Power Calculator - Free Online Tool

Power Calculator

Calculate electrical, mechanical, or thermal power using different formulas

Power Type

Voltage (V)

Current (I)

Resistance (R) (optional)

Calculated Power

600 W

Using: P = V × I

In Watts

600 W

In Kilowatts

0.6 kW

In Horsepower

0.804 HP

Power Calculation Formulas

P = V × I

P: Power in Watts (W)

V: Voltage in Volts (V)

I: Current in Amperes (A)

Other formulas: P = V²/R = I²R (DC), P = VIcosφ (AC), P = Fv (mechanical)

People Also Ask

🤔 What's the difference between Watts, kW, and HP?

Watts (W) = base SI unit, Kilowatts (kW) = 1,000W, Horsepower (HP) = 745.7W. Use W for small appliances, kW for homes/cars, HP for engines/motors.

🔍 How to calculate power from voltage and resistance?

Use P = V²/R (if voltage known) or P = I²R (if current known). Example: 12V across 6Ω → P = 12²/6 = 144/6 = 24W.

⚡ What is power factor in AC circuits?

Power factor (cosφ) = ratio of real power to apparent power. Range: 0 to 1. Low PF = inefficient, causes higher current for same power.

📏 How much power does a typical house use?

Average US home: 1.2-1.5 kW continuous, 10-15 kW peak. Monthly: 900 kWh = 1.25 kW average. Large AC: 3-5 kW, LED bulb: 10W, TV: 100W, Oven: 2-3 kW.

🎯 How to convert mechanical to electrical power?

Multiply by efficiency (η): P_elec = η × P_mech. Typical generator: 85-95% efficient. 100 HP engine → 74.6 kW mech → ~71 kW elec at 95% efficiency.

🔥 What's the difference between power and energy?

Power = rate of energy use (Watts = Joules/sec). Energy = total consumption (kWh = power × time). Example: 100W bulb × 10h = 1 kWh energy.

What is a Power Calculator?

A Power Calculator computes the rate of energy transfer or conversion in various systems. Power, measured in Watts (W), represents how quickly work is done or energy is transferred. This calculator handles electrical power (P=VI), mechanical power (P=Fv), thermal power (P=Q/t), and AC power calculations for engineers, electricians, and physics students.

Why Calculate Power?

Power calculations are essential for sizing electrical components, designing mechanical systems, estimating energy consumption, and optimizing efficiency. Understanding power requirements prevents overloads, reduces energy costs, and ensures system reliability in everything from circuit design to industrial machinery.

Common applications of power calculations:

Electrical Design: Sizing wires, circuit breakers, transformers
Energy Management: Calculating electricity bills, solar panel sizing
Mechanical Engineering: Motor selection, gearbox design
HVAC Systems: Heating/cooling capacity calculations
Renewable Energy: Wind turbine, hydroelectric power estimation

How to Use This Calculator

Our power calculator supports five calculation modes with automatic unit conversion:

Five Calculation Types:

Electrical Power (DC): P = V × I, P = V²/R, P = I²R
Mechanical Power: P = F × v (linear), P = τ × ω (rotational)
Thermal Power: P = Q / t (heat transfer rate)
AC Power (Single Phase): P = V × I × cosφ (real power)
3-Phase Power: P = √3 × V_L × I_L × cosφ

Automatic features:

Smart formula selection: Uses appropriate formula based on inputs
Complete unit conversion: W, kW, MW, HP, BTU/hr, cal/s
Real-time calculation: Updates as you type or change units
Multiple outputs: Shows power in all common units simultaneously

Power Units Conversion Reference

Different fields use different power units. Here's a comprehensive conversion table:

Unit	Symbol	Equivalent to Watts	Common Usage	Example Values
Watt	W	1 W	SI unit, electronics, physics	LED bulb: 10W, Laptop: 65W
Kilowatt	kW	1,000 W	Homes, appliances, cars	Hair dryer: 1.5kW, House: 5kW
Horsepower	HP	745.7 W	Engines, motors, mechanical	Car engine: 150HP, Lawn mower: 5HP
BTU/hour	BTU/hr	0.2931 W	HVAC, heating systems	AC unit: 12,000 BTU/hr = 3.5kW
Ton of Refrigeration	TR	3,517 W	Air conditioning	Central AC: 3-5 TR
Calorie/second	cal/s	4.184 W	Nutrition, metabolism	Human at rest: 1.2 cal/s = 5W

Quick Conversions:

1 kW = 1.341 HP, 1 HP = 0.7457 kW, 1 W = 3.412 BTU/hr, 1 BTU/hr = 0.2931 W. Use our calculator for exact conversions.

Common Questions & Solutions

Below are answers to frequently asked questions about power calculations:

Electrical Power Calculations

How do I calculate power for AC vs DC circuits?

AC and DC power calculations differ due to phase relationships in alternating current:

AC vs DC Power Comparison:

Circuit Type	Power Formula	Notes	Example
DC	P = V × I	Simple multiplication	12V × 2A = 24W
AC Resistive	P = V_rms × I_rms	Use RMS values	120V × 5A = 600W
AC Inductive/Capacitive	P = VIcosφ	Include power factor	120V×5A×0.8=480W
3-Phase AC	P = √3VIcosφ	Industrial systems	400V×20A×0.9×√3=12.5kW

Our calculator handles all these cases - just select the appropriate power type.

What is apparent, real, and reactive power?

In AC circuits, power has three components due to phase differences:

AC Power Triangle:

Apparent Power (S): S = V × I (VA) - total power supplied
Real Power (P): P = V × I × cosφ (W) - useful work done
Reactive Power (Q): Q = V × I × sinφ (VAR) - energy stored/released
Relationship: S² = P² + Q², cosφ = P/S
Power Factor: cosφ = P/S (0 to 1, 1 is ideal)

Example: 120V, 10A, PF=0.8 → S=1200VA, P=960W, Q=720VAR. Only real power does useful work.

Mechanical & Thermal Power

How do I convert between mechanical and electrical power?

Conversion requires accounting for efficiency losses in motors and generators:

Conversion	Formula	Typical Efficiency	Example	Application
Mech → Elec (Generator)	P_elec = η × P_mech	85-95%	100 HP → 71.3 kW elec	Power plants
Elec → Mech (Motor)	P_mech = η × P_elec	75-95%	10 kW → 9.5 kW mech	Industrial motors
Thermal → Elec	P_elec = η × P_thermal	30-60%	100 MW thermal → 40 MW elec	Steam turbines
Solar → Elec	P_elec = η × P_solar	15-22%	1 kW/m² → 200 W/m²	Solar panels

Always multiply by efficiency (η) to account for energy losses as heat, friction, etc.

How to calculate power for rotating machinery?

For rotational systems, power relates torque and angular speed:

Rotational Power Formulas:

Basic formula: P = τ × ω (τ = torque in N·m, ω = angular speed in rad/s)
With RPM: P (kW) = [τ (N·m) × RPM] / 9549
With HP: P (HP) = [τ (lb·ft) × RPM] / 5252
Example: 10 N·m at 3000 RPM → P = (10 × 3000)/9549 = 3.14 kW
Conversion: 1 rad/s = 9.549 RPM, 1 RPM = 0.1047 rad/s

Use our mechanical power calculator with torque and angular speed inputs.

Practical Applications & Sizing

How to size a circuit breaker based on power?

Circuit breaker sizing requires calculating current from power and voltage:

Breaker Sizing Steps:

Calculate current: I = P / V (for DC/resistive AC)
For AC with PF: I = P / (V × cosφ)
Add safety margin: Typically 125% of calculated current
Choose standard breaker: 15A, 20A, 30A, 40A, 50A, etc.
Check wire size: Match wire gauge to breaker rating

Example: 2400W appliance at 120V → I = 2400/120 = 20A → Use 25A breaker (20A × 1.25).

How much solar power do I need for my house?

Solar system sizing involves calculating daily energy needs and peak power:

Solar System Calculation:

Daily energy: E = 30 kWh/day (typical house)
Peak sun hours: 4-6 hours/day (depends on location)
System size: P = E / (sun hours × system efficiency)
Example: 30 kWh/day ÷ (5h × 0.8) = 7.5 kW system
Number of panels: 7.5 kW ÷ 0.3 kW/panel = 25 panels
Battery storage: For 2 days autonomy: 30 kWh/day × 2 = 60 kWh

Use our calculator to determine your specific power requirements first.