We will experience some solar PV installations and technology while traveling, so I provide some more details about it below.
- In order to maximize panel output, the panel should be as close to perpendicular to the sun's rays as possible, which allows it to capture the most solar energy possible at a given time. (Here is a really good interactive animation of why a perpendicular panel captures the most sunlight.) Most solar panels are fixed, i.e., they don't move. This means that you need to choose your location carefully. For fixed panels, there are two factors to consider in this regard:
- Azimuth/orientation is the compass direction that the panel faces. Typically, 0º is due north, 90º is east, 180º is south, and 270º is west.
- Tilt is the angle above the horizon to which the panel is tilted.
- Shading is a very important consideration as well. All else being equal, more shading means less output.
- In the Northern Hemisphere, the rule of thumb to maximize the output of a fixed panel is that the panel should be faced due south (180º orientation) and at "latitude tilt." You may recall from a Geography class that latitude is how far north or south a location is from the equator. The equator is 0º, the tropics are at 23.5º, and the North and South Poles are at 90º north and south, respectively.
- For example, State College, PA is at just over 40º north, so the ideal array tilt is about 40º, and the ideal azimuth is 180º (south).
- Anchorage, Alaska is at about 61º north, so the ideal tilt and azimuth are about 61º and 180º (south), respectively.
A few more terms that are important to know:
- A solar cell is the smallest current-generating part of an array. They can be any size but are normally around 6 inches by 6 inches. It's difficult to see, but in the image above there are 72 cells in each panel (12 rows, each with 6 cells).
- A solar panel (aka module) is a number of cells wired together in a single panel. They can be any size but are usually about 5 feet by 3.25 feet.
- A solar array is a number of panels wired together. The image above shows part of a solar array, which as I noted has 270 total panels.
- Capacity refers to the rated maximum output of a panel or array. Under optimal conditions, a 250 W panel will output about 250 W of electricity. Note that this is only the immediate output. By the time the electricity goes to the building or the grid, some of it is lost due to a variety of factors (usually 10% - 15%).
- The array above has 270 panels, and each panel has a capacity of 305 W. Thus, the capacity of the array is (305 W x 270 = ) 82,350 W, which is 82.35 kW.
- A solar panel outputs DC (direct current) electricity. DC means that the electricity flows in one direction. Household outlets (and the electric grid) use AC (alternating current) electricity, which rapidly alternates direction of flow (60 times per second in the U.S., in case you are interested), so electricity from an array must be converted to AC if it is to be used in a building or distributed to the grid. An inverter is a piece of equipment that converts DC to AC and is a standard part of most solar arrays.
- A grid-tied array is one that is connected to the grid. A standalone or off-grid system is not tied to the grid.
The Impact of Tilt and Orientation
Recall that the rule of thumb is that the optimal tilt is "latitude tilt," and the ideal orientation in the Northern Hemisphere is due south (180º). This begs the question: what happens if the tilt and orientation are not optimal? The answer, as you might guess, is "it depends." This impact can be quantified by something that is called tilt and orientation factor (TOF). The tilt and orientation factor is a decimal that indicates what percent of the maximum solar output you would receive throughout the year at said tilt and orientation. So if you install an array and it has a TOF of 0.85, that means that it will only be able to output about 85% of the energy it would output if it were at the ideal tilt and orientation.
Wilmington, Delaware is at about 40º north. As it turns out, the ideal tilt is closer to 35º (rules of thumb are only rules of thumb, after all!). The tables below show the TOF at different tilts and azimuths. The first table illustrates the TOFs of different tilts, all with an orientation of 180º. The second table shows the TOFs at different azimuths, all at a tilt of 35º (the ideal tilt).
|Tilt (º)||Azimuth (º)||TOF|
|Tilt (º)||Azimuth (º)||TOF|
Calculating Solar Output
Okay, now we're ready to calculate the solar output. There are a number of software programs and a formula or two that can do this, but the National Renewable Energy Laboratory (NREL) provides a free one that is well-regarded in the energy industry called PVWatts. In the video below, I demonstrate how to calculate the annual output of the array in the images above, which has the following specs:
- address: 400 Stanton Christiana Rd, Newark, DE
- 82.35 kW capacity
- 13º tilt
- 222º azimuth
A Few More Notes
Hopefully, by now you have a relatively good grasp on some of the considerations that go into designing and calculating the output of solar PV. Solar PV really took off in the early- to mid-2000s, led by residential array installations, which generally had capacities of a few kW. The solar industry in the U.S. is now dominated by utility-scale solar, which is much cheaper per W to construct because of economies of scale. Utility-scale arrays can be thousands of watts (multiple MWs) in capacity!
There are a few ways that people can use and pay for solar PV:
- Systems can be purchased and owned by individuals. This can be done out-of-pocket or using loans. Out-of-pocket purchases usually generate the highest return.
- Power purchase agreements allow people or businesses to pay a third party to build an installation on their own roof. The individual or business then pays the third party for the electricity generated at a contractually agreed-upon rate. This is the model that SolarCity (now Tesla Energy) used to rise to prominence. This is a popular model because it requires no up-front cost (among other reasons).
- Community-owned solar or just community solar is used increasingly in the U.S. and elsewhere. Community solar allows individuals to "buy into" a solar array that is installed somewhere else, or perhaps on a shared roof or field space. This provides access to individuals and businesses that would not otherwise be able to use solar due to factors such as not having a suitable site (e.g., a heavily shaded roof) or living in rented space. (The SEIA provides a good overview of community solar here, and NREL provides a more robust explanation here.)
- Most utility-scale installations effectively act as power plants, and the electricity is sold to the grid.
Incentives have played a very important role in the growth of the solar industry, and renewable energy in general. The following provides a description of some incentives.
States and countries have implemented a variety of policies meant to incentivize or encourage private investment in clean, renewable energies. The most common of these policies are tax credits, grants/rebates, and performance-based incentives (PBI), including feed-in tariffs (FIT) and renewable portfolio standards (RPS).
A tax credit is just that, a credit. When an individual or business investor earns a tax credit it means that the amount of the credit will be subtracted from a future tax bill. For example, in the United States, we have a Federal Residential Renewable Energy Tax Credit available to the residential (not commercial or industrial) sector which provides a tax credit covering 26% of the cost of an installation. A typical residential system costs about $3/Watt to install. If you put a 7 kW photovoltaic system on your roof, it should cost around $21,000 before incentives (7,000 W x $3/W). If so, you would earn a $5,460 tax credit. The government doesn’t mail you a check for this amount. It means you get to deduct that amount from your next tax payment. To realize this money, you will need to have paid at least $5,460 in taxes, but excess credits can "generally" be carried over to future tax years. Note that even if you were owed a refund, this tax credit can be used to increase your refund, as long as you paid at least $5,460 in federal income tax throughout the year. Other renewable energy technologies qualify for this credit. See the link above for details.
A rebate means that a government agency or other group (sometimes utility companies) will refund some of the investment. These are usually based on the size/capacity of the system. For example, Pennsylvania used to provide a solar rebate program that provided rebates to investors of $1.75/Watt. Rebates are checks mailed directly to the investor (or their designate). Many states still have such programs such this solar PV rebate program in Oregon(description from DSIRE of course!). Different states often have different program specifics. See DSIRE for more examples of programs. Specifics vary within states as well. The program in Oregon, for example, has different rebate levels for different utilities and for different sectors (residential, agricultural, industrial, non-profit, government). For example, if you are a residential customer of the PGE utility in Oregon, you would receive $0.25/W up to $1,750. So for a 7 kW system you would recieve a $1,750 (7,000 W x $0.25/W) rebate.
Performance-based incentives (PBIs), also known as production incentives, provide cash payments based on the actual output of the system. For wind and solar electric, this is the number of kilowatt-hours (kWh) generated. This is usually only applied to utility-scale systems.
Rnewable portfolio standard (RPS) are very imnportant policies. Breifly, an RPS requires utilities to use renewable energy credits (RECs) to account for a certain percentage of their retail electricity sales. (RPSs are established by state legislatures, and not all states have an RPS policy.) A REC is earned by a qualified grid-tied facility for every 1,000 kWh (i.e., 1 MWh) of electricity that is generated using a renewable energy resource. The RECs are then bought and sold through a market. The settlement price varies depending on REC supply and demand at any point in time, though special auctions with guaranteed pricing and incentives are sometimes used. If you live in a state that has an RPS policy, you can "sell" your credits on an annual basis and receive a payment. For example, if you generate 7,000 kWh in a years and you can sell your credits for $40/MWh, you would earn $280 (7,000 kWh = 7 MWh x $40/MWh). You would receiv a check for this at the end of the year. See below for states that have RPS policies.
Another type of production-based incentive, a feed-in-tariff (FIT) pays grid-tied renewable energy generators a specified price for the electricity they generate and guarantees them this price for a specified amount of time. This type of policy is widely used in Europe, most notably in Germany, but less so in the USA. This is also usually used on utility-scale systems.