Energy & Power Converter
Convert between joules, kWh, BTU, calories and megajoules, and between watts, kilowatts, horsepower and BTU/hr. Real-world equivalents and appliance electricity cost calculator — all in one tool.
Energy and Power Converter Tool
Calculate how much electricity an appliance costs to run. Enter the wattage, usage time and your electricity rate.
Energy and power in one tool — with real-world context
Every competitor splits energy and power across separate pages. This tool combines both, adds real-world equivalents most tools skip entirely, and includes an appliance cost calculator for immediately practical results.
How to convert energy and power units
How this energy converter compares
| Feature | LazyTools ✦ | UnitConverters.net | RapidTables | ConvertUnits.com |
|---|---|---|---|---|
| Energy + Power in one tool | ✔ Combined | Separate pages | Separate pages | Separate pages |
| Real-world equivalents | ✔ LED, phone, kettle | No | No | No |
| Appliance cost calculator | ✔ Built-in tab | No | ✔ Separate tool | No |
| All units at once | ✔ 11 energy / 8 power | ✔ Yes | One at a time | ✔ Yes |
| Appliance quick-fill presets | ✔ 14 appliances | No | No | No |
| Click to copy any result | ✔ Yes | No | No | No |
| Scientific notation for extremes | ✔ Auto | ✔ Yes | ✔ Yes | ✔ Yes |
| No ads blocking the tool | ✔ Clean layout | Heavy ads | Ads present | Heavy ads |
Energy and power conversion factors
| Unit | Equals (in joules or watts) | Common use |
|---|---|---|
| 1 Joule (J) | 1 J | SI base unit; energy of 1 N force over 1 m |
| 1 Kilojoule (kJ) | 1,000 J | Food energy, thermodynamics |
| 1 kWh | 3,600,000 J | Electricity billing worldwide |
| 1 Calorie (cal) | 4.184 J | Chemistry, physics (small calorie) |
| 1 kcal (food Cal) | 4,184 J | Nutrition, food energy labels |
| 1 BTU | 1,055.06 J | Heating/cooling, US energy |
| 1 Therm | 105,480,400 J | Natural gas billing (100,000 BTU) |
| 1 Watt (W) | 1 J/s | SI unit of power |
| 1 kW | 1,000 W | Appliances, generators |
| 1 hp (mech) | 745.699872 W | US motors, engines |
| 1 PS (metric hp) | 735.49875 W | European car engine ratings |
| 1 BTU/hr | 0.29307107 W | Air conditioning ratings (12,000 BTU/hr = 1 ton) |
Energy and Power Units Explained — A Complete Conversion Guide
Energy and power are two of the most frequently confused quantities in physics and everyday life. They are related but distinct: energy is the total amount of work done or heat transferred, while power is the rate at which energy is used or produced. Understanding this distinction — and how to convert between the many units used worldwide — is essential for engineers, students, energy consultants, homeowners comparing appliances and anyone working with electricity bills, heating systems or motor specifications.
Energy vs power — the fundamental distinction
The relationship between energy, power and time is expressed by a simple formula: Energy = Power × Time. A 2000-watt electric kettle running for 3 minutes (0.05 hours) uses 100 watt-hours or 0.1 kWh of energy. A 10-watt LED bulb running for 10 hours also uses 100 watt-hours. The power levels are very different but the energy consumed is identical. This is why electricity bills are charged in kilowatt-hours (kWh) rather than kilowatts — you pay for the total energy consumed, not the rate at which you use it. Power ratings on appliances (in watts or horsepower) tell you the rate of energy use; multiply by hours of use to get energy consumed.
The joule — the SI base unit of energy
The joule (J) is the International System of Units (SI) base unit of energy. It is defined as the energy transferred when a force of one newton acts through a distance of one metre. All other energy units can be expressed as exact multiples or fractions of joules: one kWh equals exactly 3,600,000 joules (3.6 megajoules), one calorie equals 4.184 joules and one BTU equals 1055.06 joules. Using joules as the anchor unit allows any energy conversion to be done in two steps — convert to joules, then convert to the target unit — which is exactly how this tool calculates every result.
Kilowatt-hours — the unit that matters for electricity bills
The kilowatt-hour (kWh) is by far the most important energy unit for consumers because it is the unit used on electricity bills worldwide. One kWh is the energy consumed by a 1000-watt device running for one hour. In practical terms, 1 kWh can power a 10W LED bulb for 100 hours, charge a typical smartphone about 50 times, run a laptop for roughly 6–8 hours or boil approximately 5 full kettles. The UK average domestic electricity price in 2024 was approximately 24–28p per kWh, meaning 1 kWh costs about 25 pence. Understanding kWh allows you to calculate the running cost of any appliance: watts ÷ 1000 × hours × rate per kWh = cost.
BTU and therms — the US and heating industry units
The British Thermal Unit (BTU) is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Despite its name, BTU is primarily used in the United States for air conditioning (a 12,000 BTU/hr unit is called "1 ton" of cooling), heating systems, and natural gas. One BTU equals 1055.06 joules or about 0.293 watt-hours. The therm is a larger unit used for natural gas billing, equal to 100,000 BTU or approximately 29.3 kWh. When comparing the running cost of gas versus electric heating, converting BTU and therms to kWh is essential for a fair cost comparison at current fuel prices.
Horsepower — mechanical vs metric
Horsepower is a unit of power with two distinct versions that are frequently confused. Mechanical horsepower (hp or bhp) is defined as exactly 33,000 foot-pounds per minute, which works out to 745.699872 watts. It is used in the United States for engine and motor ratings. Metric horsepower (PS, from the German Pferdestärke) is defined as the power needed to raise 75 kilograms by 1 metre per second, equal to 735.49875 watts — approximately 1.4% less than mechanical horsepower. European car specifications typically use PS, while US specifications use hp. For a 200PS European engine, the mechanical horsepower equivalent is approximately 197hp.
Calories — food versus physics
The word "calorie" causes enormous confusion because it is used to mean two very different quantities. The small calorie (cal) used in chemistry and physics equals 4.184 joules — the energy needed to raise 1 gram of water by 1°C. The kilocalorie (kcal), also confusingly called the "large calorie" or "food calorie" (written as Cal with a capital C on nutrition labels), equals 4,184 joules — a thousand times larger. When a food label says "200 Calories", it means 200 kcal or 836,800 joules. This tool uses the small calorie (4.184 J) for the cal unit and the large kilocalorie (4,184 J) for the kcal unit.