Watt Calculator

Calculate electrical power, current, voltage, resistance, and energy using Ohm's Law and power formulas.

Convert between different electrical parameters using fundamental electrical formulas. Perfect for electronics, electrical engineering, and energy calculations.

Examples

Click on any example to load it into the calculator.

Household Light Bulb

Household

A typical LED light bulb consuming 9W at 120V household voltage.

Voltage: 120 V

Power: 9 W

Time: 4 h

Laptop Charger

Electronics

A laptop charger delivering 19.5V at 3.33A for charging.

Voltage: 19.5 V

Current: 3.33 A

Time: 2 h

Electric Heater

Heating

A 1500W electric space heater running on 120V household power.

Voltage: 120 V

Power: 1500 W

Time: 3 h

Car Battery Charger

Automotive

A 12V car battery with 0.5A charging current.

Voltage: 12 V

Current: 0.5 A

Time: 8 h

Other Titles
Understanding the Watt Calculator: A Comprehensive Guide
Master electrical power calculations with our comprehensive guide to watts, voltage, current, and energy. Learn the fundamental principles of electrical engineering and how to apply them in real-world scenarios.

What is the Watt Calculator?

  • Core Concepts
  • Why Electrical Power Matters
  • Fundamental Formulas
The Watt Calculator is an essential tool for anyone working with electrical systems, from hobbyists to professional engineers. It provides instant calculations for electrical power, current, voltage, resistance, and energy consumption using fundamental electrical formulas. Whether you're designing circuits, calculating energy bills, or troubleshooting electrical problems, this calculator helps you understand the relationships between these key electrical parameters.
The Fundamental Relationship: Ohm's Law
At the heart of electrical calculations lies Ohm's Law: V = I × R, where V is voltage (volts), I is current (amperes), and R is resistance (ohms). This fundamental relationship governs how electrical circuits behave and forms the basis for all power calculations. Understanding this relationship is crucial for designing safe and efficient electrical systems.
Power: The Rate of Energy Transfer
Electrical power (P) is the rate at which electrical energy is transferred or consumed. It's measured in watts (W) and is calculated using the formula P = V × I. This means that power equals voltage multiplied by current. For example, a device operating at 120V with 2A of current consumes 240W of power.
Energy: Power Over Time
While power measures the rate of energy transfer, energy (E) measures the total amount of electrical energy consumed over time. Energy is calculated as E = P × t, where t is time in hours. This gives us energy in watt-hours (Wh) or kilowatt-hours (kWh), which is what utility companies use for billing.

Key Electrical Units Explained:

  • Volt (V): Unit of electrical potential difference - the 'pressure' driving current
  • Ampere (A): Unit of electrical current - the flow rate of electrical charge
  • Ohm (Ω): Unit of electrical resistance - opposition to current flow
  • Watt (W): Unit of electrical power - rate of energy transfer
  • Watt-hour (Wh): Unit of electrical energy - total energy consumed over time

Step-by-Step Guide to Using the Calculator

  • Gathering Your Data
  • Inputting Values
  • Interpreting Results
Using the Watt Calculator is straightforward, but understanding what values to input and how to interpret the results is crucial for accurate calculations. Follow these steps to get the most reliable results.
1. Determine What You Know
Start by identifying which electrical parameters you already know. You need at least two values to calculate the others. Common scenarios include: knowing voltage and current to find power, knowing power and voltage to find current, or knowing voltage and resistance to find current and power.
2. Input Your Known Values
Enter your known values in the appropriate fields. Leave unknown fields empty. The calculator will automatically determine which calculations are possible based on your inputs. For example, if you enter voltage and current, it will calculate power, resistance, and energy consumption.
3. Add Time for Energy Calculations
If you want to calculate energy consumption, enter the time duration in hours. This is particularly useful for estimating electricity costs or understanding long-term energy usage patterns.
4. Analyze and Apply the Results
Review all calculated values to ensure they make sense. Check that the calculated resistance is reasonable for your application, and verify that the power consumption aligns with typical values for similar devices.

Common Calculation Scenarios:

  • Voltage + Current → Power, Resistance, Energy
  • Voltage + Power → Current, Resistance, Energy
  • Current + Power → Voltage, Resistance, Energy
  • Voltage + Resistance → Current, Power, Energy
  • Power + Resistance → Voltage, Current, Energy

Real-World Applications and Practical Uses

  • Home Electronics
  • Industrial Applications
  • Energy Efficiency
The Watt Calculator has countless practical applications in everyday life and professional settings. Understanding electrical power calculations helps you make informed decisions about energy consumption, equipment selection, and system design.
Home Energy Management
Use the calculator to understand your home's energy consumption. Calculate the power draw of appliances, estimate monthly electricity costs, and identify energy-efficient alternatives. For example, compare a 100W incandescent bulb (0.1 kW) running 4 hours daily (1.2 kWh/day) with a 15W LED bulb (0.06 kWh/day) to see the energy savings.
Electronics Design and Troubleshooting
Electronics engineers and hobbyists use power calculations to design circuits, select appropriate components, and troubleshoot problems. Calculate if a power supply can handle the load, determine if components will overheat, and ensure circuits operate within safe parameters.
Renewable Energy Systems
Solar panel and wind turbine installations require careful power calculations. Determine the power output of renewable energy systems, calculate battery storage requirements, and estimate system efficiency. This is crucial for designing off-grid systems and optimizing renewable energy investments.

Typical Power Consumption Examples:

  • LED Light Bulb: 9-15W
  • Laptop Computer: 45-65W
  • Refrigerator: 150-400W
  • Microwave Oven: 600-1200W
  • Electric Water Heater: 3000-4500W
  • Central Air Conditioner: 3500-5000W

Common Misconceptions and Electrical Myths

  • Power vs. Energy
  • Voltage vs. Current
  • Efficiency Myths
Electrical concepts can be confusing, and many common misconceptions exist about power, voltage, current, and energy. Understanding these differences is crucial for safe and effective electrical work.
Myth: Higher Voltage Always Means More Power
This is not always true. Power depends on both voltage and current (P = V × I). A high voltage with very low current might result in less power than a lower voltage with higher current. For example, 12V at 10A (120W) delivers more power than 120V at 0.5A (60W).
Myth: Current is What Kills, Not Voltage
While current is what causes physiological effects, voltage is what drives current through the body. Ohm's Law shows that for a given body resistance, higher voltage means higher current. Both voltage and current are dangerous - respect all electrical hazards.
Myth: Power and Energy are the Same Thing
Power (watts) is the rate of energy transfer, while energy (watt-hours) is the total amount transferred over time. A 100W device running for 2 hours consumes 200Wh of energy. Understanding this difference is crucial for energy billing and efficiency calculations.

Safety Guidelines:

  • Always turn off power before working on electrical circuits
  • Use appropriate personal protective equipment
  • Never exceed component ratings or circuit capacities
  • Consult qualified electricians for complex installations
  • Regularly inspect electrical equipment for damage or wear

Mathematical Derivation and Advanced Concepts

  • Power Formula Derivation
  • Efficiency Calculations
  • Three-Phase Power
Understanding the mathematical foundations of electrical power calculations helps you apply these concepts more effectively and troubleshoot complex problems.
Deriving the Power Formula
The power formula P = V × I comes from the definition of power as the rate of work done. In electrical terms, work is done when charge moves through a potential difference. Since current is charge per unit time (I = Q/t) and voltage is work per unit charge (V = W/Q), multiplying them gives us power: P = V × I = (W/Q) × (Q/t) = W/t.
Alternative Power Formulas
Using Ohm's Law (V = I × R), we can derive alternative power formulas: P = V²/R and P = I² × R. These are useful when you know voltage and resistance, or current and resistance, respectively. For example, if you have a 12V circuit with 6Ω resistance, power = 12²/6 = 24W.
Efficiency and Power Factor
Real-world electrical systems have efficiency losses. Efficiency = (Output Power / Input Power) × 100%. Additionally, AC circuits have a power factor (cos φ) that affects apparent vs. real power. For purely resistive loads, power factor = 1, but inductive or capacitive loads have power factors less than 1.

Advanced Calculation Examples:

  • Efficiency: 90% efficient motor consuming 1000W input = 900W output
  • Power Factor: 0.8 power factor with 1000VA apparent power = 800W real power
  • Three-Phase: √3 × V × I × cos φ for balanced three-phase systems
  • Energy Cost: 15¢/kWh × 2.4 kWh = 36¢ for running a 1200W device for 2 hours