A reader (whose identity we are not going to disclose since they chose to contact us through the Contact Us form) asked whether there we have info on how much energy is used in manufacturing of solar and wind systems, the production of the components and the installation, full cost of maintaining a system like that.

Very interesting question. It refers to the so called net energy balance or net energy result – the difference between energy invested in the creation of an energy source and the useful energy it brings back.

This indeed is an interesting area for research and will definitely get on SolarByTheWatt.Com’s priority list.

The net energy balance is specifically interesting topic with fuels because they are intended to be used once per unit of their quantity – i.e. every pound, gallon, ton, kilogram or whatever unit is used for to measure the quantity of the fuel is only used once to bring useful energy in certain amount (kWh, a galon of gas equivalent of energy, BTU, a barrel of oil worth of energy – whatever unit we use).

It was a very controversial topic when ethanol was becoming popular as a new fuel and there were studies and comments that a gallon of ethanol (or the equivalent of a gallon of gas worth of ethanol) needs more than a gallon of gas to be produced. So though economic efficiency can be achieved, either through temporary market prices differences or subsidies, in reality we can not efficiently convert the raw solar energy stored in biomass (corn, other feedstock for ethanol) in to a usable fuel.

Economic efficiency (see our article on economics of solar) definitely has similarities with the net energy balance issues.  A strongly economically efficient technology will definitely have a positive net energy balance since the cost to produce the energy producing facility (fuel or electricity generating plant) will partially cover the cost of used energy and more. If the economic efficiency is close to a net zero, the way it is with solar at the moment where economic efficiency is still achieved only where there are subsidies, cheap capital (low interest loans) and plenty of good insolation, the net energy balance has to be questioned.

Related sources:

Grain ethanol strikes back | Cleantech Group

Green news harvest: Recycling solar, electric car calculator ..

Greenline » Villa Åkarp – A Positive Net Energy House in Malmö, Sweden

Radical New Hypothesis on the Effect of Greenhouse Gases

The Conservation Imperative

The Energy Blog: Maize for Biofuels: The Ultimate Energy Crop

Renewables & biofuels – Apr 13 | Energy Bulletin

Estimating the Net Energy Balance of Corn Ethanol

Is ethanol energy-efficient?: Journey to Forever

Estimating the Net Energy Balance of Corn Ethanol

The Oil Drum | The 2009 EIA Energy Conference: Day 1

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SolarByTheWatt.Com Solar By The Watt Dot Com

(jump directly to calculation tables)

We want to look at a very simple model of economic efficiency where are we interested in the annual return on investment. A project involving a solar photovoltaic installation requires certain amount of capital to be built and will expect certain amount of annual revenue. The simple indicator of efficiency that we want to study is this annual return on invested capital.

There are more complex models that also include the cost of capital. In a model that studies the cost of the electricity produced for example the cost of capital and factored in return for the investor could be very important factors. Cost of capital on its own can be a complex concept too. Whether the capital is borrowed and the cost will be defined by the interest rate of the loan or is cash participation from the investor and the cost will be defined by other opportunities the investor has to invest in (including risk free investments) or the combination of the two, cost of capital is very significant factor in such models.

We simplify our model to the level of looking at potential return from invested amount of capital. Further investment efficiency study and study on the cost of production of electricity should involve cost of capital.

We will also try to come to tables of calculated efficiency depending on different factors but not the size of the installation. The size of the installation definitely impacts the efficiency of a project. This is the traditional concept of economy if scales. A larger project will be expected to be more efficient over all because of the spread fixed expenses that the project has. But we will be building our model around the idea of dollar per watt, dollar per watt peak, so that it is applicable to different. The efficiency that comes from scale can be reflected in the factors going in to our model – specifically cost to build.

Another factor not included in our model is the maintenance cost of the solar installation. We are hoping that the reader can find resources which will indicate dollar per watt peak what the expected cost to maintain the station are.

The factors that expect to influence our model are: cost to build the power installation, insolation at the geographic location, electricity market price.

The cost to build the power installation includes:

• the price of the core photovoltaic components employed: modules, panels, arrays (what exactly list of such components we deal with depends on the design and configuration of the installation),
• the price of the mounting equipment (trackers and, or positioning frames) and wiring;
• price of inverters and grid connection equipment
• service and labor cost to assemble
• management and administration costs and costs of permits

The insolation at the geographic location. The insolation is the amount of solar light energy falling on the surface of the earth at a given geographic location per certain amount of area in the span of certain unit of time. More specifically the unit of measure used is watt per square meter (W/m²) or also kilowatt hour per square meter per day (kWh/m²/day). Insolation in Europe, just for reference purposes, approximately, ranges between 1,000 to 2,000 kWh per year per installed kWp, in the continental US – between 800 to 2,100 kWh per year per installed kWp for different regions. Exact numbers can be found in different reference tables and insolation maps on the internet and elsewhere.

In the photovoltaic industry a measure of insolation is employed which helps us more directly figure out how much electricity in kilowatt hours (kWh) we can produced annually from an installed nominal watt (Wp) – that is kilowatt hours annually per installed kWp (kWh/kWp/y).

The market price of the electricity produced. Since we are trying to figure out economic efficiency – annual return on investment we need to involve in the model:

• the price of the electricity we produce and sell to the grid

or conversely

• the price of the amount of electricity we offset that we were otherwise going to pay for at such market price.

In many places in the world – by country or by specific territory (state, other subdivision) there are financial incentives for investors in photovoltaic power stations in the way of a special feed-in-tariff. These usually are higher price the grid pays to energy producers who utilize photovoltaic power stations. If we have to give some examples these countries have the following feed-in-tariffs (unit of measure is Euros per kWh since classically many of the notable incentives started in Europe and that is the currency used there): Austria 0.57 – 0.59, Germany 0.52 – 0.57, Italy, 0.36 – 0.49, France 0.30 – 0.55, Spain 0.19 – 0.44, Switzerland 0.10 – 0.50, Israel 0.14 – 0.16, Bulgaria 0.40.

The formula.

The annual return on investment in a solar power station will be the annual income from sold (or offset electricity) divided by the amount of the investment.

Annual return on investment = annual income ($) / investment amount ($).

The annual income is the market price of the electricity in dollar ($) per kilowatt hour (kWh) times the amount of energy produced in an year in kilowatt hours (kWh).

Annual income ($) = production per year (kWh) x market price ($/kWh)

The production per year is the solar insolation in kilowatt hours per year per kilowatt peak installed times the installed capacity/power in kilowatt peak (kWp).

Production per year (kWh) = insolation (kWh/y/kWp) x installed capacity (kWp) .

So that we can work on dollar per watt basis we will express the investment amount by the investment cost per watt.

Investment amount ($) = total cost per watt ($/Wp) x installed capacity (kWp) x 1,000

– the 1,000 factor  is to adjust the dollar per watt unit to the kWp in which we are measuring the installed power.

So the complete formula for the annual return on investment will be:

Annual return on investment = annual income ($) / investment amount ($)

If we use the formula for the annual income we get:

= production per year (kWh) x market price ($/kWh) / (total cost per watt ($/Wp) x installed capacity (kWp) x 1,000)

If we use the formula for the production per year we come to:

= insolation (kWh/y/kWp) x installed capacity (kW) x market price ($/kWh) / (total cost per watt ($/Wp) x installed capacity (kWp) x 1,000)

Here we can get rid of the installed capacity (kWp) in the numerator and denominator and get to:

Annual return on investment
= insolation (kWh/y/kWp)   x   market price ($/kWh)
/  ( total cost per watt ($/Wp)   x   1,000 )

Let’s use the formula in an example where $7 per watt were spend in the building of the station, the insolation is 1,200 kWh per installed kWp and there is a $0.52 feed-in-tariff for the electricity produced.

Annual return on investment = 1,200 x 0.52 / (7 x 1,000) = 624 / 7,000 = 8.92%.

This calculated rate now can be compared to cost of capital and maintenance cost can be factored in to look for some more complete number of real economic return.

In addition to this formula the following tables can be used to get to approximate numbers:

The first table is given for insolation of 1,000 (this can be easily factored if we know the specific insolation, i.e. for 1,500 we just have to multiply by 1.5, for 2,200 we simply multiply by 2.2, etc.)

The investment cost per watt is given in a very wide range of $3 to $10 per watt but it is not implied that these are possible, practical or easy to obtain prices in reality. Our research shows that large industrial project can reach down to $6 per watt of such cost and smaller commercial and residential projects can go up to $10 and even $15 per watt peak installed.

The price for kWh of electricity is given in a range of $0.05 to $0.60 to reflect different market prices for electric energy and all possible feed in tariffs.

Annual Return on Investment At Insolation of 1,000 kWh/kWp:

Annual Return on Investment At Insolation of  1,500 kWh/kWp:

Annual Return on Investment At Insolation of 2,000 kWh/kWp:

To factor in the cost of capital  in the numbers from these tables one needs to subtract the cost of capital as a percentage. For example if the cost of capital is 4%  that has to be subtracted from the numbers in the tables above. Similar adjustment can be made for to factor in the maintenance costs.

Configuration efficiency, (performance ratio). We do not know whether this term is discussed in other research in length but we want to mention it for this reason: projects with equal labeled peak power – watt peak (Wp), nominal power, if taken directly from the labeled peak power of the modules involved can have different overall output electricity production – kilowatt hour (kWh) per year at the same geographic location. This could be because of how different modules perform when positioned with trackers or mounted stationary, or how well they perform in different cloud conditions etc. etc. So, realistically the nominal installed power (Wp) that can be used with insolation numbers to figure out annual production can be different for different installations mainly because of the type of modules used a the specific conditions (insolation, cloudiness, positioning etc.) For the different projects this factor has to be specifically researched based on data from similar configuration used in existing projects. By information we can get to such factor can vary between 0.75 and 1.4 – i.e. the electricity produced in a year from the installation in kWh may vary from the above calculation for known insolation from –25% to +40% from the indicated.

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Resources:

http://en.wikipedia.org/wiki/Photovoltaic

http://en.wikipedia.org/wiki/Insolation

http://en.wikipedia.org/wiki/Feed-in_Tariff

http://en.wikipedia.org/wiki/PV_financial_incentives

First lets look at what we are using currently to meet our energy needs, coal and nuclear energy, and how ecologically damaging they are.

To start, most existing coal plants release many different toxins directly into the air we breathe, from sulfur to lead and mercury. Even the newer plants coming on line, which reduce toxins dramatically, still produce massive quantities of CO2, a greenhouse gas and a direct cause of global warming. Natural gas is far more benign but still produces large quantities of CO2 when used to produce electricity. How much CO2 is released – a 5-kilowatt solar system will prevent the release of nearly 10,400 pounds of CO2 every year for the life of the system. The average home uses 8,000-kilowatt hours per year. A coal power plant producing that much electricity emits about 18,000 pounds of CO2 per year.

When coal is burned, carbon dioxide, sulfur dioxide, nitrogen oxides, and mercury compounds are released. For that reason, coal-fired boilers are required to have control devices to reduce the amount of emissions that are released. Mining, cleaning, and transporting coal to the power plant generate additional emissions. For example, methane, a potent greenhouse gas that is trapped in the coal, is often vented during these processes to increase safety.

Secondly, large quantities of water are needed to remove impurities from coal at the mine. These large quantities of water are used for producing steam and for cooling systems. When coal-fired power plants remove water from a lake or river, the fish and other aquatic life can be affected, as well as animals and people who depend on these aquatic resources. At the same time, pollutants build up in the water used by the power plant boiler and cooling system. If the water used in the power plant is discharged to a lake or river, the pollutants in the water can harm fish and plants.

Last, the burning of coal creates solid waste, called ash, which is composed primarily of metal oxides and alkali. On average, the ash content of coal is 10 percent. Solid waste is also created at coal mines when coal is cleaned and at power plants when air pollutants are removed from the stack gas. Much of this waste is deposited in landfills and abandoned mines, although some amounts are now being recycled into useful products, such as cement and building materials.

So, you may say, what about nuclear power? While nuclear power plants do not emit carbon dioxide, sulfur dioxide, or nitrogen oxides – fossil fuel emissions are associated with the uranium mining and uranium enrichment process as well as the transport of the uranium fuel to the nuclear plant. Nuclear power plants also use large quantities of water for steam production and for cooling. When nuclear power plants remove water from a lake or river, fish and other aquatic life can be affected. Water pollutants, such as heavy metals and salts build up in the water used in the nuclear power plant systems. These water pollutants, as well as the higher temperature of the water discharged from the power plant, can negatively affect water quality and aquatic life.

Waste generated from uranium mining operations and rainwater runoff can contaminate groundwater and surface water resources with heavy metals and traces of radioactive uranium. Every 18 to 24 months, nuclear power plants must shut down to remove and replace the “spent” uranium fuel. This spent fuel has released most of its energy as a result of the fission process and has become radioactive waste.

All of the nuclear power plants in the United States together produce about 2,000 metric tons per year of radioactive waste. Currently, the radioactive waste is stored at the nuclear plants at which it is generated, either in steel-lined, concrete vaults filled with water or in above-ground steel or steel-reinforced concrete containers with steel inner canisters. This waste will remain radioactive for many thousands of years.

As you can see, the more solar power is used, the more it helps our environment. By investing in solar today you are investing in your future and your children’s future and you do this by combating global warming and reduce our nation’s dependence of foreign energy sources. And, you are helping in the reduction of CO2 emissions and protecting clean water sources. It’s amazing how such a small change in one’s life can do so much. ARI Green Energy is a manufacturer of wind generator technologies. Visit them today for a full line of wind turbines and solar technology solutions. Think green.

Article Source: http://www.articlealley.com/article_830778_45.html

About the Author:  Author: Robert Bell

Alternative energy sources like solar power and wind power, can reduce the burden on non-renewable resources. There are many things you can do as a home owner or user of energy to conserve energy, save the environment, and save some money too. Here are a few ideas to consider.

Solar water heaters can save you a lot of money on the energy used to heat your water for showers, laundry, dishes, and everything else you need heated water for. They are more expensive to install than a traditional natural gas or electric water heater, but over time they can pay for themselves with the energy savings you will achieve. You can either buy a water heater designed to use solar energy or you can modify your existing water heater to take advantage of the suns rays to heat your water. This is a really great use of the suns energy!

If you happen to have a swimming pool which uses a heater, you can also install a solar water heater on that system. These systems are so cost effective that if you live in an area where you can get sun to the heating system, you are foolish not to use solar energy to heat your pool up. If you really want to be environmentally friendly, make sure you convert to a salt water pool to avoid the chlorine and install a solar panel to produce electricity for your pump. Once you do that, you can have an eco-friendly pool that is powered completely by alternative solar energy.

Lighting your home is another simple way to use the suns rays to reduce your consumption of electricity. During the day, make sure your blinds are open to let the sunlight in and provide some heat. You can also install skylights or light tubes that will let sunlight into your house so you can turn off some lights. Alternatively, you can keep your blinds closed during the summer to block the heat from coming in to help keep your house cool. Just use a little common sense on when you want to let the light and heat in and when you want to keep the heat out. This use of solar energy costs almost nothing to use unless you decide to go all out and install a bunch of skylights. It might not even cost you anything if you already have blinds on your windows.

One last bit of alternative solar energy to talk about is solar panels. These come in many sizes and do things as simple as store energy during the day to power your sidewalk lights at night or as complex as harness the suns rays to create electricity to power your house. The technology is the same. Basically you convert solar energy into electrical energy and store it in a battery or series of batteries for use later. Large solar panels can be quite expensive, but with the cost of electricity today, there can be a very substantial payback on your monthly electricity bill. If you really produce a lot of electricity, you can even sell some of it back to the electricity company in some areas.

Now you might be asking what you can do if you cannot install solar panels or modify your house or apartment to put in a solar water heater or skylights. You may be able to purchase environmentally friendly energy from your electric company. In many areas, electric companies offer a green energy option which promotes the use of green or renewable energy like alternative solar energy. One such company that offers a green energy option to electricity customers in Texas and New York is Ambit Energy. Check with your company to see if you have the option to select green energy or check out the cheap electricity Ambit Energy offers.

Now that you know a little more about what is out there as far as renewable energy, go start saving money! Check out the video below for a little more information on saving energy and money!

We want to jump here directly to a graph comparing the different levels of electricity capacity currently generated from oil, coal, natural gas and the rest (nuclear, hydro etc) with the capacity wind power fields, farms need, the cost of investment needed and the land area used.

The assumptions are $1.3 per installed watt – much less than solar for now. And 3.8 acres per 1.5MW turbine – this is only the land used by the actual turbines and utility roads etc. not the actual land required for the field in which land can be used for agriculture or other purposes.

Click here or on the image to see full resolution graph.

The highest level displayed is not that important because it already includes renewable energy and nuclear (the argument about how clean it is and it’s safety are a subject of a separate discussion). Natural gas though not renewable is clean and still in abundant quantities in reserves.  The more important part of the analysis is the lower levels – oil and coal.  Look at how affordable the numbers look. Compare to the money currently the government is spending on trying to jump-start the economy anyway.

The scale is also not impossible to achieve quickly. Wind as solar is an accelerating, fast growing industry.  Just to create the correct impression here of how quickly the first level – oil can be covered – it requires about 56 GW of wind power. Only in 2007 the newly installed wind power is 5.3 GW (see resources used at the bottom). So even at the same rate of investment, construction we only need 10 years to replace oil used for electricity. This is not counting the potential of solar, other renewables and the potential for the very likely accelerated addition of new wind capacity every year.

Resources Used

US Department of Energy
Energy Efficiency and Renewable Energy (EERE)
http://www1.eere.energy.gov/windandhydro/pdfs/43025.pdf

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Physical efficiency for example is the measure of what portion of the light energy is being transformed to useful electric energy. That currently runs for different types of photovoltaic cell technologies between 8 and 30 and by some reports 38%. We will spare a separate article on the different technologies and their respective efficiency readings.

Economic efficiency or investment efficiency is the dollar cost per nominal generating watt power or simply dollar per watt peak ($/Wp). The investment efficiency can be calculated at the different stage of completeness of a solar photovoltaic system or component – cells, panels, installed systems, power stations. We want to dedicate special studies to the importance of these measures of efficiency. Be prepared to hear our opinion that economic, or investment efficiency is much more important to the useful implementation of this technology than physical efficiency is.

Land area use efficiency is the measure of how much land area is needed for certain capacity of nominal (peak) power of a solar photovoltaic power station (or smaller system). The units of measure used are usually:

acres per megawatt or acres per MW – the land area needed in acres per MW of generation; the way to calculate that is to divide the area used in a project in acres by the peak power of the project in MW;

kilowatt per acre or kW per acre – this is the amount of installed peak power in kW (1 MW = 1,000 kW) divided by the land area needed in acres.

The equivalent units of measure used more often overseas (incl. Europe) are using the derivatives of the metric system for land area – hectares (10,000 sq.meters) and decares (1,000 sq.meters) – hectares per MW, decares per MW and kW or MW per hectare or decare.

We wanted to find out what is the approximate land area efficiency of existing completed and planed projects or ones in progress. We looked first at some of the biggest solar photovoltaic projects around the world, tried to find out quoted figures for their peak capacity and land are used and calculate the land are use efficiency for them.

We also looked at publications discussing proposed project especially for technologies that are just emerging.

This is what we found out.

Some of the largest completed solar photovoltaic power plants, also called solar parks or fields, have area efficiency of about 4.5 to 8 acres per MW. This is when crystalline (mono-crystalline or poly-crystalline) technology cells, modules are used. With thin-film technologies the efficiency is about 10 to 13.5 acres per MW.

For comparison we also looked at data for some solar thermal project and we estimated land area use efficiency of about 5 to 11 acres per MW. The lower values were for projects involving Sterling engines as opposed to the more traditional technology of parabolic mirror throws heating oil in a tube.

Here is a table of our calculations with numbers in the other units of measure and also a list of the resources used.

Resources used:

http://en.wikipedia.org/wiki/Moura_photovoltaic_power_station

http://www.solarpay.com.au/mega-watts-to-nega-watts.html

http://www.jccm.es/fondosestructurales/fileadmin/user_upload/_temp_/00-NOTICIAS/2008_MARZO/07-03-08_LA_TRIBUNA-Renovalia_invierte_en_parque_solar_Puertollano.pdf

http://www.bizjournals.com/sanjose/stories/2009/02/09/daily59.html?ana=from_rss

http://www.gizmag.com/go/4422/

http://www.renewableenergyworld.com/rea/news/article/2007/07/pg-e-signs-agreement-with-solel-for-553-mw-of-solar-power-49448

http://www.pv-tech.org/news/_a/first_solar_finishes_10_mw_pv_power_plant_for_sempra_pge_to_buy_electricity/

http://www.abengoasolar.es/sites/solar/en/technologies/photovoltaic/concentration/index.html

http://www.energy.ca.gov/reti/steering/2008-06-18_meeting/SOLAR_FS-Utility-Scale_Photovoltai

http://i-r-squared.blogspot.com/2008/08/massive-solar-pv-deal.html

http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel3/60/654/00017907.pdf?arnumber=17907

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SolarByTheWatt.com

We want to look at the needs of energy of the United States and the world and compare with the practical size installations of solar power stations needed to replace that.

The question of completely replacing all fossil fuel or all other sources of electricity and other energy is much more complex than just looking at the numbers. Probably replacing the other sources in just electricity production is simpler but still in addition to simply adding capacity, changes of large scale are needed in the national grid configuration.

Replacing resources used to produce other energy, other than electricity, is even more complex. That will mean changing how energy is being transported and used. While electricity is a type of energy easy to transport what may  need to be changed is the way it is utilized at the end consumer – industrial, residential or transportation. That will involve changes and costs again more than the ones related to replacing the source with solar power stations.

The current electricity generation capacity in the US is about 1TW (1,000,000 MW). About 395 GW (1GW = 1,000 MW) is from natural gas, 315 GW from coal, 100 GW from nuclear, 100 GW from hydroelectric 56GW from oil products, 30 GW from renewables  other than hydro, and other small components.

We will look at these numbers separately to see what it will take to replace the more urgent ones of them – coal, oil etc.

The US total energy consumption, not just electricity is 3.5 TW. Most of the difference between just electricity and this number is made up of energy produced from oil so we are not going to try to break this down. We will look at this as one of our theoretical targets.

The world total energy consumption is at around 16 TW of which 4.5TW is electricity.

So, lets build a scale of the different stages that theoretically can be achieved in solar replacing other energy sources.

For our calculations we will assume installation / construction cost for one 1MW of $5m. This is a number ($5/Wp) we think smaller than the recent historic numbers for cost to install solar capacity. But we want to factor for future falling prices of solar modules and other components and overall improving efficiency in the industry. Anyone is free to factor the $ numbers we have if they believe $7/Wp or $10Wp is a better number.

For surface area needs we will assume 200 kW per acre. With different design and technology 800kW per acre is possible but we will go with the low number on this to be on the safe side. Since 1 sq mile is 640 acres, with our assumption we will have 128 MW per sq mile.

We tried to display this comparison between energy currently produced by different sources and capacity needed in solar in gigawatts (GW), cost of  installation in billion dollars ($bn), land area needed in sq. miles, marks of different countries GDP, and land areas of different geographic locations (certain US states in this case).

Click here or on the graph below to see it in full scale.

On the above diagram, since it uses a linear scale for the capacity, cost and land and the large values (world energy needs) are much larger than the smaller ones (oil and coal for electricity in the US), the small values are not given a good presentation. So here we put the same numbers on a logarithmic scale for the solar capacity, cost and land requirements. We have much better clarity on the comparisons now for the smaller values.

Click here or on the graph below to see it in full scale.

We think in general our study is much more important in reference to the smaller values. They deal with some levels of scale of capacity that are practically easier to achieve in terms of replacing with solar. We are talking about the unquestionably very polluting coal and oil burning for electricity. Again – look at the numbers to replace oil products used for electricity generation in the US we only need $280b and spending that will create jobs, help growth in technology and give all sorts of other economic and social benefits.

The numbers given in our study in reference to the higher levels – world electricity and total energy consumption are simply for getting a better view of the comparative size of all levels.

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newnet news – Scalable electric power from solar energy

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Resources:

http://www.eia.doe.gov/cneaf/electricity/epa/epa_sum.html

http://en.wikipedia.org/wiki/Energy_use_in_the_United_States

http://www.eia.doe.gov/oiaf/ieo/ieoecg.html

http://en.wikipedia.org/wiki/List_of_U.S._states_by_area

http://en.wikipedia.org/wiki/List_of_countries_and_outlying_territories_by_area

http://en.wikipedia.org/wiki/List_of_metropolitan_areas_by_population

http://en.wikipedia.org/wiki/List_of_countries_by_GDP_(nominal)

SolarByTheWatt.com

What do we mean? Much too often watt hour (Wh) and the derivatives kilowatts hour (kWh) and megawatts hour (MWh) – as a measure of produced energy – are used wrongly instead of watt (W) or it’s derivatives kilowatt (kW) and megawatt (MW) and the other way around.

Let’s deal first with the well known kilo- and mega- prefixes. These are used to make a 1,000 and 1,000,000 of any unit. So 1kW equals 1,000W and 1,000,000W equals 1,000kW which also equals 1MW. Same goes for Wh – 1MWh = 1,000 kWh = 1,000,000 Wh. This was the easy part – kilo = thousands, mega = millions.

Now let’s look at the difference between W (watt) and Wh (watt hour). This is what confuses people more.

The watt (W) is a unit of measure for the power output a machine can produce. Strictly speaking from science point of view this is the amount of work (as in the scientific term) a machine can do (the amount of energy it can output) in a unit of time. Since the more standard scientific units for work (energy) and time are joule (J) and second (s) the standard unit of measure for power is actually a joule per second  (J/s). That is what is used more often in reference to mechanical devices and to chemical phenomena. In electricity the equivalent  is Watt (W) and mathematically 1W is exactly equal 1J/s. When a machine outputs 1 joule of energy every second it is said that the machine is working at 1W power.

So, a watt (W) is a measure of power – the ability for a machine to do work or produce energy in an amount of time. Another way of looking at it is as the speed at which a machine can output energy. If it can output energy at a rate of 1 joule per second that is power of 1 watt.

This is why power and watts (W), correspondingly kilowatts (kW) and megawatts (MW) should be seen as the potential of a machine to output certain amount of energy per unit of time. In energy production industries, like the solar energy industry, machines/generators (solar power stations, any other solar system) are given a number called watts peak (Wp) which is the nominal power they can output. So if a system is rated at 500 Wp that means the system can produce 500 joules of energy per second. If it is easier for you to think of energy produced per hour you have to multiply by the number of seconds in an hour – 36,600.

For those who are good at volts (V) and amperes (A) – if a generators outputs 1A current at 1V voltage the machine is working at 1W. Yes, that simple – power in watts is equal voltage in volts by current in amps.

The other unit – watt hour (Wh) is the unit of measure for amount of energy. If a machine with a power of 1W works for an hour it will produce 1Wh of energy. Now, wait a second, why did we suddenly switch from seconds to hours. Well if in the previous statement we replaced the word hour with second everything would have been correct but the unit of measure for amount of energy would also have to be watt second Wc – this is not used practically. The unit in use in the energy industry and electricity is watt hour (Wh). This is also how usually electricity is sold and priced. If you look at your utility bill you will see you are paying certain cents per Wh, well actually to confuse you now more you are paying cents per kWh but you already know why the k- is in front – the price is for 1,000 watt hours not for one small watt hour.

If we have to look again at the example with volts and amperes – if an electrical system is outputting 1A of electric current at 1V of voltage it outputs 1W of power. If that same system  works for 1 hour it would produce 1Wh energy, for 1,000 hours – 1kWh. At the regular household voltage of 110V a light bulb that uses 0.5A current consumes 55W (110V x 0.5A), in a thousand hours it will burn 55kWh and if you are paying $0.09 for kWh that will cost you $4.95.

Also in respect of generating electricity by solar panels, module – if a solar panel’s rating is at 100W and 25V that means at peak conditions the panel will be outputting 4A current (25Vx4A=100W) and every hour of peak performance will be producing 100W x 1h = 100Wh energy.

OK,  so let’s summarize and see how we should correctly use the units of measure.

Power is measured in watts (W) and systems usually have a rated peak power in watts (W). This of course could be in kilowatts (kW) and megawatts (MW) if it is a large number. Power should be perceived as the rate at which it produces energy (joules per second).

So correct expressions with it will be: “… the station has peak power of 300kW … ” or “.. the solar park’s capacity is 2MW … ” etc.

Energy, or amount of energy, is measured in watt hours (Wh) or the corresponding kilowatt hour (kWh) and megawatt hour (MWh) and it is the useful amount of work output by the system. This is why consumers or the grid pays in cents per watt hour (Wh). If a system works at 100 kW power for an hour it will produce 100 kWh, if this is paid at $0.09 per kWh that would earn $9.00.

Correct expressions using energy and Wh will be: “… the system will produce 500 MWh per year …” or “… the system will generate 700kWh per day …”

What are examples of wrongly put statements. “The power station will produce 15MW per year”. MW is power it is the rate at which energy is produced so it can not be per year, or per day, or per hour. If we know it will work at a capacity of 15MW we should simply say “… will produce 15MW …” may be add “…of electric power…”.

If we want to say “per year” (or per other period) we should calculate what the expected nominal insulation is (in the case of solar power stations and working, up-hours hours for other systems) for that period (that will usually be hours for that period, like in 1,400 hours insolation per year), multiply by the nominal power in watts and we will get watt hours. So then we can say “… the system will generate 700 MWh of electric energy per year…”.

Also often in news reports journalists will write that certain system will be able to power that many households. And this is the correct expression – “able to power (or feed with electricity) certain number of households”. A household needs certain power in kilowatts (kW) let’s say 4kW. So a 400kW station will be bale to feed 100 households. The incorrect statement is “… the system will power 100 households per year …”. We do not need “per year”. It will power the 100 house holds as long as it is operational – 10 years, 20 years – whatever the plan is.

As with all articles on SolarByTheWatt.com, please, do comment on this one, especially if you have recommendation of what to improve to make the content more useful and readable for you.

Related resources:

Wikipedia – Watt

Wikipedia – Kilowatt Peak

Wikiepedia – Kilowatt Hour

How Stuff Works – Amps, Watts and Volts

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In the latest version of the Resources page we have interesting reference to information on UL listing of solar equipment.

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