Energy is the capacity to do work. This guide covers what it is, forms, measurement units, conversions, and practical efficiency tips.
What is Energy?
Energy is the ability to do work or produce heat. It transforms between forms and is conserved (First Law of Thermodynamics).
Key Concept:
SI unit: joule (J). 1 J = work by 1 N over 1 m.
Forms of Energy
Kinetic Energy
Energy of motion
- Moving car
- Falling objects
- Wind
Potential Energy
Stored energy
- Water behind a dam
- Stretched spring
- Chemical bonds
Thermal Energy
Heat energy
- Geothermal
- Body heat
- Sun heat
Electrical Energy
From electric charges
- Lightning
- Batteries
- Power lines
Measuring Energy
Units vary by context:
Metric Units
- Joule (J) — SI unit
- Kilojoule (kJ) — 1,000 J
- Calorie (cal) — 4.184 J
- Kilocalorie (kcal) — 4,184 J
- Kilowatt-hour (kWh) — 3.6 MJ
Imperial/Other Units
- BTU — 1,055 J
- Foot‑pound (ft·lbf) — 1.356 J
- Electronvolt (eV) — 1.602×10⁻¹⁹ J
- Ton of TNT — 4.184 GJ
- Barrel of oil eq. — ~6.12 GJ
Energy Conversion Factors
| Unit | Joules (J) | Common Uses |
|---|---|---|
| 1 Joule (J) | 1 | Lift a small apple 1 m |
| 1 Calorie (cal) | 4.184 | Heat 1 g water by 1°C |
| 1 kWh | 3,600,000 | Household electricity per hour |
| 1 BTU | 1,055 | Heat 1 lb water by 1°F |
Energy in Everyday Life
Food Energy
- Banana ~400 kJ (95 kcal)
- Slice of pizza ~1,100 kJ (260 kcal)
- Daily adult ~8,400–12,600 kJ
Household Energy
- LED 10W × 1 h: 36 kJ
- Microwave 1,000W × 1 min: 60 kJ
- Oven 2,000W × 1 h: 7,200 kJ
Transportation
- Gasoline 1 L: ~32,000 kJ
- EV battery 60 kWh: 216,000 kJ
- Cycling 1 h: ~1,500 kJ
Did You Know?
Solar energy reaching Earth in 1 hour exceeds global annual consumption.
Energy Efficiency and Conservation
Energy‑Saving Tips
- Use LED lighting
- Set thermostats wisely
- Unplug idle devices
- Choose efficient appliances
- Insulate your home
Practical Tip:
Compare EER/COP across appliances; higher means better efficiency and lower cost.