Energy and power are easy to confuse because both appear on appliance labels, utility bills, and engine specs. Energy is a quantity — the capacity to do work or transfer heat — while power is the rate at which energy is transferred. The energy converter expresses a known energy amount in joules, small calories, kilowatt-hours, or International Table BTU. The power converter expresses a known rate in watts, kilowatts, mechanical horsepower, or megawatts. Neither page infers its quantity from other inputs; each converts a value you already have, using its own unit conventions.
What each calculator does
The energy converter converts a nonnegative energy amount among joules, small calories, kilowatt-hours, and International Table BTU, using the joule as the base: it multiplies the input by the source factor to get joules, then divides by the target factor. One small calorie is exactly 4.184 J, one kilowatt-hour is 3,600,000 J, and one BTU_IT is 1,055.05585262 J. Results are displayed to two decimals for BTU and most units, while kilowatt-hours use six decimals — a 2 kWh input converts to 6,824.28 BTU. The page also flags the food-Calorie pitfall: a nutrition Calorie is 1000 small calories.
The power converter converts a nonnegative power value among watts, kilowatts, mechanical horsepower, and megawatts, using the watt as the base. One kilowatt is 1,000 W, one mechanical horsepower is 745.6998715822702 W, and one megawatt is 1,000,000 W. Displayed results use two decimals — 2 kW converts to 2.68 hp — and the page notes that the form has no BTU per hour, refrigeration-ton, volt-ampere, or metric-horsepower units. Its FAQ makes the same energy-versus-power distinction the energy converter makes: a one-kilowatt device running for one hour uses one kilowatt-hour of energy.
Side-by-side
| Energy converter | Power converter | |
|---|---|---|
| Quantity | Energy: capacity to do work or transfer heat | Power: rate of energy transfer |
| Units | Joules, small calories, kilowatt-hours, International Table BTU | Watts, kilowatts, mechanical horsepower, megawatts |
| Base unit | Joule (J) | Watt (W), defined as one joule per second |
| Key factors | 1 cal = 4.184 J; 1 kWh = 3,600,000 J; 1 BTU_IT = 1,055.05585262 J | 1 kW = 1,000 W; 1 hp = 745.6998715822702 W; 1 MW = 1,000,000 W |
| Typical sources | Utility bills, battery capacity, heating loads, chemistry tables | Appliance labels, engine and pump ratings, grid-scale equipment |
| Input rule | Nonnegative energy amount | Nonnegative power value |
| Display rounding | Two decimals (kWh uses six) | Two decimals |
| Units excluded | Watts, horsepower, and other rates | BTU per hour, volt-amperes, refrigeration tons, metric horsepower |
When to use which
Use the power converter when your question is about a rate: what an appliance draws, how an engine compares in horsepower, or how a generator’s megawatt rating scales. Its own article points to the energy converter for the accumulated amount behind a rate, since a 100 MW plant running for two hours produces 200 MWh of energy.
Use the energy converter when your question is about an amount: a kilowatt-hour figure on a bill, a battery’s stored energy, a BTU heating load, or a chemistry table in small calories. Its article cross-links the power converter for rates and notes that keeping energy distinct from power prevents many HVAC and metering mistakes. If your actual goal is a cost or a charging plan rather than a unit conversion, both pages point onward — the electricity cost calculator for bills and the battery charge time calculator for charging scenarios.
Limits and disclaimer
Both converters only rewrite units of a value you supply; neither infers energy from mass, speed, voltage, or time, and both reject negative inputs, even though signed energy and power can be meaningful in advanced contexts. The small calorie is not a food Calorie — multiply nutrition-label Calories by 1000 first. Horsepower conventions differ: this site’s power converter uses mechanical horsepower, not metric, boiler, or electrical horsepower, and it excludes apparent and reactive power, which depend on AC power factor. Display rounding can hide small values, as when 2 kW shows as 0.00 MW. Both pages are educational unit converters, not billing, engineering, or safety tools.