| Watt | W | 1 W | 10⁰ W |
| Kilowatt | kW | 1,000 W | 10³ W |
| Megawatt | MW | 1,000,000 W | 10⁶ W |
| Gigawatt | GW | 1,000,000,000 W | 10⁹ W |
| Terawatt | TW | 1,000,000,000,000 W | 10¹² W |
| Electric Horsepower | hp(E) | 746 W | 7.46 × 10² W |
Practical Applications
Home Electrical
Calculate power consumption of appliances to estimate electricity bills. A 100W bulb running for 10 hours uses 1 kWh (kilowatt-hour).
Energy (kWh) = Power (kW) × Time (hours)
Battery Systems
Calculate watt-hours for battery capacity. A 12V battery with 100Ah capacity stores 1,200 Wh of energy.
Wh = Voltage × Amp-hours
Electronics Design
Size power supplies and calculate heat dissipation in circuits. A resistor dissipating more than its rated power will overheat.
Power rating must exceed calculated P
Solar & Renewable
Size solar panels and inverters based on power requirements. A 300W panel in 5 hours of sunlight produces 1,500 Wh daily.
Daily Energy = Panel Watts × Sun Hours
Watts vs Watt-Hours: Understanding Energy
Watts (W) measure instantaneous power—the rate at which energy is used at any moment. Watt-hours (Wh) measure total energy consumed over time.
The Relationship
Energy (Wh) = Power (W) × Time (hours)
Examples
- 60W light bulb for 5 hours:
60W × 5h = 300 Wh = 0.3 kWh - 1500W heater for 2 hours:
1500W × 2h = 3000 Wh = 3 kWh - Smartphone (5W) charging for 2 hours:
5W × 2h = 10 Wh = 0.01 kWh
Electricity Cost Calculation
To calculate electricity cost:
Cost = kWh consumed × Price per kWh
Example: Running a 1000W appliance for 5 hours at $0.12/kWh costs: 1kW × 5h × $0.12 = $0.60
Frequently Asked Questions
How do I calculate watts from volts and amps?
Multiply voltage by current: P = V × I. For example, a 120V device drawing 2 amps uses 120 × 2 = 240 watts.
What is the difference between watts and volts?
Voltage (volts) is the electrical pressure or potential difference that pushes current through a circuit. Power (watts) is the rate at which energy is consumed or produced. Voltage is like water pressure; watts are like the flow rate.
How many watts is 1 amp at 120 volts?
Using P = V × I: 120V × 1A = 120 watts. Similarly, at 240V, 1 amp equals 240 watts.
How do I convert watts to amps?
Divide power by voltage: I = P ÷ V. A 1200W device at 120V draws 1200 ÷ 120 = 10 amps.
What is 1 kilowatt equal to?
1 kilowatt (kW) = 1,000 watts. It's commonly used to measure larger power consumption like appliances and HVAC systems.
How do I calculate power in a resistor?
Use any of these formulas: P = V × I, P = I² × R, or P = V² ÷ R. Choose based on which values you know.
What is a gigawatt?
1 gigawatt (GW) = 1 billion watts (10⁹ W). It's used to measure power plant output. For reference, the famous "1.21 gigawatts" from Back to the Future equals 1,210,000,000 watts.
How does resistance affect power?
At constant voltage, lower resistance means higher power (P = V²/R). At constant current, higher resistance means higher power (P = I²R). This is why proper resistor sizing is critical in electronics.
What is electric horsepower?
Electric horsepower (hp(E)) = 746 watts. It's used to rate electric motors. A 1 hp motor consumes approximately 746 watts of electrical power.
How do I size a power supply?
Calculate total power needed by summing all component requirements, then add 20-30% headroom. For example, if components need 400W, choose at least a 500W power supply for safe operation.
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