Power to Energy Converter
Our free thermal & energy converter handles power energy conversions. See tables, ratios, and examples for quick reference.
Power to Energy Converter
Calculator
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Formula: Energy = Power x Time | E (J) = P (W) x t (s) | E (kWh) = P (kW) x t (h)
Worked example โ 12 kWh = 43.2 MJ = 10,325 kcal, costing $1.44
Formula
Energy = Power x Time | E (J) = P (W) x t (s) | E (kWh) = P (kW) x t (h)
Energy is the product of power and time. In SI units, 1 watt for 1 second equals 1 joule. For electricity billing, the kilowatt-hour is standard: 1 kW for 1 hour = 1 kWh = 3,600,000 joules. This converter handles the unit conversions automatically, letting you input power in any unit and time in any duration.
Worked Examples
Example 1: Home Appliance Energy Use
Problem:A 1,500 W space heater runs for 8 hours. How much energy does it consume?
Solution:Energy = Power x Time Energy = 1,500 W x 8 h = 12,000 Wh = 12 kWh In joules: 1,500 W x 28,800 s = 43,200,000 J = 43.2 MJ Cost at $0.12/kWh: 12 x $0.12 = $1.44
Result:12 kWh = 43.2 MJ = 10,325 kcal, costing $1.44
Example 2: Industrial Motor Energy
Problem:A 10 HP motor runs continuously for 24 hours. Calculate total energy in kWh.
Solution:Convert HP to kW: 10 x 0.7457 = 7.457 kW Energy = 7.457 kW x 24 h = 178.97 kWh In MJ: 178.97 x 3.6 = 644.3 MJ Cost at $0.12/kWh: 178.97 x $0.12 = $21.48
Result:10 HP for 24 hours = 178.97 kWh = 644.3 MJ
Frequently Asked Questions
How do you calculate energy from power and time?
Energy equals power multiplied by time. In SI units, energy (joules) = power (watts) x time (seconds). For practical purposes, electricity is measured in kilowatt-hours: energy (kWh) = power (kW) x time (hours). For example, a 100-watt bulb running for 10 hours uses 100 x 10 = 1,000 Wh = 1 kWh of energy. This relationship is fundamental to all energy calculations and electricity billing.
What is the difference between power and energy?
Power is the rate at which energy is transferred or converted, measured in watts (joules per second). Energy is the total amount of work done or heat transferred, measured in joules or kilowatt-hours. Think of power as speed and energy as distance. A 1,000-watt microwave uses energy faster than a 100-watt bulb, but total energy consumed depends on how long each runs. Power is instantaneous; energy is cumulative over time.
How do I calculate my electricity cost from power usage?
First calculate energy in kWh: multiply the device power in kW by hours of use. Then multiply by your electricity rate (cost per kWh). For example, a 1,500 W space heater running 8 hours: 1.5 kW x 8 h = 12 kWh. At $0.12/kWh: 12 x $0.12 = $1.44 per day. To find monthly cost, multiply daily cost by 30 days: $1.44 x 30 = $43.20 per month.
How do I convert horsepower to energy?
First convert horsepower to watts: 1 mechanical horsepower = 745.7 watts. Then multiply by time in seconds to get joules, or by time in hours to get watt-hours. For example, a 5 HP motor running for 2 hours: 5 x 745.7 = 3,728.5 W = 3.7285 kW. Energy = 3.7285 kW x 2 h = 7.457 kWh. This is useful for calculating energy costs of motors, pumps, and other mechanical equipment.
How many joules are in one kilowatt-hour?
A kilowatt-hour is 1,000 watts sustained for 3,600 seconds, so it equals 3,600,000 joules, or 3.6 megajoules. The relationship is exact, since a watt-second and a joule are the same SI unit. It is the key bridge between utility billing, which uses kilowatt-hours, and physics or engineering work, which uses joules and megajoules, and it explains why the joule is such a small quantity in household terms.
How do I convert between kilowatt-hours, megajoules and BTU?
One kilowatt-hour equals exactly 3.6 megajoules and about 3,412 BTU, since one BTU is roughly 1,055 joules. Electricity is billed in kilowatt-hours, gas is often metered in megajoules or therms, and heating equipment is rated in BTU, so comparing fuels for the same task means putting them all on one energy basis first and only then applying prices and conversion efficiencies.
How much energy does a 60 watt lamp use in eight hours?
Multiply power by time in matching units: 60 watts is 0.06 kilowatts, and 0.06 kW x 8 h equals 0.48 kilowatt-hours. In SI terms that is 0.48 x 3.6 = 1.728 megajoules. Running the same lamp every day for 30 days uses 14.4 kilowatt-hours. Scaling by days, or by the number of identical devices, is the usual way to turn a single appliance rating into a monthly consumption figure.
How do I handle a device that only runs part of the time?
Use the duty cycle, the fraction of the period during which the device actually draws power, and apply it to the rated power before multiplying by time. A 1,500 watt heater on a thermostat that keeps it energised 40 percent of the time averages 600 watts, so over 5 hours it uses 3 kilowatt-hours rather than 7.5. Compressors, pumps and heaters almost always cycle, so nameplate power alone overstates their energy use.
What is a watt-hour and how does it relate to a battery's amp-hour rating?
A watt-hour is one watt sustained for one hour, or 3,600 joules, the same relationship as the kilowatt-hour on a smaller scale. A battery's amp-hour rating is a charge capacity, so it only becomes energy once multiplied by voltage: a 12 volt, 100 amp-hour battery stores about 1,200 watt-hours, or 1.2 kilowatt-hours. Comparing batteries that run at different voltages is only meaningful in watt-hours.
Does an appliance's rated power mean input or output?
Electrical appliances are normally labelled with the input power drawn from the supply, which is what the meter records. Motors and some heating devices may instead quote useful output, in which case the input is higher by the inverse of the efficiency. A motor delivering 750 watts of shaft power at 85 percent efficiency draws about 882 watts. Calculate billed energy from input power and reserve output figures for sizing the work being done.
References
Reviewed for accuracy by Manoj Kumar, Mathematics Educator ยท Editorial policy
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