Free online watt calculator to calculate electrical power (watts), voltage (volts), current (amperes), and resistance (ohms). Use formulas P = V × I, P = V²/R, and P = I²R with step-by-step solutions.
Calculate power, voltage, current, or resistance using electrical formulas
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Electrical power is one of the most fundamental concepts in physics and electrical engineering. Measured in watts (W), power represents the rate at which electrical energy is transferred or consumed. Our Watt Calculator makes it easy to calculate power, voltage, current, or resistance using the fundamental electrical formulas: P = V × I, P = V²/R, and P = I²R.
Whether you're working on electrical circuits, designing electronic devices, or solving physics problems, understanding how to calculate power is essential. Our calculator automatically selects the appropriate formula based on the values you provide, making complex electrical calculations simple and accurate.
Our Watt Calculator is designed for simplicity and accuracy. Follow these steps to calculate electrical power or related values:
The calculator supports multiple power formulas, so you can solve for any missing value regardless of which two values you know.
Electrical power can be calculated using three main formulas, each useful in different scenarios:
P = V × I
Where: P = Power (W), V = Voltage (V), I = Current (A)
This is the most commonly used formula. It directly relates power to voltage and current, making it perfect when you know both electrical potential and current flow.
P = V²/R
Where: P = Power (W), V = Voltage (V), R = Resistance (Ω)
This formula is useful when you know the voltage across a component and its resistance, but not the current. It's derived from combining P = VI with Ohm's Law (V = IR).
P = I²R
Where: P = Power (W), I = Current (A), R = Resistance (Ω)
This formula is ideal when you know the current flowing through a component and its resistance. It shows that power is proportional to the square of current, which is why high-current circuits generate more heat.
You can rearrange these formulas to solve for any variable:
Our calculator automatically applies the correct rearrangement based on which values you provide.
Electrical power calculations are essential in countless applications:
A device operates at 120 volts with a current of 2.5 amperes. What is its power consumption?
P = V × I = 120 V × 2.5 A = 300 W
A resistor with 50 ohms resistance has 10 volts across it. What power is dissipated?
P = V²/R = (10 V)² ÷ 50 Ω = 100 ÷ 50 = 2 W
A circuit element with 8 ohms resistance carries 3 amperes of current. What is the power?
P = I²R = (3 A)² × 8 Ω = 9 × 8 = 72 W
A 60-watt light bulb draws 0.5 amperes. What voltage does it require?
V = P/I = 60 W ÷ 0.5 A = 120 V
A 1500-watt heater operates at 120 volts. What current does it draw?
I = P/V = 1500 W ÷ 120 V = 12.5 A
It's crucial to understand the standard electrical units used in power calculations:
Important: Always use consistent units throughout your calculations. Our calculator uses standard SI units, so all results are in volts, amperes, ohms, and watts.
The power formulas are closely related to Ohm's Law, which states that V = I × R. By combining Ohm's Law with the basic power formula (P = V × I), we derive the alternative power formulas:
From V = I × R and P = V × I:
P = V × I = (I × R) × I = I²R
P = V × I = V × (V/R) = V²/R
This relationship shows why all three power formulas are equivalent and can be used interchangeably depending on which values you know.
Watts (W) measure power (energy per unit time), volts (V) measure voltage (electrical potential), and amperes (A) measure current (charge flow rate). Power is the product of voltage and current: P = V × I.
Yes! Use the formula P = V²/R. Our calculator automatically applies this formula when you enter voltage and resistance values.
Use the formula I = P/V. Simply divide power (in watts) by voltage (in volts) to get current in amperes. Our calculator does this automatically.
Power is related to resistance through two formulas: P = V²/R (when you know voltage) and P = I²R (when you know current). Higher resistance with the same voltage results in lower power, while higher resistance with the same current results in higher power.
This formula shows that power is proportional to the square of current. It comes from combining P = V × I with Ohm's Law (V = I × R), giving P = (I × R) × I = I²R. This is why high-current circuits generate significant heat.
Our calculator uses the standard DC (direct current) power formulas. For AC circuits, you would need to consider RMS (root mean square) values for voltage and current, but the basic formulas remain the same: P = V_rms × I_rms.
The calculator will detect inconsistencies and display an error message. For example, if you enter voltage, current, and power values that don't satisfy P = V × I, it will alert you to check your inputs.
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Understanding electrical power is fundamental to physics, electrical engineering, and everyday electronics. Our Watt Calculator simplifies these essential calculations, allowing you to quickly determine power, voltage, current, or resistance using any combination of known values.
Whether you're a student learning electrical principles, an engineer designing circuits, or a homeowner calculating energy consumption, this tool provides accurate, step-by-step solutions for all your electrical power calculations.
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