# How much energy is it?

Computes energy from power and time (E = P × t), from force and distance (W = F × d), or the mechanical energy of a moving, raised mass (½mv² + mgh), in joules, kWh, kcal, BTU and ft·lbf.

- Page: https://www.acalculator.org/physics/energy-calculator
- JSON spec: https://www.acalculator.org/physics/energy-calculator.json
- Version: 6f0ceb582d65

## Default answer

Example with the default inputs (Energy from Power × time, Power 1,500 W, Time 2 h): That is 10,800,000 J of energy, or 3 kWh.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| from | Energy from | Work out the energy from power and time, from force and distance, or from mass, speed and height. |
| P | Power | The steady power used or delivered. |
| t | Time | How long the power runs. |
| F | Force | The steady force, pushing along the direction of motion. |
| d | Distance | How far the force moves the object. |
| m | Mass | The mass of the object. |
| v | Speed | How fast the object moves. |
| h | Height | How high the object is above the level you measure from. |
| g | Gravity (m/s²) | The acceleration of gravity in m/s²: 9.80665 on Earth. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| joules | Energy (J) | The energy in joules. |
| kwh | Energy (kWh) | The same energy in kilowatt-hours (1 kWh = 3,600,000 J). |
| kcal | Energy (kcal) | The same energy in kilocalories, or food Calories (1 kcal = 4,184 J). |
| btu | Energy (BTU) | The same energy in British thermal units (1 BTU = 1,055.05585262 J). |
| ftlbf | Energy (ft·lbf) | The same energy in foot-pounds (1 ft·lbf = 1.3558179483314004 J). |
| kinetic | Kinetic energy (J) | In the motion mode: ½ m v². |
| potential | Potential energy (J) | In the motion mode: m g h. |

## Method

E = P × t; W = F × d; E = ½ m v² + m g h.

## Assumptions

- Power × time: the power is steady over the whole time. Force × distance: a steady force that points along the motion.
- Motion and height: kinetic energy ½ m v² (classical, well below light speed) plus gravitational potential energy m g h near a planet’s surface.
- Exact unit sizes: 1 hp = 550 ft·lbf/s; 1 BTU = 1,055.05585262 J; 1 kcal = 4,184 J; 1 kWh = 3,600,000 J; 1 lbf = 4.4482216152605 N; 1 ft = 0.3048 m; 1 mph = 0.44704 m/s.

## Worked examples

1. from = power, P = 1,500, t = 7,200 gives joules = 10,800,000, kwh = 3. Source: OpenStax, University Physics Volume 1, §7.4 Power (P = W ÷ t), https://openstax.org/books/university-physics-volume-1/pages/7-4-power.
2. from = work, F = 100, d = 10 gives joules = 1,000, kcal = 0.239006. Source: OpenStax, University Physics Volume 1, §7.1 Work (W = F d for a constant force along the motion), https://openstax.org/books/university-physics-volume-1/pages/7-1-work.
3. from = motion, m = 1,000, v = 20, h = 10, g = 9.80665 gives joules = 298,066.5, kinetic = 200,000, potential = 98,066.5. Source: OpenStax, University Physics Volume 1, §8.3 Conservation of Energy (E = K + U), https://openstax.org/books/university-physics-volume-1/pages/8-3-conservation-of-energy.
4. from = power, P = 1,055.055853, t = 3,600 gives btu = 3,600, kwh = 1.055056. Source: OpenStax, University Physics Volume 1, §7.4 Power (P = W ÷ t), https://openstax.org/books/university-physics-volume-1/pages/7-4-power; NIST SP 811 (1 BTU = 1,055.05585262 J), https://www.nist.gov/pml/special-publication-811.

## FAQ

### How do I calculate energy from power and time?

Multiply them: E = P × t. With the power in watts and the time in seconds, the energy is in joules. A 1,500 W heater running for 2 hours uses 1,500 × 7,200 = 10,800,000 J, which is 1.5 kW × 2 h = 3 kWh.

### How is work related to energy?

Work is energy moved by a force: W = F × d for a steady force along the motion. Pushing with 100 N for 10 m does 1,000 J of work.

### What is mechanical energy?

The sum of kinetic energy (½ m v²) and potential energy (m g h). A 1,000 kg car at 20 m/s and 10 m up has 200,000 J + 98,066.5 J = 298,066.5 J.

### How many joules are in a kWh?

One kilowatt-hour is 1,000 W for 3,600 s: 3,600,000 J, or 3.6 MJ.

### How many joules are in a calorie?

A small calorie is 4.184 J. A food Calorie is a kilocalorie, 4,184 J. So 1,000 J is about 0.239 kcal.

### What is a BTU?

The British thermal unit, used for heating and cooling, is 1,055.05585262 J exactly (the International Table BTU). One kWh is about 3,412 BTU.

## Sources

- OpenStax, University Physics Volume 1, §7.4 Power (P = W ÷ t, so W = P t for a steady power). https://openstax.org/books/university-physics-volume-1/pages/7-4-power (retrieved 2026-10-03)
- OpenStax, University Physics Volume 1, §7.1 Work (W = F d for a constant force along the displacement). https://openstax.org/books/university-physics-volume-1/pages/7-1-work (retrieved 2026-10-03)
- OpenStax, University Physics Volume 1, §8.3 Conservation of Energy (mechanical energy E = K + U). https://openstax.org/books/university-physics-volume-1/pages/8-3-conservation-of-energy (retrieved 2026-10-03)
- NIST SP 811, Appendix B.8 Factors for Units Listed Alphabetically (British thermal unit (IT), calorie (thermochemical), kilowatt hour, horsepower, foot-pound-force). https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8 (retrieved 2026-10-03)