Electrical Energy Calculator finds the energy a load uses in kWh and joules from voltage, current, resistance or power and its running time, with the cost at your electricity rate.
Enter any two of voltage, current and resistance, or the wattage from the nameplate, then how long it runs.
AC Circuit
Three-phase uses line-to-line voltage and the current in one line.
Electricity Rate
Energy and Cost by Running Time
| Running Time | Energy | Cost |
|---|---|---|
| 1 h | 1.500 kWh | $0.27 |
| 2 h | 3.000 kWh | $0.54 |
| 4 h | 6.000 kWh | $1.08 |
| 6 h | 9.000 kWh | $1.62 |
| 8 h | 12.00 kWh | $2.16 |
| 12 h | 18.00 kWh | $3.24 |
| 24 h | 36.00 kWh | $6.48 |
| 7 days | 252.0 kWh | $45.36 |
| 30 days | 1,080 kWh | $194.40 |
| 1 year | 13,140 kWh | $2,365.20 |
Step-by-Step Working
- Power: P = V × I = 120 V × 12.5 A = 1.500 kW.
- Time: t = 6 h, or 21,600 s.
- Energy: E = P × t = 1.500 kW × 6 h = 9.000 kWh.
- In joules: 1.500 kW × 21,600 s = 32.40 MJ.
- Cost: 9.000 kWh × 18¢ per kWh = $1.62.
Electrical Energy Calculator Formula
The Electrical Energy Calculator multiplies the power a load draws by the time it runs. To calculate electrical energy by hand, that single step is the whole electrical energy calculation formula, and everything else is finding the power first.
E is energy, P is power and t is time. With power in watts and time in seconds the answer is in joules. With power in kilowatts and time in hours it is in kilowatt-hours, the unit on an electricity bill.
Power itself comes from any two of voltage, current and resistance. These are the three formulas of power, and Calculate From picks the one that matches what you know.
The tool opens on 120 V and 12.5 A for 6 hours. Power is 120 times 12.5, which is 1,500 W, so the energy is 9 kWh. The line under the result prints that sum, and Step-by-Step Working lists each stage down to the cost. The second card shows the power behind it, 1.5 kW or 2.011 hp, and the 9.6 Ω load resistance those two values imply.
Current and Resistance suits a heating element or a resistor. A current of 10 A through 5 Ω is 500 W, and in 2 hours that is exactly 1 kWh. Voltage and Resistance works the same way, and 240 V across 28.8 Ω is 2 kW.
Power and Time skips the circuit and takes the wattage from a nameplate. For that mode the second card shows the horsepower equivalent and the current the load would draw at 120 V and 240 V. Used as an electrical power calculator online, the Electrical Power Calculator goes further into power alone, and the Ohm's Law Calculator solves for a missing voltage or resistance.
kWh, Units and Joules
One unit of electricity is 1 kWh. A meter counts units, and one unit is the energy a 1 kW load uses in one hour. That answers whether 1 kW equals 1 unit, since it does only when the load runs for a full hour.
To convert 1 kW to kWh, multiply by the hours. One kW for 1 hour is 1 kWh, for 30 minutes it is 0.5 kWh and for a 24 hour day it is 24 kWh. There are 1,000 watts in 1 kW, so a watt figure is divided by 1,000 first.
To calculate 1 unit of electricity for any appliance, multiply its watts by the hours and divide by 1,000. A kilowatt on its own is a rate and has no per hour, so the question of how many kW in one hour really asks for kWh. In the same way, 200 units of electricity means 200 kWh.
| Load | Units in 1 Hour | Time to Use 1 Unit | Units in 24 Hours |
|---|---|---|---|
| 1 W | 0.001 kWh | 1,000 h | 0.024 kWh |
| 40 W | 0.04 kWh | 25 h | 0.96 kWh |
| 100 W | 0.1 kWh | 10 h | 2.4 kWh |
| 1,000 W | 1 kWh | 1 h | 24 kWh |
| 1,500 W | 1.5 kWh | 40 min | 36 kWh |
| 3,000 W | 3 kWh | 20 min | 72 kWh |
So 3000 watts consumes 3 units in 1 hour, and 40 watts is 0.04 units per hour, or one unit every 25 hours. A single watt takes 1,000 hours to use one unit. The fourth card works as a 1 unit kWh calculator for any load, showing the time it takes to use 1 kWh and the energy per day and per 30 days.
The first card in the Electrical Energy Calculator gives the same energy in joules, BTU and kilocalories. One watt-hour is 3,600 joules, so 1 kWh is 3.6 MJ. The 9 kWh example is 32.40 MJ, 30,710 BTU or 7,744 kcal, which matters when the electricity ends up as heat.
Electricity Cost per kWh
Open Electricity Rate in the Electrical Energy Calculator and enter what you pay per kWh. The default of 18 cents sits in the range of the U.S. residential average in the EIA price table, which showed 17.78 cents for November 2025. Your own bill gives a better figure.
The third card makes the tool an electricity cost calculator in kWh terms. The 9 kWh example costs $1.62, and the rows show 27 cents per hour and $6.48 if the same load runs for 24 hours. Energy and Cost by Running Time lists the cost from 1 hour to 1 year.
The cost of 1 kW running for an hour is your rate, because that is 1 kWh of electricity. At 18 cents, 44 units of electricity cost $7.92, 200 units cost $36, a bill for 500 units is $90 and 1,000 units is $180.
How many kWh a day is normal depends on the home. The EIA puts the average U.S. household at about 10,500 kWh a year, close to 29 kWh a day. A home using 20 kWh of electricity per day is below that, and costs $3.60 a day or $108 over 30 days at 18 cents.
Used as an electrical energy calculator in India, the tool counts the same unit, since one unit there is also 1 kWh. The cost of 200 units of electricity in rupees is 200 times the tariff per unit, which is 1,600 rupees at 8 rupees a unit. Type the tariff in paise in the rate field and read the dollar figures as rupees.
The tool handles one load at a time. As a household electricity consumption calculator for a whole list of appliances with different hours, the Energy Consumption Calculator is the better fit. On a phone the page behaves like an electrical energy calculator app, with Copy Results and Share Results under the cards.
AC Power: Single-Phase and Three-Phase
With Voltage and Current selected, the AC Circuit panel in the Electrical Energy Calculator adds power factor. The formula for AC power in a single-phase circuit is volts times amps times power factor, which is also how to calculate kW in 1 phase.
A 240 V load drawing 20 A at a power factor of 0.9 uses 4.32 kW. To calculate 3 phase AC power, the line-to-line voltage and the current in one line are multiplied by the square root of 3 as well.
The same 20 A on 3 phase 240V power at 0.9 is 7.482 kW, and 8 hours of that is 59.86 kWh. The result already covers all three phases, so it is not multiplied by three again. The Three-Phase Power Calculator breaks that power down further.
A meter bills real energy. When power factor is below 1, the note under the cards also gives the apparent energy in kVAh, 66.51 kVAh for that three-phase example. For the largest service sizes, 200 amp in kW is 48 kW at 240 V single-phase, which is 48 units for every hour at full load. The Current Calculator turns a wattage back into amps.
Air Conditioner Energy and Cost
How many kW is 1.5 ton AC?
A ton of cooling is 12,000 BTU per hour, so a 1 ton AC load is 3.517 kW of heat removed and a 1.5 ton unit removes 5.275 kW. The electricity it draws is far less. At an EER of 10 a 1.5 ton unit takes 1.8 kW from the supply, and the nameplate gives the exact figure.
How much AC bill in 1 hour?
Enter 1.8 kW under Power and Time in the Electrical Energy Calculator. One hour uses 1.8 kWh, which is 32.4 cents at 18 cents per kWh. The unit uses one kWh every 33.33 minutes while the compressor runs, and less in practice because it cycles off once the room is cool.
Can I run AC full night?
You can, and the cost is the only question the math answers. A full night of 8 hours is 14.4 kWh, or $2.59. If you use the AC for 12 hours it is 21.6 kWh and $3.89. These are upper figures for a compressor that never cycles off.
What is the 3 minute rule for AC?
It is the wait before a compressor restarts after switching off. LG states that its air conditioners hold the outdoor unit for 3 minutes after each stop to protect the compressor, and the count restarts every time the unit is switched off and on. It changes the energy used very little.