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Solar Photovoltaic - Considering a Small Electric System

Renewable Energy > Solar Thermal

Solar Thermal | Evaluating Your Site | Maintenance & Repair | Solar Pool Heating |
Solar Water Heaters | Siting a Solar Collector | Codes & Regulations | Things to Consider |

Considering a Small Solar Electric System
To help evaluate whether a small solar electric system will work for you, you should consider the following:
Your available solar resource—do you have clear and unobstructed access to sunlight for most or all of the day, throughout the year?
The system size—do you have a roof or area large enough to accommodate it?
The economics—is it worth the investment?
Local permits and covenants—are there any issues with installing a system?
 
 
Evaluating Your Site's Solar Resource for Solar Electricity
The solar resource across the U.S. is ample for solar electric systems—also known as photovoltaic (PV) systems—because they can use both direct and scattered sunlight. However, the amount of electricity generated at a particular site depends on how much of the sun's energy reaches it. Thus, PV systems function most efficiently in the southwestern United States, which receives the greatest amount of solar energy.
Before you buy a PV system, you'll want to be sure your site has enough solar energy to meet your electricity needs efficiently and economically. Your local system supplier can perform a solar site analysis for you or show you how to do so on your own.
When evaluating your site, you'll also need to consider both the geographic orientation and the tilt of your solar panels—PV modules—as both can affect your system's performance.
 
Solar Radiation Basics
Solar radiation is a general term for the electromagnetic radiation emitted by the sun. We can capture and convert solar radiation into useful forms of energy, such as heat and electricity, using a variety of technologies. The technical feasibility and economical operation of these technologies at a specific location depends on the available solar radiation or solar resource.
Basic Principles
Every location on Earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one "spot" on the Earth's surface varies according to these factors:
  • Geographic location
  • Time of day
  • Season
  • Local landscape
  • Local weather.
Because the Earth is round, the sun strikes the surface at different angles ranging from 0º (just above the horizon) to 90º (directly overhead). When the sun's rays are vertical, the Earth's surface gets all the energy possible. The more slanted the sun's rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year.
The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth's surface receives a little more solar energy. The Earth is nearer the sun when it's summer in the southern hemisphere and winter in the northern hemisphere. However the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.
The 23.5º tilt in the Earth's axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other six months. Days and nights are both exactly 12 hours long on the equinoxes, which occur each year on or around March 23 and September 22.
Countries like the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun's rays are far more slanted during the shorter days of the winter months. Cities like Denver, Colorado, (near 40º latitude) receive nearly three times more solar energy in June than they do in December.
The rotation of the Earth is responsible for hourly variations in sunlight. In the early morning and late afternoon, the sun is low in the sky. Its rays travel further through the atmosphere than at noon when the sun is at its highest point. On a clear day, the greatest amount of solar energy reaches a solar collector around solar noon.
Diffuse and Direct Solar Radiation
As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by the following:
    • Air molecules
    • Water vapor
    • Clouds
    • Dust
    • Pollutants
    • Forest fires
    • Volcanoes.
This is called diffuse solar radiation. The solar radiation that reaches the Earth's surface without being diffused is called direct beam solar radiation. The sum of the diffuse and direct solar radiation is called global solar radiation. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days.
Measurement
Scientists measure the amount of sunlight falling on specific locations at different times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total radiation on a horizontal surface, or as total radiation on a surface tracking the sun.
Radiation data for solar electric (photovoltaic) systems are often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may also be expressed as watts per square meter (W/m2).
Radiation data for solar water heating and space heating systems are usually represented in British thermal units per square foot (Btu/ft2).
 
Sizing Your Small Solar Electric System
Accurately sizing the components of your solar electric system, also known as a photovoltaic (PV) system, helps ensure that your system will produce the amount of power you want it to produce. This is especially important for stand-alone systems, which are not connected to the electricity grid. However, because PV is modular, you can always add to your solar energy collector should you need more power down the road.
First, consider what portion of your current electricity needs you would like your PV system to meet. For example, suppose that you would like to meet a certain percentage of your electricity needs with your PV system. You could work with your PV provider to examine past electric bills and determine the size of the PV system needed to achieve that goal. You can contact your utility company and request the total electricity usage, measured in kilowatt-hours (kWh), for your household or business over the past 12 months or consult your electric bills if you save them.
If you reduce your electricity loads, you can generally buy a smaller, less expensive PV system.
In addition to how much electricity you'd like to generate, the size of your system also depends on these factors:
The site's solar resource or available sunlight
The system's orientation and tilt
The system's efficiency at converting sunlight to electricity
Other electricity sources, like a utility, a wind turbine, or a fossil fuel generator.
PV systems are classified by their rated power output (the peak power they produce when exposed to solar radiation of 1000 watts per square meter at a module temperature of 25°C). Systems rated between 1 and 5 kilowatts are generally sufficient to meet most of the needs of home and small business owners.
The table below provides an estimate of the roof area needed for several systems. Your system supplier/installer can make, or help you make, more precise calculations at your site before you purchase a system.
 Roof Area Needed in Square Feet </> 
PV Module Efficiency (%)
PV Capacity Rating (Watts)
 
100
250
500
1,000
2,000
4,000
10,000
4
30
75
150
300
600
1,200
3,000
8
15
38
75
150
300
600
1,500
12
10
25
50
100
200
400
1,000
16
8
20
40
80
160
320
800
For example, to generate 2,000 watts from a 12%-efficient system, you need 200 square feet of roof area.
 
The Economics of a Small Solar Electric System
The economics of the small solar electric or photovoltaic (PV) system are determined by both the capital and operating costs. Capital costs include the initial costs of designing and installing a PV system. Operating costs include the costs associated with maintaining and operating the PV system over its useful life.
The factors that affect both capital and operating costs include:
System components
System size
Whether a system is grid-connected or stands alone (off-grid)
Solar resource at your location (amount of sunlight).
Electricity Consumption
Before selecting system components and sizing a PV system for an existing home, you should evaluate your energy consumption patterns and try to reduce your home's electricity use. You can start by performing a load analysis, which includes these tasks:
Looking at your utility bills over the past year
Calculating energy consumption
Recognizing consumption trends.
By understanding your "energy habits" and becoming more energy efficient, you can reduce the size of the PV system you'll need, lowering both your capital and operating costs.
If you're designing a new home, you should work with the builder and the solar professional to incorporate your PV system into your whole-house system design—an approach for building an energy-efficient home.
PV Cost Considerations
Ask your PV provider how much electricity your new PV system will produce per year (measured in kilowatt-hours) and compare that number to your annual electricity usage (called demand) to get an idea of how much you will save. As a rule, the cost per kilowatt-hour goes down as you increase the size of the system.
You should also compare the purchase price of utility-generated electricity to the higher costs of smaller PV systems. PV-generated electricity is usually more expensive than conventional, utility-supplied electricity. However, these costs will vary by geographic location.
Solar rebate programs, subsidies, and other incentives can help make PV more affordable. Tax incentives may include a sales tax exemption on the PV system purchase, a property tax exemption, or state personal income tax credits, all of which provide an economic benefit to consumers by lowering high capital costs.
Some solar rebate programs are capped at a certain dollar amount. Therefore, a solar electric system that matches this cap maximizes the benefit of the solar rebate.
Many homeowners use PV systems because other considerations—such as environmental benefits and energy independence—tip the balance in their favor.
 
Small Solar Electric System Permits and Covenants
Before purchasing a small solar electric system, you should research your local permit and neighborhood covenant requirements.
Local Permits
You will probably need to obtain permits from your city or county building department. These include a building permit, an electrical permit, or both. Typically, your photovoltaic (PV) provider will take care of this, rolling the price of the permits into the overall system price. However, in some cases, your PV provider may not know how much time or money will be involved in obtaining a permit. If so, this task may be priced on a time-and-materials basis, particularly if additional drawings or calculations must be provided to the permitting agency. In any case, make sure the permitting costs and responsibilities are addressed at the start with your PV provider before installation begins.
Code requirements for PV systems vary somewhat from one jurisdiction to the next, but most are based on the National Electrical Code (NEC). Article 690 in the NEC spells out requirements for designing and installing safe, reliable, code-compliant PV systems.
If you are one of the first people in your community to install a PV system, your local building department may not have experience in approving one of these systems. If this is the case, you and your PV provider can speed up the process by working closely with building officials to educate them on the technology.
Neighborhood Covenants
If you live where a homeowners association must approve a solar electric system, you or your PV provider will likely need to submit your plans and get approval before you begin installing your PV system. However, some state laws stipulate that you have the right to install a solar electric system on your home.
For more information, see state and community codes and requirements for small renewable energy systems.