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Energy is vital for our lives: our bodies run on the energy we get from the food we eat, which requires energy to farm, process, transport and cook. Our industries, commerce, education, health-care, communications and recreation all consume energy. But most of this energy comes from burning fossil fuels - coal, oil and gas - which release Carbon Dioxide into the atmosphere, which causes global warming and dissolves into water causing ocean acidification.

Energy and Power

The terms Energy and Power tend to be used interchangeably to refer to the electricity, oil, gas etc we all consume, and we talk about "wind energy" and "solar power" in the same context. However the terms have specific, different, meanings: energy is the ability to do a certain amount of work e.g. boil a particular quantity of water or move a car a certain distance, and power is the rate at which work is done - how quickly the water is heated or the car travels. Energy and work can be thought of as cause and effect: putting a certain amount of energy into a kettle or a car results in an equivalent amount of heating or movement happening in it.

Units of measurement


There are various different units for measuring energy (or work) and power. For power probably the most common units are watts (W) (and kilowatts (KW), megawatts (MW), gigawatts (GW) and even terawatts (TW)). For energy common units are watt hours (or more commonly kilowatt hours (KWh)). The "unit" of electricity (used on British electricity meters and bills) is the same as one KWh. Sometimes a quantity of energy (e.g. the amount supplied by a solar panel over a certain period) is wrongly and confusingly stated as so many kilowatts rather than kilowatt hours.

kWh/y and Joules

Another unit of power based on watts is kilowatt hours per year. Since there are 8,766 hours in a year a power of 1KW (roughly the consumption of a 1 bar electric fire) is 8,766 KWh/y. A unit of energy more often used in scientific work is the Joule, which is one watt second, so 3,600 (60 times 60) joules are a watt hour, and 3.6 megajoules are 1KWh.

Horespower, BTUs and TOEs

An older unit of power is the horsepower, which is about 746 Watts. The British Thermal Unit (BTU or BThU) is another old unit of energy, which is mostly obsolete in the UK (though still used in the US). A common measure of energy, usually used on a large, even national, scale, is the Tonne of Oil Equivalent and its multiples such as the mega-tonne of oil equivalent (MTOE).

Homes (and other units)

Publicity material and news articles about energy projects often talk about the number of homes they can power. One figure for the amount of power this represents is given by DUKES (source) of 4370KWh per household per annum. This equates to about 0.5kW or 500Watts, so halving the number of "homes" gives the equivalent kilowatts. According to David MacKay the British Wind Energy Association uses the figure 4700kWh per year, equivalent to 0.54kW or 540Watts, and other organizations use 4000kWh/y per household -- 0.46kW or 460Watts.

North American usage is higher: Canada's Ontario Power Generation cites "monthly domestic usage of 972 kWh per home" or 1.33kW.

MacKay also points out that the “home” unit only covers average domestic electricity consumption of a household, not gas or oil used for home heating, cooking and hot water, the energy that occupants use in their workplaces and for transport, or all the other energy-consuming things that society does for them; all of which add up to roughly 24 times more than a "home".

MacKay also discusses other units including "power stations", "cars taken off the road", "calories" barrels, gallons, tons, BTUs, quads, cups of tea double decker buses, Albert Halls and Wembley Stadiums. He also provides this chart for translating power units.


The IEA provides an online energy units conversion calculator.

Heat and Electricity: Thermal Equivalent

Some power sources such as solar photovoltaic, wind, hydroelectric, wave and tide produce electricity directly. Others such as coal, oil, gas, biomass and nuclear produce heat. Heat energy may be used directly - for example for heating buildings, or in industrial processes - or it may be used to generate electricity, in which case only about 1/3 to 1/2 of the heat energy gets turned into electricity. When comparing heat with electricity this conversion may be factored in to allow meaningful comparison of, say, how much fossil fuel is saved by a given amount of hydro, wind or nuclear. This conversion is particularly likely to have been used when "tonnes of oil equivalent" ("toe"s) and their multiples (Mtoes, Gtoes) are quoted, for example in the BP Statistical Review (which uses a conversion factor of 38% - "the average for OECD thermal power generation").

Energy density and population density

Different parts of the world have different densities of population, and those people use energy at different rates (so in first world cities more energy is consumed in a given area than in rural areas in the developing world).

Different sources of energy also have different densities: a 1GW coal, gas or nuclear power station may require a few tens of square kilometres (including the mines or wells needed to supply it) whereas to generate the same power from say, biomass, requires thousands of square kilometres to grow energy crops. When considering what sources of energy could power a given country or region we can compare the energy densities of supply and demand, and David MacKay's Map of the World provides a convenient way of doing so visually for various countries and sources of low-carbon energy.


Capacity Factor

Generators - whether fossil fuelled, nuclear, hydro, wind, solar etc - don't produce electricity (or other forms of useful energy) continuously, 24*7. All mechanical plants can break down, and most are stopped from time to time for inspection and maintenance. Most present day nuclear power stations have to be shut down for refuelling. And hydro, wind, solar, wave and tidal generators can only produce energy when there is the water, wind, sun, wave and tidal conditions they need. The percentage of a generator's maximum ("nameplate") output (pr "installed capacity") which it achieves over a representative period in practice is known as its "capacity factor"; this is typically around 10-20% for solar, 30-40% for wind and around 90% for nuclear and other thermal power stations.

Because of the significant difference in capacity factors between - in particular - solar and wind and nuclear, it is misleading to compare installed capacities without compensating for capacity factors.

Energy Return on Energy Investment / EROI, Carbon Intensity, Cost of Energy and Life Cycle Analysis

Energy is required in building, operating, maintaining and eventual decommissioning any power generating plant. As long as the plant generates more energy in its lifetime than it consumes it will provide a surplus for useful purposes. The greater the surplus the more it allows society to provide other goods and services - to grow food, to build, heat and cool homes and workplaces, run water and sewerage services, provide education, health care, transport, recreation etc - besides building, maintaining and decommissioning its energy generators. The ratio of energy generated / energy consumed is known as Energy Return On Energy Invested (EROEI, or sometimes EROI) and it is estimated that for a modern civilisation to function the EROEI of its energy supplies in aggregate must be in the region of 10.

Even an EROEI less than unity - a system that consumes more energy than it produces - may be useful if it provides services such as storing energy for use when and where it is needed but unavailable, such as to supply energy when demand temporarily exceeds supply, or to convey energy from large fixed generators to electric vehicles. However the overall, aggregate, EROEI of the system of generators and storage systems must still be high enough to allow for a functioning civilisation.

ERoEI for Beginners Euan Mearns; Energy Matters; 25 May 2016

many more references in comments

Science for Energy Scenarios

3rd Science and Energy Seminar at Ecole de Physique des Houches, March 6th-11th 2016
presentations on EROI, power-gas-power, intermmittency, grids, etc

Unsubsidized Levelized Cost of Energy Comparison Lazard

Stanford scientists calculate the carbon footprint of grid-scale battery technologies

Stanford scientists have developed a novel way to calculate the energetic cost of building large batteries and other storage technologies for the electrical grid.

Solar energy in the context of energy use, energy transportation and energy storage David MacKay; Philosophical Transactions of the Royal Society

In a decarbonized world that is renewable-powered, the land area required to maintain today's British energy consumption would have to be similar to the area of Britain.

The Energy Return of Solar PV Euan Mearns; Energy Matters; 9 May 2016

A new study by Ferroni and Hopkirk estimates the ERoEI of temperate latitude solar photovoltaic (PV) systems to be 0.83.

The Energy Return of Solar PV – a response from Ferroni and Hopkirk Euan Mearns; Energy Matters; 20 May 2016

Last week’s post on The Energy return of Solar PV caused quite a stir. Yesterday I received a response to some of the comments from Ferroccio Ferroni and Robert Hopkirk addressing some of the queries raised by readers. Their response is given below the fold. But first I have a few comments to add.

Energy payback time (EPBT) and energy return on energy invested (EROI) of solar photovoltaic systems: A systematic review and meta-analysis Khagendra P. Bhandari, Jennifer M. Collier, Randy J. Ellingson, Defne S. Apul; Renewable and Sustainable Energy Reviews; 28 Feb 2015

There is a fast growing interest in better understanding the energy performance of PV technologies as evidenced by a large number of recent studies published on this topic. The goal of this study was to do a systematic review and a meta-analysis of the embedded energy, energy payback time (EPBT), and energy return on energy invested (EROI) metrics for the crystalline Si and thin film PV technologies published in 2000–2013. A total of 232 references were collected of which 11 and 23 passed our screening for EPBT/EROI and embedded energy analysis, respectively. Several parameters were harmonized to the following values: Performance ratio (0.75), system lifetime (30 years), insolation (1700 kWh m2 yr1), module efficiency (13.0% mono-Si; 12.3% poly-Si; 6.3% a:Si; 10.9% CdTe; 11.5% CIGS). The embedded energy had a more than 10-fold variation due to the variation in BOS embedded energy, geographical location and LCA data sources. The harmonization narrowed the range of the published EPBT values. The mean harmonized EPBT varied from 1.0 to 4.1 years; from lowest to highest, the module types ranked in the following order: cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), amorphous silicon (a:Si), poly-crystalline silicon (poly-Si), and mono-crystalline silicon (mono-Si). The mean harmonized EROI varied from 8.7 to 34.2. Across different types of PV, the variation in embedded energy was greater than the variation in efficiency and performance ratio suggesting that the relative ranking of the EPBT of different PV technology today and in the future depends primarily on their embedded energy and not their efficiency.

The Destruction of Scottish Power Posted on January 6, 2016 by Euan Mearns

How Long Does It Take to Pay Off a Tesla Powerwall?

kite generator / high altitude

The ERoEI of High Altitude Wind Power Euan Mearns; Energy Matters; 29 Jun 2016 For several weeks I have been researching and writing a review post on high altitude wind power. It has grown into a 6000 word monster that should hopefully fly on Monday. While doing this it has been difficult to find time to write other posts. Hence this is a preview of one section on Energy Return on Energy Invested (ERoEI) which makes a nice post in its own right. KiteGen have presented a back of the envelope style ERoEI calculation for their 3 MW stem indicating a value of 562 which is incredibly high. I have done my own calculation using a variant of their methodology and my own input variables. The idea is to try and estimate the energy intensity of a wind turbine structure and to interpolate that into a KiteGen stem. This involves making many weak assumptions but should be good for arriving at a ball park number.


Negative Electricity Prices Are Not A Sign Of Renewable Success Michael Lynch; Forbes; 19 Feb 2016

Advocates of wind and solar power often point to low or negative prices for electricity in wholesale markets with a heavy reliance on those sources as representing success, but this reflects their ignorance of the utility system and basic economics and is misleading as to the true cost of power from these sources.
a report from Der Spiegel in September 2013: “For society as a whole, the costs have reached levels comparable only to the euro-zone bailouts. This year, German consumers will be forced to pay €20 billion ($26 billion) for electricity from solar, wind and biogas plants — electricity with a market price of just over €3 billion.”
In Texas, problems caused by excess windpower were reduced by construction of $7 billion worth of new transmission lines

Germany's Energy Poverty: How Electricity Became a Luxury Good staff; Der Speigel; 4 Sep 2013

Germany's agressive and reckless expansion of wind and solar power has come with a hefty pricetag for consumers, and the costs often fall disproportionately on the poor


Energy Analysis of Power Systems (World Nuclear Association)

  • Life Cycle Analysis, focused on energy, is useful for comparing net energy yields from different methods of electricity generation.
  • Nuclear power shows up very well as a net provider of energy, and only hydro electricity is closely comparable.
  • External costs, evaluated as part of life cycle assessment, strongly favour nuclear over coal-fired generation.
  • Energy Return on (energy) Investment is a way of measuring relative inputs and outputs.


The Catch-22 of Energy Storage John Morgan; Brave New Climate; 22 Aug 2014

Several recent analyses of the inputs to our energy systems indicate that, against expectations, energy storage cannot solve the problem of intermittency of wind or solar power. Not for reasons of technical performance, cost, or storage capacity, but for something more intractable: there is not enough surplus energy left over after construction of the generators and the storage system to power our present civilization.
The problem is analysed in an important paper by Weißbach et al.

Energy intensities, EROIs, and energy payback times of electricity generating power plants D. Weißbach, G. Ruprecht, A. Huke, K. Czerski, S. Gottlieb, A. Hussein; ; 6 Apr 2013 [preprint]

The Energy Returned on Invested, EROI, has been evaluated for typical power plants representing wind energy, photovoltaics, solar thermal, hydro, natural gas, biogas, coal and nuclear power. The strict exergy concept with no ”primary energy weighting”, updated material databases, and updated technical procedures make it possible to directly compare the overall efficiency of those power plants on a uniform mathematical and physical basis. Pump storage systems, needed for solar and wind energy, have been included in the EROI so that the efficiency can be compared with an ”unbuffered” scenario. The results show that nuclear, hydro, coal, and natural gas power systems (in this order) are one order of magnitude more effective than photovoltaics and wind power.

safety / deaths per TWh

IPCC wg3 ch 7 p550

figures for ff, re, nuclear etc

Fossil fuels are far deadlier than nuclear power Phil McKenna; New Scientist; 23 Mar 2011

A 2002 review by the IAE put together existing studies to compare fatalities per unit of power produced for several leading energy sources. The agency examined the life cycle of each fuel from extraction to post-use and included deaths from accidents as well as long-term exposure to emissions or radiation. Nuclear came out best, and coal was the deadliest energy source. The explanation lies in the large number of deaths caused by pollution. “It’s the whole life cycle that leads to a trail of injuries, illness and death,” says Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School. Fine particles from coal power plants kill an estimated 13,200 people each year in the US alone, according to the Boston-based Clean Air Task Force (The Toll from Coal, 2010). Additional fatalities come from mining and transporting coal, and other forms of pollution associated with coal. In contrast, the International Atomic Energy Agency and the UN estimate that the death toll from cancer following the 1986 meltdown at Chernobyl will reach around 9000.


DEAD IN FIRE WIND TURBINE OOLTGENSPLAAT Audrey Graanoogst; NLtimes; 30 Oct 2013

A wind turbine caught fire Tuesday afternoon in Ooltgensplaat on Goeree-Overflakkee, costing the lives of two mechanics.Four mechanics were at work in the wind turbine on the Mariadijk, about 80 meters above ground, Tuesday afternoon. By a cause, yet unknown, a fire started in the engine room. Two mechanics managed to get themselves to safety in time, reported a police spokesperson. Rescuers found the body of a deceased mechanic next to the wind turbine on the ground.

This Photo Of Two Engineers Hugging Atop A Burning Turbine Before They Died Is Heart-Breaking! archipelagofiles

This photograph tugs at your heartstrings. Two engineers hug and cling to each other as fire and smoke creeps toward them. They both died after. According to news reports, one of them jumped off the turbine while the other succumbed to the fire. What makes this more heartbreaking is that the two engineers are just aged 19 and 21.


Solar panel installer dies after falling through roof Installer Online; Feb 2015?

A solar panel installer has died after he fell through a barn roof. His employer Eco Generation Ltd has been fined £45,000 after it was found the company had failed to provide vital safety equipment. Gregorz Sobko had been working on the roof of a cowshed when one of the clear plastic panels, designed to let in light, gave way. The 34-year-old from Southport fell five metres to the concrete floor below and died in hospital ten days later.


Banqiao Dam wikipedia

The Banqiao Reservoir Dam is a dam on the River Ru in Zhumadian City, Henan province, China. Its failure in 1975 caused more casualties than any other dam failure in history. It was subsequently rebuilt. The Banqiao dam and Shimantan Reservoir Dam are among 62 dams in Zhumadian that failed catastrophically or were intentionally destroyed in 1975 during Typhoon Nina. These dam failures killed an estimated 171,000 people; 11 million people lost their homes. It also caused the sudden loss of 18 GW of power, the power output equivalent of roughly 9 very large modern coal-fired thermal power stations.

The Catastrophic Dam Failures in China in August 1975 San José State University Department of Economics

Sources: Yi Si, "The World's Most Catastrophic Dam Failures: The August 1975 Collapse of the Banqiao and Shimantan Dams," in Dai Qing, The River Dragon Has Come!, M.E. Sharpe, New York, 1998.

2009 Sayano–Shushenskaya power station accident wikipedia

The 2009 Sayano–Shushenskaya hydroelectric power station accident occurred at 00:13 GMT on 17 August 2009, (08:13 AM local time) when turbine 2 of the Sayano–Shushenskaya hydroelectric power station near Sayanogorsk in Khakassia, Russia, broke apart violently. The turbine hall and engine room were flooded, the ceiling of the turbine hall collapsed, 9 of 10 turbines were damaged or destroyed, and 75 people were killed.


Coal-burning EU countries make their neighbours sick WWF; 5 Jul 2016

Coal pollution and its health impacts travel far beyond borders, and a full coal phase-out in the EU would bring enormous benefits for all citizens across the continent. That is according to a new report published today by the Health and Environment Alliance (HEAL), Climate Action Network (CAN) Europe, the WWF European Policy Office and Sandbag. The report, ‘Europe’s dark cloud: How coal-burning countries make their neighbours sick’, analyses the health impacts from air pollution of all EU coal-fired power stations for which data is available (257 out of 280). It reveals that in 2013 their emissions were responsible for over 22,900 premature deaths, tens of thousands of cases of ill-health from heart disease to bronchitis, and up to EUR 62.3 billion in health costs.

Report: Germany suffers more coal-linked deaths than rest of EU James Crisp;; 5 Jul 2016

Germany – home to the much-hailed ‘Energiewende’ green revolution – suffered more premature deaths linked to coal plant pollution than any other EU member state, research by health and environment campaigners has found. Analysis of 257 of 280 coal-fired power plants in the EU found that their 2013 emissions caused over 22,900 deaths, tens of thousands of illnesses from heart disease to bronchitis, and up to €62.3 billion in health costs. 3,630 people in Germany died from coal-related illnesses in 2013, according to the report by the Health and Environment Alliance, Climate Action Network Europe, WWF European Policy Office and Sandbag.

5 years after coal-ash spill, little has changed Duane W. Gang; USA TODAY; 23 Dec 2013

A dike failure at TVA's Kingston Fossil Plant led to the largest spill in history.

Other resources

EIA Glossary of energy-related terms

Energy Education website