Was ist ein/eine Watt-hour (Wh)?
The watt-hour (symbol: Wh) is a unit of energy equal to the energy delivered by one watt of power sustained for one hour. One watt-hour equals exactly 3,600 joules (since one hour = 3,600 seconds and one watt = one joule per second). The kilowatt-hour (kWh), equal to 1,000 Wh, is the standard unit for electricity billing worldwide.
Practical Electrical Energy Unit
The watt-hour bridges the gap between power (watts) and energy (joules) in an intuitive way. A 100-watt light bulb running for one hour uses 100 Wh (0.1 kWh). A 2,000-watt heater running for 3 hours uses 6,000 Wh (6 kWh). This power-times-time relationship makes watt-hours easy to calculate from device specifications.
Battery Capacity
Battery capacity is commonly specified in watt-hours or milliwatt-hours (mWh). A smartphone battery might hold 10-15 Wh, a laptop battery 50-100 Wh, and an electric vehicle battery 40,000-100,000 Wh (40-100 kWh). Watt-hours directly indicate how long a battery can power a given load.
Etymology
James Watt (1736-1819)
The "watt" component is named after Scottish inventor James Watt, whose improvements to the steam engine were fundamental to the Industrial Revolution. Watt introduced the concept of horsepower to market his engines and became synonymous with power measurement.
Compound Unit
The watt-hour is a compound unit combining power (watts) and time (hours). While the SI system uses the joule (= watt-second) for energy, the watt-hour emerged as a practical unit for electrical energy because consumption is more naturally described over hours rather than seconds. One kilowatt-hour (3.6 MJ) represents a more manageable number than 3,600,000 joules.
Adoption by Electric Utilities
Electric utilities adopted the kilowatt-hour in the late 19th century when commercial electricity generation began. Thomas Edison's Pearl Street Station (1882) metered customer usage in what would become the kWh. The watt-hour meter, invented in the 1880s, became the standard instrument for measuring electricity consumption.
Precise Definition
One watt-hour is defined as the energy delivered by one watt of power for one hour. Since 1 watt = 1 joule per second and 1 hour = 3,600 seconds: 1 Wh = 3,600 J = 3.6 kJ.
Key Conversions
1 Wh = 3,600 J = 3.6 kJ = 860.4 cal = 0.8604 kcal = 3.412 BTU. 1 kWh = 1,000 Wh = 3,600,000 J = 3.6 MJ = 3,412 BTU. Inversely: 1 J ≈ 0.000278 Wh, 1 kJ ≈ 0.278 Wh, 1 kcal ≈ 1.163 Wh, 1 BTU ≈ 0.293 Wh.
Kilowatt-Hour Dominance
The kilowatt-hour (kWh = 1,000 Wh) is by far the most commonly used multiple. Electricity bills, grid energy statistics, solar panel output, and EV charging are all measured in kWh. Larger multiples include megawatt-hour (MWh = 1,000 kWh) for power plants and gigawatt-hour (GWh = 1,000,000 kWh) for national energy statistics.
Geschichte
Edison and Early Electrification
The watt-hour became a practical unit with the advent of commercial electricity in the 1880s. Thomas Edison needed a way to bill customers for electricity consumption at his Pearl Street Station in Manhattan (1882). The watt-hour meter — an integrating device that measures cumulative energy consumption — was developed to fill this need.
Samuel Gardiner's Watt-Hour Meter
Early watt-hour meters were electrolytic (measuring the amount of metal deposited in a chemical cell proportional to energy consumed) or motor-driven. Elihu Thomson developed the first commercially practical recording watt-hour meter in 1888 for the Thomson-Houston Electric Company. This spinning-disk meter became the standard utility meter for over a century.
Global Electricity Standard
As electrical grids expanded globally throughout the 20th century, the kilowatt-hour became the universal unit for electricity billing. Every country, regardless of its measurement system (metric or imperial), uses the kWh for electricity. This makes the watt-hour one of the few non-SI units used universally in commerce.
Battery Era
The rise of portable electronics and electric vehicles in the 21st century has given the watt-hour renewed prominence. Smartphone specs list battery capacity in mWh, laptop batteries in Wh, and EV batteries in kWh. These watt-hour ratings directly inform consumers about device runtime and vehicle range.
Aktuelle Verwendung
Electricity Billing
Every electricity bill in the world is calculated in kilowatt-hours. Residential rates range from about $0.05/kWh (subsidized markets) to $0.40+/kWh (expensive markets like Germany and Denmark). An average US household uses about 900 kWh per month; European households average 300-500 kWh per month.
Battery Specifications
Battery energy capacity is specified in Wh or kWh: a smartphone battery holds 10-20 Wh, a laptop 50-100 Wh, a power bank 20-100 Wh, a home battery (like Tesla Powerwall) 13.5 kWh, and an EV battery 40-100 kWh. These figures directly indicate usable energy storage.
Solar Energy
Solar panel output is rated in watts (peak capacity) but actual production is measured in watt-hours. A 400-watt residential panel in a sunny location might produce about 1,600 Wh (1.6 kWh) per day. A typical residential solar system (6-10 kW) might produce 7,000-12,000 kWh per year.
Grid Energy Statistics
National and global energy statistics use MWh, GWh, and TWh. Global electricity generation is about 28,000 TWh per year (28 trillion kWh). The US generates about 4,000 TWh annually. These statistics underpin energy policy, climate targets, and infrastructure planning.
Everyday Use
Understanding Your Electric Bill
The watt-hour helps decode electricity costs. A 1,500-watt space heater running for 8 hours uses 12 kWh (about $1.20-$4.80 at typical rates). A 10-watt LED bulb running 10 hours per day uses 100 Wh (0.1 kWh) per day — about $1-$4 per year. Understanding watt-hours helps identify energy-hungry appliances.
Device Battery Life
Battery capacity in Wh predicts device runtime. A laptop with a 60 Wh battery running at 15 watts lasts about 4 hours (60/15). A smartphone with 15 Wh running at 3 watts average lasts about 5 hours of active use. Airlines limit carry-on batteries to 100 Wh (spare batteries) for safety.
EV Range
Electric vehicle range is directly related to battery capacity in kWh. Most EVs consume about 15-25 kWh per 100 km. A 75 kWh battery provides roughly 300-500 km of range depending on driving conditions and efficiency. Charging costs about $0.03-$0.08 per km at home rates.
Home Appliance Energy
Common appliance consumption: a refrigerator uses about 1-2 kWh per day, a washing machine cycle about 0.5-1 kWh, a dishwasher cycle about 1.5-2 kWh, an air conditioner about 1-3 kWh per hour, and a TV about 0.1-0.3 kWh per hour.
In Science & Industry
Energy Storage Research
Battery research measures energy density in Wh/kg (gravimetric) and Wh/L (volumetric). Lithium-ion batteries achieve about 150-300 Wh/kg, while theoretical lithium-air batteries could reach 1,000+ Wh/kg. For comparison, gasoline contains about 12,700 Wh/kg. Closing this gap is a central challenge of energy storage research.
Grid Engineering
Power grid engineering uses MWh for power plant output, transmission capacity planning, and storage requirements. Grid-scale batteries (like the Hornsdale Power Reserve in Australia) are rated in MWh (150 MWh). Grid balancing, peak shaving, and frequency regulation are all analyzed in watt-hour terms.
Climate and Energy Policy
Carbon intensity of electricity is measured in grams of CO₂ per kWh (gCO₂/kWh). Coal generates about 900 gCO₂/kWh, natural gas about 400 gCO₂/kWh, solar about 40 gCO₂/kWh (lifecycle), and nuclear about 12 gCO₂/kWh. These metrics drive decarbonization policy.
Space Technology
Spacecraft power budgets are planned in watt-hours. The International Space Station generates about 240 kWh per day from its solar arrays. Mars rovers rely on limited battery and solar capacity measured in Wh — the Perseverance rover's radioisotope generator produces about 2.6 kWh per day.
Interesting Facts
One kilowatt-hour of electricity costs about $0.10-$0.40 in most countries but represents 3.6 million joules of energy — more than a strong person can produce in a full day of physical labor.
The Tesla Model S Long Range has a 100 kWh battery — enough energy to power an average US home for about 3 days if used as a backup power source.
A human body at rest produces about 80-100 watts of heat, meaning over an 8-hour sleep you 'generate' about 640-800 Wh (roughly equivalent to a large laptop battery charging twice).
The lithium-ion battery in a typical smartphone (about 15 Wh) stores roughly the same energy as burning a single wooden match — but delivers it much more usefully over many hours.
Global electricity generation is about 28,000 TWh per year. If all of this came from solar panels, it would require covering about 0.3% of Earth's land surface with panels.
Airline regulations limit carry-on lithium batteries to 100 Wh per battery. This limit explains why you cannot bring very large portable power banks on flights.
A typical lightning bolt delivers about 250 kWh of energy, but the flash lasts only milliseconds, meaning the power is in the terawatt range — far beyond any human technology.
Energy density of batteries has roughly doubled every decade: from about 80 Wh/kg in the 1990s to about 250+ Wh/kg in modern lithium-ion cells. Solid-state batteries aim for 500+ Wh/kg.
Regional Variations
Universal Electricity Unit
The kilowatt-hour is used for electricity billing in every country worldwide, regardless of other measurement preferences. This makes it arguably the most universally adopted commercial energy unit.
BTU in the US
For heating energy (natural gas, heating oil), the United States uses BTU (1 BTU ≈ 0.293 Wh) and therms (1 therm = 100,000 BTU ≈ 29.3 kWh). Heating efficiency ratings in the US use BTU. The rest of the world uses kWh, kJ, or MJ for heating energy.
Joule Preference in Science
Scientific publications prefer joules (1 Wh = 3,600 J) as the SI energy unit. The watt-hour is technically a non-SI unit, though it is universally accepted in engineering and commerce. Some physics purists insist on expressing all energies in joules, but the watt-hour's practicality ensures its continued use.
Emerging Markets
In developing countries where electrification is ongoing, the kWh is often the first metric unit people encounter in daily life, through electricity bills and prepaid meters. Pay-as-you-go solar systems in Africa and Asia sell energy in kWh increments.