The majority of green pricing programs charge a higher price per kilowatt-hour to support an increased percentage of renewable sources or to buy discrete kilowatt-hour blocks of renewable energy. Other programs have fixed monthly fees, round up customer bills, charge for units of renewable capacity, or offer renewable energy systems for lease or purchase.


In 2010, the International Energy Agency predicted that global solar PV capacity could reach 3,000 GW or 11% of projected global electricity generation by 2050—enough to generate 4,500 TWh of electricity.[40] Four years later, in 2014, the agency projected that, under its "high renewables" scenario, solar power could supply 27% of global electricity generation by 2050 (16% from PV and 11% from CSP).[2]
Some of the second-generation renewables, such as wind power, have high potential and have already realised relatively low production costs. At the end of 2008, worldwide wind farm capacity was 120,791 megawatts (MW), representing an increase of 28.8 percent during the year,[30] and wind power produced some 1.3% of global electricity consumption.[31] Wind power accounts for approximately 20% of electricity use in Denmark, 9% in Spain, and 7% in Germany.[32][33] However, it may be difficult to site wind turbines in some areas for aesthetic or environmental reasons, and it may be difficult to integrate wind power into electricity grids in some cases.[10]
Solar electricity is inherently variable and predictable by time of day, location, and seasons. In addition solar is intermittent due to day/night cycles and unpredictable weather. How much of a special challenge solar power is in any given electric utility varies significantly. In a summer peak utility, solar is well matched to daytime cooling demands. In winter peak utilities, solar displaces other forms of generation, reducing their capacity factors.
Biomass, biogas and biofuels are burned to produce heat/power and in doing so harm the environment. Pollutants such as sulphurous oxides (SOx), nitrous oxides (NOx), and particulate matter (PM) are produced from this combustion; the World Health Organisation estimates that 7 million premature deaths are caused each year by air pollution.[102] Biomass combustion is a major contributor.[102][103][104]

Wind energy research dates back several decades to the 1970s when NASA developed an analytical model to predict wind turbine power generation during high winds.[136] Today, both Sandia National Laboratories and National Renewable Energy Laboratory have programs dedicated to wind research. Sandia’s laboratory focuses on the advancement of materials, aerodynamics, and sensors.[137] The NREL wind projects are centered on improving wind plant power production, reducing their capital costs, and making wind energy more cost effective overall.[138] The Field Laboratory for Optimized Wind Energy (FLOWE) at Caltech was established to research renewable approaches to wind energy farming technology practices that have the potential to reduce the cost, size, and environmental impact of wind energy production.[139] The president of Sky WindPower Corporation thinks that wind turbines will be able to produce electricity at a cent/kWh at an average which in comparison to coal-generated electricity is a fractional of the cost.[140]
A wide range of concentrating technologies exists: among the best known are the parabolic trough, the compact linear Fresnel reflector, the Stirling dish and the solar power tower. Various techniques are used to track the sun and focus light. In all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.[11] Thermal storage efficiently allows up to 24-hour electricity generation.[12]
A recent UK Government document states that "projects are generally more likely to succeed if they have broad public support and the consent of local communities. This means giving communities both a say and a stake".[194] In countries such as Germany and Denmark many renewable projects are owned by communities, particularly through cooperative structures, and contribute significantly to overall levels of renewable energy deployment.[195][196]
The PV industry is beginning to adopt levelized cost of electricity (LCOE) as the unit of cost. The electrical energy generated is sold in units of kilowatt-hours (kWh). As a rule of thumb, and depending on the local insolation, 1 watt-peak of installed solar PV capacity generates about 1 to 2 kWh of electricity per year. This corresponds to a capacity factor of around 10–20%. The product of the local cost of electricity and the insolation determines the break even point for solar power. The International Conference on Solar Photovoltaic Investments, organized by EPIA, has estimated that PV systems will pay back their investors in 8 to 12 years.[73] As a result, since 2006 it has been economical for investors to install photovoltaics for free in return for a long term power purchase agreement. Fifty percent of commercial systems in the United States were installed in this manner in 2007 and over 90% by 2009.[74]
Solar power - The most prevalent type of renewable energy, solar power is typically produced using photovoltaic cells, which capture sunlight and turn it into electricity. Solar energy is also used to heat buildings and water, provide natural lighting and cook food. Solar technologies have become inexpensive enough to power everything from small hand-held gadgets to entire neighborhoods.
With Georgetown emerging as a brave new model for a renewable city, it makes sense to ask if others can achieve the same magical balance of more power, less pollution and lower costs. In fact, cities ranging from Orlando to St. Louis to San Francisco to Portland, Oregon, have pledged to run entirely on renewable energy. Those places are much larger than Georgetown, of course, and no one would expect misty Portland to power a light bulb for long with solar energy, which is crucial to Georgetown’s success. But beyond its modest size, abundant sunshine and archetype-busting mayor, Georgetown has another edge, one that’s connected to a cherished Lone Star ideal: freedom.
Due to increased technology and wide implementation, the global glass fiber market might reach US$17.4 billion by 2024, compared to US$8.5 billion in 2014. Since it is the most widely used material for reinforcement in composites around the globe, the expansion of end use applications such as construction, transportation and wind turbines has fueled its popularity. Asia Pacific held the major share of the global market in 2014 with more than 45% volume share. However China is currently the largest producer. The industry receives subsidies from the Chinese government allowing them to export it cheaper to the US and Europe. However, due to the higher demand in the near future some price wars have started to developed to implement anti dumping strategies such as tariffs on Chinese glass fiber.[58]
Most small wind turbines do not perform quite as well as their manufacturers want you to believe. That should come as no surprise at this point. What may be surprising is that even the turbines of the more honourable manufacturers that are honest about performance fall short, more often than not. The likely cause is turbulence and improper site selection.
The generator, which is approximately 34% of the wind turbine cost, includes the electrical generator,[38][39] the control electronics, and most likely a gear box (e.g. planetary gear box),[40] adjustable-speed drive or continuously variable transmission[41] component for converting the low-speed incoming rotation to high-speed rotation suitable for generating electricity.
Common battery technologies used in today's home PV systems include, the valve regulated lead-acid battery– a modified version of the conventional lead–acid battery, nickel–cadmium and lithium-ion batteries. Lead-acid batteries are currently the predominant technology used in small-scale, residential PV systems, due to their high reliability, low self discharge and investment and maintenance costs, despite shorter lifetime and lower energy density. However, lithium-ion batteries have the potential to replace lead-acid batteries in the near future, as they are being intensively developed and lower prices are expected due to economies of scale provided by large production facilities such as the Gigafactory 1. In addition, the Li-ion batteries of plug-in electric cars may serve as a future storage devices in a vehicle-to-grid system. Since most vehicles are parked an average of 95 percent of the time, their batteries could be used to let electricity flow from the car to the power lines and back. Other rechargeable batteries used for distributed PV systems include, sodium–sulfur and vanadium redox batteries, two prominent types of a molten salt and a flow battery, respectively.[114][115][116]
Commercial concentrated solar power plants were first developed in the 1980s. As the cost of solar electricity has fallen, the number of grid-connected solar PV systems has grown into the millions and utility-scale solar power stations with hundreds of megawatts are being built. Solar PV is rapidly becoming an inexpensive, low-carbon technology to harness renewable energy from the Sun.

Although many older thermoelectric power plants with once-through cooling or cooling ponds use more water than CSP, meaning that more water passes through their systems, most of the cooling water returns to the water body available for other uses, and they consume less water by evaporation. For instance, the median coal power plant in the US with once-through cooling uses 36,350 gal/MWhr, but only 250 gal/MWhr (less than one percent) is lost through evaporation.[139] Since the 1970s, the majority of US power plants have used recirculating systems such as cooling towers rather than once-through systems.[140]

The incentive to use 100% renewable energy, for electricity, transport, or even total primary energy supply globally, has been motivated by global warming and other ecological as well as economic concerns. The Intergovernmental Panel on Climate Change has said that there are few fundamental technological limits to integrating a portfolio of renewable energy technologies to meet most of total global energy demand. Renewable energy use has grown much faster than even advocates anticipated.[148] At the national level, at least 30 nations around the world already have renewable energy contributing more than 20% of energy supply. Also, Professors S. Pacala and Robert H. Socolow have developed a series of "stabilization wedges" that can allow us to maintain our quality of life while avoiding catastrophic climate change, and "renewable energy sources," in aggregate, constitute the largest number of their "wedges".[149]
In 2006 California approved the 'California Solar Initiative', offering a choice of investment subsidies or FIT for small and medium systems and a FIT for large systems. The small-system FIT of $0.39 per kWh (far less than EU countries) expires in just 5 years, and the alternate "EPBB" residential investment incentive is modest, averaging perhaps 20% of cost. All California incentives are scheduled to decrease in the future depending as a function of the amount of PV capacity installed.
Projections vary. The EIA has predicted that almost two thirds of net additions to power capacity will come from renewables by 2020 due to the combined policy benefits of local pollution, decarbonisation and energy diversification. Some studies have set out roadmaps to power 100% of the world’s energy with wind, hydroelectric and solar by the year 2030.
Go-anywhere rechargeable battery pack keeps your handheld gear Go-anywhere rechargeable battery pack keeps your handheld gear going strong. Charge AA/AAA batteries from the sun or any USB port then power your phone MP3 GPS or perk up your tablet in a pinch. Kit included Nomad 7m v2 Solar Panel and Guide 10 Plus power pack. This ultra-lightweight kit ...  More + Product Details Close
The trouble with rated power is that it does not tell you anything about energy production. Your utility company charges you for the energy you consume, not power. Likewise, for a small wind  turbine you should be interested in the energy it will produce, for your particular site, with your particular annual average wind speed. Rated power of the turbine does not do that. To find out about energy production take a look at the tables presented earlier.
The home wind Generator systems are designed for reliable power output for the next 30 years or so. With every price increase of the utility company power your investment gets better all the time. Utility costs are rising all over and will accelerate over the next few years. We expect the cost of electricity to rise and double over Obamas term in office due to cap and trade and increased regulation and market pressure.

The journal, Renewable Energy, seeks to promote and disseminate knowledge on the various topics and technologies of renewable energy systems and components. The journal aims to serve researchers, engineers, economists, manufacturers, NGOs, associations and societies to help them keep abreast of new developments in their specialist fields and to apply alternative energy solutions to current practices.


Taken together, the generation and distribution of electric power in the United States is an astonishingly complex undertaking. Utilities may generate their own power or buy it from other utilities; that power travels over a grid of transformers and high- and low-voltage lines to your house. Ownership of utilities varies from nonprofits to cooperatives to for-profits. Federal regulators ultimately oversee the grid. Amazingly, when you flip a switch, electricity is there.

Most installers overrate the available wind resource. The majority of small wind turbine installations underperforms their predictions, often by a wide margin. Since wind speed is the most important parameter for turbine energy production, getting that wrong has large consequences (the power in the wind goes with the cube of the wind speed, so double the wind speed and the power in it is 2 * 2 * 2 = 8x as much). You have to be realistic about your annual average wind speed.
In an electricity system without grid energy storage, generation from stored fuels (coal, biomass, natural gas, nuclear) must be go up and down in reaction to the rise and fall of solar electricity (see load following power plant). While hydroelectric and natural gas plants can quickly follow solar being intermittent due to the weather, coal, biomass and nuclear plants usually take considerable time to respond to load and can only be scheduled to follow the predictable variation. Depending on local circumstances, beyond about 20–40% of total generation, grid-connected intermittent sources like solar tend to require investment in some combination of grid interconnections, energy storage or demand side management. Integrating large amounts of solar power with existing generation equipment has caused issues in some cases. For example, in Germany, California and Hawaii, electricity prices have been known to go negative when solar is generating a lot of power, displacing existing baseload generation contracts.[107][108]
Since having the Peimar Solar Panels installed and listening to the advice of the owner I have saved a lot of money on my electric bill. Texas Solar Integrated did the work as quickly and efficiently as promised. If the panels look dirty, since I live around cement plants, I just get my high pressure water hose and spray them off. Thank you to this company and the installers. The owner or another contractor in the office is always ready to answer your questions before and after installation.... read more
Any solar PV system that’s tied to the grid will use a bi-directional meter. When you use electricity from the grid, you’ll see your meter move forward. But when your solar PV system produces electricity, any excess will go back into the grid and your meter will move backward. This is called “net metering,” and the utility company will credit your bill for the excess electricity generated.
Where the reputable, and more expensive manufacturers are good in honouring their warranties, you are likely on your own with the cheap stuff. Even with a good warranty, take our word for it that you would much rather not make use of it. Even if the manufacturer supplies replacement parts, it is still expensive to install them. Not to mention that your turbine will not be making energy meanwhile.
If you can turn a wrench and operate an electric drill, you can build this simple generator in two days: one day for chasing down parts, and one day for assembling the components. The four major components include a vehicle alternator with a built-in voltage regulator, a General Motors (GM) fan and clutch assembly (I used one from a 1988 GM 350 motor), a tower or pole on which to mount the generator (15 feet of used 2-inch tubing cost me $20), and the metal to build a bracket for mounting the generator on the tower or pole. If you’re a Ford guy or a Mopar gal, that’s fine — just make sure your alternator has a built-in voltage regulator. You’ll also need some electrical cable or wires to hook the alternator up to your storage batteries. I used 8-gauge, 3-conductor cable pilfered from the oil patch. (And they said the transition from fossil fuels to renewables would take years. Pfft!)
On most horizontal wind turbine farms, a spacing of about 6–10 times the rotor diameter is often upheld. However, for large wind farms distances of about 15 rotor diameters should be more economical, taking into account typical wind turbine and land costs. This conclusion has been reached by research[62] conducted by Charles Meneveau of the Johns Hopkins University,[63] and Johan Meyers of Leuven University in Belgium, based on computer simulations[64] that take into account the detailed interactions among wind turbines (wakes) as well as with the entire turbulent atmospheric boundary layer.
With Georgetown emerging as a brave new model for a renewable city, it makes sense to ask if others can achieve the same magical balance of more power, less pollution and lower costs. In fact, cities ranging from Orlando to St. Louis to San Francisco to Portland, Oregon, have pledged to run entirely on renewable energy. Those places are much larger than Georgetown, of course, and no one would expect misty Portland to power a light bulb for long with solar energy, which is crucial to Georgetown’s success. But beyond its modest size, abundant sunshine and archetype-busting mayor, Georgetown has another edge, one that’s connected to a cherished Lone Star ideal: freedom.

Single small turbines below 100 kilowatts are used for homes, telecommunications dishes, or water pumping. Small turbines are sometimes used in connection with diesel generators, batteries, and photovoltaic systems. These systems are called hybrid wind systems and are typically used in remote, off-grid locations where a connection to the utility grid is not available.

Dale Ross, the mayor of Georgetown, Texas, has a big smile, a big handshake and a big personality. In last year’s election, he won big, with 72 percent of the vote. The key to his success? “Without being too self-reflective,” he says, “I just like people.” He’s a Republican, and his priorities are party staples: go light on regulation, be tough on crime, keep taxes low. But the thing that is winning him international renown is straight out of the liberal playbook—green power. Thanks to his (big) advocacy, Georgetown (pop. 67,000) last year became the largest city in the United States to be powered entirely by renewable energy.


Run-of-the-river hydroelectricity plants derive energy from rivers without the creation of a large reservoir. The water is typically conveyed along the side of the river valley (using channels, pipes and/or tunnels) until it is high above the valley floor, whereupon it can allowed to fall through a penstock to drive a turbine. This style of generation may still produce a large amount of electricity, such as the Chief Joseph Dam on the Columbia river in the United States.

A wind turbine is made up of two major components and having looked at one of them, the rotor blade design in the previous tutorial, we can now look at the other, the Wind Turbine Generator or WTG’s which is the electrical machine used to generate the electricity. A low rpm electrical generator is used for converting the mechanical rotational power produced by the winds energy into usable electricity to supply our homes and is at the heart of any wind power system.

I mounted this turbine in my back yard on the recommended schedule 40 galvanized pipe at about 20' high. My location does not get consistent wind from one direction which is the only way this turbine will spin. Even in gusty conditions of 15-20 mph the turbine rarely spins more than a few revolutions and has not produced any measurable power after a month. If you don't have a steady wind from one direction this turbine will not produce any power at all. You would be better off with a vertical turbine or one with larger blade surface area. The specs say 8 mph start up, that means a consistent 8 mph wind from a single direction. For the money you would be better off with a single 80 watt solar panel.
Prior to the development of coal in the mid 19th century, nearly all energy used was renewable. Almost without a doubt the oldest known use of renewable energy, in the form of traditional biomass to fuel fires, dates from 790,000 years ago. Use of biomass for fire did not become commonplace until many hundreds of thousands of years later, sometime between 200,000 and 400,000 years ago.[31] Probably the second oldest usage of renewable energy is harnessing the wind in order to drive ships over water. This practice can be traced back some 7000 years, to ships in the Persian Gulf[32] and on the Nile.[33] Moving into the time of recorded history, the primary sources of traditional renewable energy were human labor, animal power, water power, wind, in grain crushing windmills,[32] and firewood, a traditional biomass. A graph of energy use in the United States up until 1900 shows oil and natural gas with about the same importance in 1900 as wind and solar played in 2010.
At GE, product evolution is at our core, and we are continuously working to develop the next generation of wind energy. Beginning in 2002 with one wind turbine model, we now offer a full suite of turbines created for a variety of wind environments. We offer increased value to customers with proven performance, reliability, and availability. Our portfolio of turbines feature rated capacities from 1.7 MW to 5.3 MW (Onshore) and 6 MW to 12 MW (Offshore), we are uniquely suited to meet the needs of a broad range of wind regimes. 
Some of the second-generation renewables, such as wind power, have high potential and have already realised relatively low production costs. At the end of 2008, worldwide wind farm capacity was 120,791 megawatts (MW), representing an increase of 28.8 percent during the year,[30] and wind power produced some 1.3% of global electricity consumption.[31] Wind power accounts for approximately 20% of electricity use in Denmark, 9% in Spain, and 7% in Germany.[32][33] However, it may be difficult to site wind turbines in some areas for aesthetic or environmental reasons, and it may be difficult to integrate wind power into electricity grids in some cases.[10]
Biomass, biogas and biofuels are burned to produce heat/power and in doing so harm the environment. Pollutants such as sulphurous oxides (SOx), nitrous oxides (NOx), and particulate matter (PM) are produced from this combustion; the World Health Organisation estimates that 7 million premature deaths are caused each year by air pollution.[102] Biomass combustion is a major contributor.[102][103][104]
Technology advances are opening up a huge new market for solar power: the approximately 1.3 billion people around the world who don't have access to grid electricity. Even though they are typically very poor, these people have to pay far more for lighting than people in rich countries because they use inefficient kerosene lamps. Solar power costs half as much as lighting with kerosene.[136] As of 2010, an estimated 3 million households get power from small solar PV systems.[137] Kenya is the world leader in the number of solar power systems installed per capita. More than 30,000 very small solar panels, each producing 1[138]2 to 30 watts, are sold in Kenya annually. Some Small Island Developing States (SIDS) are also turning to solar power to reduce their costs and increase their sustainability.

Green power is a subset of renewable energy and represents those renewable energy resources and technologies that provide the highest environmental benefit. The U.S. Environmental Protection Agency defines green power as electricity produced from solar, wind, geothermal, biogas, biomass and low-impact small hydroelectric sources. Customers often buy green power for avoided environmental impacts and its greenhouse gas reduction benefits.[9]
In Denmark by 1900, there were about 2500 windmills for mechanical loads such as pumps and mills, producing an estimated combined peak power of about 30 (MW). The largest machines were on 24-meter (79 ft) towers with four-bladed 23-meter (75 ft) diameter rotors. By 1908 there were 72 wind-driven electric generators operating in the United States from 5 kW to 25 kW. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year, mostly for water-pumping.[9]
Materials that are typically used for the rotor blades in wind turbines are composites, as they tend to have a high stiffness, high strength, high fatigue resistance, and low weight.[46] Typical resins used for these composites include polyester and epoxy, while glass and carbon fibers have been used for the reinforcing material.[47] Construction may use manual layup techniques or composite resin injection molding. As the price of glass fibers is only about one tenth the price of carbon fiber, glass fiber is still dominant.

In 2015, Ross wrote an op-ed for Time magazine about his city’s planned transition to renewables. “A town in the middle of a state that recently sported oil derricks on its license plates may not be where you’d expect to see leaders move to clean solar and wind generation,” he wrote. Lest readers get the wrong idea, he felt compelled to explain: “No, environmental zealots have not taken over City Council.”
Energy harnessed by wind turbines is intermittent, and is not a "dispatchable" source of power; its availability is based on whether the wind is blowing, not whether electricity is needed. Turbines can be placed on ridges or bluffs to maximize the access of wind they have, but this also limits the locations where they can be placed.[72] In this way, wind energy is not a particularly reliable source of energy. However, it can form part of the energy mix, which also includes power from other sources. Notably, the relative available output from wind and solar sources is often inversely proportional (balancing)[citation needed]. Technology is also being developed to store excess energy, which can then make up for any deficits in supplies.

Because one obstacle to adopting wind and solar power is reliability—what happens on calm, cloudy days?—recent improvements in energy-storage technology, a.k.a. batteries, are helping accelerate adoption of renewables. Last May, for example, Tucson Electric Power signed a deal for solar energy with storage, which can mitigate (if not entirely resolve) concerns about how to provide power on gray days. The storage upped the energy cost by $15 per megawatt hour. By the end of the year, the Public Service Company of Colorado had been quoted a storage fee that increased the cost of a megawatt hour by only $3 to $7, a drop of more than 50 percent. In a landmark achievement, Tesla installed the world’s largest lithium-ion battery in South Australia last December, to store wind-generated power. But by then Hyundai Electric was at work in the South Korean metropolis of Ulsan on a battery that was 50 percent bigger.

In 2015, Ross wrote an op-ed for Time magazine about his city’s planned transition to renewables. “A town in the middle of a state that recently sported oil derricks on its license plates may not be where you’d expect to see leaders move to clean solar and wind generation,” he wrote. Lest readers get the wrong idea, he felt compelled to explain: “No, environmental zealots have not taken over City Council.”
Through collaboration, smaller buyers can benefit from economies of scale, while larger buyers can continue to see cost benefits while achieving their renewable energy goals. Aggregation allows companies to procure in a mutually beneficial way with relatively little give and take. For that reason, RMI believes this marks “the beginning of a trend,” Haley said.  
Those not satisfied with the third-party grid approach to green energy via the power grid can install their own locally based renewable energy system. Renewable energy electrical systems from solar to wind to even local hydro-power in some cases, are some of the many types of renewable energy systems available locally. Additionally, for those interested in heating and cooling their dwelling via renewable energy, geothermal heat pump systems that tap the constant temperature of the earth, which is around 7 to 15 degrees Celsius a few feet underground and increases dramatically at greater depths, are an option over conventional natural gas and petroleum-fueled heat approaches. Also, in geographic locations where the Earth's Crust is especially thin, or near volcanoes (as is the case in Iceland) there exists the potential to generate even more electricity than would be possible at other sites, thanks to a more significant temperature gradient at these locales.
It is hard to beat the advantages of solar: No moving parts. Warranties of 25 years are common for PV modules. No maintenance, other than the occasional hosing-off if you live in a dusty place. The installed price of a 6 kW wind turbine on a good height tower is about $50,000 (and we are not even counting the money you are going to sink into maintenance of that wind turbine). At the time of this writing, half that money will buy you about 7 kW of installed solar panels. In our not-so-sunny Ottawa location those solar modules will produce around 8,000 kWh of electrical energy per average year, and they will do that for 30 years or more.
It is possible to use any type of solar thermal panel (sheet and tubes, roll-bond, heat pipe, thermal plates) or hybrid (mono/polycrystalline, thin film) in combination with the heat pump. The use of a hybrid panel is preferable because it allows covering a part of the electricity demand of the heat pump and reduce the power consumption and consequently the variable costs of the system.
Manufacturers often claim that their vertical axis turbine is superior to a horizontal one, because it always faces the wind. So does any horizontal axis turbine, thanks to their tail or yaw mechanism. If the airflow is such that wind directions change drastically from one second to the next it means you have lots of turbulence, and that means it is a poor place to put any wind turbine, HAWT or VAWT.
Efficiency can decrease slightly over time, one of the main reasons being dust and insect carcasses on the blades which alters the aerodynamic profile and essentially reduces the lift to drag ratio of the airfoil. Analysis of 3128 wind turbines older than 10 years in Denmark showed that half of the turbines had no decrease, while the other half saw a production decrease of 1.2% per year.[19] Ice accretion on turbine blades has also been found to greatly reduce the efficiency of wind turbines, which is a common challenge in cold climates where in-cloud icing and freezing rain events occur.[20] Vertical turbine designs have much lower efficiency than standard horizontal designs.[21]
The overwhelming majority of electricity produced worldwide is used immediately, since storage is usually more expensive and because traditional generators can adapt to demand. However both solar power and wind power are variable renewable energy, meaning that all available output must be taken whenever it is available by moving through transmission lines to where it can be used now. Since solar energy is not available at night, storing its energy is potentially an important issue particularly in off-grid and for future 100% renewable energy scenarios to have continuous electricity availability.[106]
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