electricity

Energy is the capacity to work.
We actually mean "Potential energy", or "stored energy", which is the ability of a system to do work due to its position or internal structure. The most common stored energy forms are chemical energy in wood, coal, natural gas and crude oil and its derivates. Gravitational potential energy is a stored energy determined by an object's position in a gravitational field, such as water in hydro dams. Elastic potential energy is the energy stored in a spring, after it has been compressed.

Energy units are BTU (British Thermal Units), kWh (Kilowatthours), kcal (Kilocalories), kJ (Kilojoules), etc.. The SI basic units for potential energy are the joule (J) or newton-meter (N*m). (OECD units )

Some statistical data (source: OECD Factbook (2005) Economic, Environmental and Social Statistics

World energy supply (2002) Mtoe (million tonnes of oil equivalent):
The world’s total primary energy supply: 10230 Mtoe = 118975 Million MWh. (1971: 5470 Mtoe).
The share of oil was 34.9% = 3571 Mtoe (1971: 42.8%)
The share of coal was 23.5% = 2204 Mtoe (1971: 26.4%)
The share of combustible renewables and waste – mainly wood and charcoal: 11% = 1125 Mtoe (1971: 11%)
The share of natural gas: 21.2% = 2169 Mtoe (1971: 16.4%)
The share of nuclear: 6.8% = 696 Mtoe (1971: 0.5%)
The increase was 87% over 31 years, which is 2% a year. By comparison, world population yearly growth was 1.6% and Gross Domestic Product was 2.9% over the same period.

[Compare World energy resources and consumption]

An adult person uses about 2500 kcal a day, of which 1600 kcal are used for the functioning of the human body. Approx. 900 kcal are available for work. This is about 1 kWh - the amount of energy (electricity) that a 60 Watt light bulb consumes in 15 hours.

Energy forms that we can use are solar energy, the energy available in people and animals, hydraulic and wind energy, geothermal, as well as the energy that is stored in fossil fuels such as oil, gas, coal and uranium.
(Energy conversions on-line )

Unit 1	=	Unit 2	Comparative value
1 J	=	1 Ws
1000 kJ	=	238.8 kcal
1000 kJ	=	0.278 kWh
1 kWh	=	3412 BTU
1000 kJ	=	0.278 kWh
1 kWh	=	3597 kJ
1 barrel of crude oil	=	1714 kWh	the daily work performance of 1714 average adults = 13712 person-hours
1 barrel of crude oil	=	50 US$ (JAN2009)	109,696 US$ adult work at 8$/h
1 kcal	=	4.1868 kJ
1 kcal	=	1.1628 Wh
2500 kcal	=	2.907 kWh	the daily energy need of an average adult person.
1 kWh	=	860 kcal	the daily work performance of an average adult person.
1 m3 Erdgas	=	8.816 kWh
1 ton of coal equivalent TCE	=	8140 kWh
1 ton of oil equivalent TOE	=	11630 kWh
1 ton of oil equivalent TOE	=	1.428 TCE
1000 BTU	=	0.2931 kWh
1 QUAD	=	1x1015 BTUs = 1 quadrillion	The energy contained in eight billion gallons of gasoline, a year's supply for ten million automobiles.
1 QUAD	=	293074646 MWh
1 QUAD	=	1.055 × 1018 joules
1 QUAD	=	1.055 exajoules or EJ
1 barrel	=	42 gallons
1 barrel of crude oil	=	5,848,000 BTU

Vermögen (Leistung) ist die Energiemenge, die pro Zeiteinheit erbracht werden kann.
Einheiten sind PS (Pferdestärke), kW (Kilowatt).