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Power Calculator – P = W/t for Physics & Engineering

Engineering Physics calculator physics power watt work
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Power Calculator

Calculate power, work, or time using P = W / t

P = W ÷ t

Power = Work ÷ Time  |  Physics & Engineering

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Variable 1
Power (P)
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Variable 2
Work / Energy (W)
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Variable 3
Time (t)
RESULT
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Enter two values above to calculate the third
Quick Scenarios
Common Power References
💡 LED Light Bulb5 – 15 W
📱 Smartphone Charger5 – 25 W
💻 Laptop30 – 100 W
🍳 Microwave Oven600 – 1,200 W
🚗 Compact Car Engine80 – 150 kW
🏠 Household (US avg.)~1.2 kW continuous
🏋️ Human (sustained)~75 – 100 W
✈️ Boeing 747 (cruise)~180 MW
Frequently Asked Questions

The fundamental formula is P = W / t, where P is power (measured in watts, W), W is work or energy transferred (measured in joules, J), and t is the time taken (measured in seconds, s). This equation tells you how fast energy is being used or work is being done. Power is the rate of energy transfer.

The SI unit of power is the watt (W), named after James Watt. One watt equals one joule per second (1 W = 1 J/s). For larger quantities, kilowatts (kW = 1,000 W) and megawatts (MW = 1,000,000 W) are commonly used. In mechanical contexts, horsepower (hp) is also widely used—1 mechanical horsepower ≈ 745.7 watts.

Rearrange the formula: W = P × t. Multiply the power (in watts) by the time (in seconds) to get the work or energy transferred (in joules). For example, a 60W light bulb running for 1 hour (3,600 seconds) consumes W = 60 × 3,600 = 216,000 J (or 216 kJ, or 0.06 kWh) of energy.

Energy (or work) is the total amount of work done or heat transferred—measured in joules (J), kilowatt-hours (kWh), or calories. Power is the rate at which energy is transferred or work is done—measured in watts (W). Think of it like a car: energy is the total distance traveled, while power is the speed at any given moment. A 100W bulb uses energy at a rate of 100 joules per second.

1 mechanical horsepower (hp) = 745.699872 watts (approximately 745.7 W). 1 metric horsepower (used in Europe and Asia) = 735.49875 watts. The unit was originally defined by James Watt to compare steam engine output to the work done by a draft horse. Today, it's still commonly used for car engines, electric motors, and industrial equipment.

Yes, mathematically power can be negative. A negative power value indicates that energy is being absorbed or work is being done on the system rather than by the system. For example, when braking a vehicle, the brakes absorb kinetic energy—this is negative power from the vehicle's perspective. In most practical engineering calculations, we use the magnitude of power and specify the direction of energy flow separately.

Power: 1 kW = 1,000 W | 1 MW = 1,000,000 W | 1 hp = 745.7 W | 1 BTU/h ≈ 0.293 W | 1 cal/s = 4.184 W
Energy/Work: 1 kJ = 1,000 J | 1 kcal = 4,184 J | 1 kWh = 3,600,000 J | 1 BTU ≈ 1,055 J
Time: 1 min = 60 s | 1 hour = 3,600 s | 1 ms = 0.001 s
Always convert all values to base SI units (W, J, s) before applying P = W/t, then convert the result back to your desired unit.
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