Calculate electric power, energy consumption (kWh), and estimated electricity cost
Voltage U (V)
Current I (A)
Usage Time (hours)
Electricity Rate (per kWh, optional)
Results
Electric Power
Energy Consumption
Estimated Cost
Step-by-Step Derivation
Power & Energy Formulas
Electric Power: P = U × I (watts W)
Energy: E = P × t (kilowatt-hour kWh)
Cost = E × Electricity Rate (currency)
Electric power measures how fast an appliance consumes energy. Energy is the total electricity used over time. 1 kWh = 1 kilowatt-hour = 3.6×10⁶ J.
⚠Power (P) is in watts (W). To compute energy in kWh, divide watts by 1000. Electricity rates vary by region — please use your local rate for accurate estimates.
What Are Electric Power and Energy?
Electric power is the rate at which electrical work is done, reflecting how fast an appliance consumes energy. Energy (electricity) is the total amount consumed over time. The power rating marked on appliances (e.g., 100W light bulb) is the rated power.
Electric Power
P = UI, measured in watts (W). Higher power means faster energy conversion. Typical values: LED bulb 5-20W, refrigerator 100-200W, air conditioner 1000-3000W.
Electric Energy
E = P × t, measured in kilowatt-hours (kWh). 1 kWh = 1 "unit" of electricity. Example: a 100W bulb running for 10 hours consumes 1 kWh.
Joule's Law
Heat generated by current: Q = I²Rt (joules). Heating appliances (kettles, heaters) use this principle. Higher power means faster heating.
Energy Saving Tips
Standby power (1-5W) can accumulate to tens of kWh per year. Use switches to cut off standby power and save on electricity bills.
💡 Example: An air conditioner runs at 220V, 4.5A, used 8 hours/day. Power P = 220×4.5 = 990W ≈ 1kW. Daily energy E = 1kW × 8h = 8 kWh. At rate $0.15/kWh, daily cost = 8 × 0.15 = $1.20.
Applications
Household Bill EstimationAppliance SelectionPower Supply DesignEnergy Efficiency AuditPhysics Education
Frequently Asked Questions
What is the difference between electric power and energy?▼
Electric power is the rate at which electrical energy is consumed or generated per unit time, measured in watts (W). Energy is the total amount of electricity consumed over a period, measured in kilowatt-hours (kWh) or joules (J). Relationship: Energy = Power × Time. 1 kWh = 1000W × 3600s = 3.6×10⁶ J.
How many watts are in 1 kWh? What can 1 kWh do?▼
1 kWh = 1000 watt-hours. It can: light a 100W bulb for 10 hours, run a 2000W air conditioner for 0.5 hours, power a 100W refrigerator for 10 hours, run a 100W TV for 10 hours, or charge a 5W phone for 200 hours. Typical residential electricity price ranges from $0.10 to $0.30 per kWh.
How do I calculate the electricity cost for an appliance?▼
Cost = Power (kW) × Time (h) × Rate ($/kWh). Example: A 2000W AC runs for 8 hours at $0.15/kWh: Cost = 2 kW × 8 h × 0.15 = $2.40. Note: actual AC power fluctuates due to compressor cycling; this is an approximation.
What does the rated power on a power adapter mean?▼
Rated power is the maximum safe output power a power supply can deliver under normal conditions. Example: "Output 5V/2A" means rated power = 10W. Ensure load power does not exceed the rating, otherwise overheating, voltage drop, or damage may occur. Typically, leave a 20% margin.
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