Solar PV (Electric) Energy Self Assessment
The Solar PV (Electric) energy self assessment tool was developed to increase renewable energy awareness and utilization in agriculture. Solar electric systems, often referred to as solar photovoltaic or PV systems, can provide reliable electricity for a home, farm or business. The Solar PV energy calculator is designed to estimate the size of a solar electric system according to either the amount of electricity you would like to generate or the maximum investment you are willing to make. Before considering a solar electric system, it is recommended that you maximize the efficiency of electricity use on your operation by upgrading lighting and installing energy efficient appliances for the residence and equipment for the farm operation. The energy self assessment tools can help you identify potential energy efficiency improvements.
This tool does not provide a site-specific renewable energy assessment. This tool does not account for trees, topography or buildings that can shadow the collectors. Before purchasing a solar electric system, a site assessment by an experienced site assessor is recommended to identify the best location for solar electricity production. We also recommend getting multiple installation quotes, as prices vary widely.
Solar PV Basics
Simple solar electric systems require a solar electric module, which generates direct current (DC) electricity, and an application to use the DC electricity. These simple photovoltaic systems are typically independent of utility connection, and operate mechanisms that are used while the sun is shining, such as water pumps or pond aerators.
A photovoltaic (PV) collection device, or solar electric module is used to convert photons of sunlight to electricity. The solar electric module can take many forms, but is typically a framed collector, often referred to as a solar panel. Solar panel sizes can range from a few square inches, such as those used for single light fixtures, up to the size of a large picture window. Arrays can use tens of thousands of solar panels and cover acres of land.
Solar electric modules can be mounted in many ways. Some applications work well with solar electric modules sited directly on a roof, while others are mounted at an angle using a tracking system. Some sites have solar electric panels mounted to poles with tracking devices to follow the sun as it tracks across the sky.
Applications that require electricity use at night, or during periods of low sunlight, will require battery storage or connection to the utility grid. A system controller is required to regulate the amount of power that goes into storage versus what is used immediately for an off-grid system. Control systems are also employed for safety reasons. Since most appliances and the utility grid run on alternating current (AC) electricity, the direct current (DC) generated by solar electric systems requires conversion to AC electricity. Inverters are devices used to convert the direct current (DC) electricity produced by solar electric modules to alternating current (AC) which can be used by appliances.
Solar electric modules are made of semiconductors, usually made of a silicon based material. When the sunlight strikes these materials, the semiconductors absorb the energy of the sun’s rays causing an electron in the material to be knocked loose. Solar electric modules have electrical conductors that force freed electrons to flow along a certain path. The flow of electrons is available as electricity.
Most electric devices require the use of alternating current (AC), the type of current supplied through power lines. DC electricity generated by PV panels needs to be converted to AC electricity. This can be achieved with the installation of a power inverter, which converts DC current into AC current.
Inverters allow the use of standard AC powered appliances and lights to be used. Many off-grid systems will convert DC to AC electricity so they can use AC appliances because AC appliances are less expensive than DC appliances, and there is also a wider selection of quality AC appliances versus DC appliances.
Grid-Connected Solar Electric Systems:
Most solar electric systems are connected to the electric grid, which allows the sale of electricity to the utility when electricity is being produced in greater quantities than is being used. Conversely, if a solar electric system experiences an electrical demand in excess of what is being produced or during hours of darkness when electricity is not being produced, the home or business is assured to have constant grid-powered electricity. Many states have “net metering” laws that require utilities to pay retail rates to small renewable energy producers for any excess power produced over a month or year time frame. This essentially allows the utility grid to be used as a battery. Figure 1 illustrates a grid connected solar PV system. The sun hits the solar panels causing DC electricity to be generated. The inverter converts the DC electric to AC electric which can be used by standard appliances and lights in the home. The electricity can flow from the inverter through the home’s circuit breaker panel to power the home or to the utilities power lines if more power is being generated than is being used in the home.
Off-Grid Solar PV Systems:
A solar electricity system that is independent of the utility is called an off-grid system. Off-grid systems can operate small loads distant from the farm’s electricity service, such as solar powered lights or water pumps. Typically battery storage is used with an off-grid system to store energy when excess is available and to provide energy when the system is not producing enough to meet the demand of electricty use. Off-grid systems are often used when the cost to run utility power lines to the point of use exceeds the cost of an off-grid system. The figure shows a solar electric panel which creates electric current from solar energy. The solar-generated direct current (DC) electricity is converted to alternating current (AC) for immediate use through a power inverter or transferred to battery storage. When electricity is demanded and there is not enough solar energy, DC electricity flows from the batteries through a power inverter, where it is converted to AC electricity and then flows to appliances and lights.
It is important to optimize battery storage size to maximize battery life and storage capability and minimize expense. This may vary according to site and system. In general, it is best to store electricity output from PV panels in “deep cycle” type batteries, or those batteries that can discharge up to 80% of their stored energy without damage to the battery. Battery selection should be based on the expected kWh need and the solar electric system’s maximum electrical output. Batteries are rated for their capacity in amp hours at a given discharge rate. For example, three 12-volt batteries, each with a 50 Ampere-hour capacity, could store 1,800 kWh. For some small off-grid sites a gel cell battery is commonly used. A responsible solar PV contractor will be able to help design and optimize a battery storage system. A battery system will significantly increase system costs compared to a utility connected system and will also require more maintenance and daily, weekly and monthly attention.
|City||Energy kWh (AC)|
|Kansas City, MO||1312|
|Key West, FL||1414|
|International Falls, MN||1185|
|San Diego, CA||1498|
The latitude has some effect on the energy production by a solar system , as well as the number of cloudy days that typically occur. For example, International Falls, MN is at a latitude of 47.57°N and Seattle, WA is at 47.45°N . Based on the latitude both cities should receive the same amount of solar energy. Because Seattle has more overcast weather, a solar panel located in Seattle is estimated to produce about 20% less energy than a solar panel located in International Falls. Solar electric modules are available with varying generation potential. Common module sizes range from 160 to 220 watts but can be as low as 5 watts for small applications.
Solar power can energize fences for pasture management and livestock handling. They utilize solar-generated electricity which is stored in batteries for nighttime fencer operation. Depending on the application, electric fences cost between $75 and $400, which includes everything except grounding rods and fence wiring supplies. Solar electric fencers with battery storage can be used in remote locations or locations without utility service. These units are rated by the mile of effective range. When using a solar fencer, it is best to use poly wire instead of traditional solid steel wire or barbed wire. Traditional solid wire increases resistance and reduces the effectiveness or shocking power of the fence.
Solar electric systems can be used to pump water for various applications. For instance, water can be pumped from ponds or steams to a stock tank to keep livestock out of waterways and manage nutrient runoff to streams or lakes. Solar water pumps provide for animals’ natural tendency to drink more on sunny days. Water can be pumped from a well or water line using a solar pump. A solar pump may also be used to pump water for crop irrigation. In these applications, photovoltaic electricity is routed directly to a well or water pump. These systems are typically independent from the utility grid.
Aerators aid in oxygenating ponds and keep water open during the winter months in ponds and livestock water tanks. Oxygenated ponds facilitate fish growth and manage algae blooms. Solar electric aeration system costs vary largely according to the volume of the managed pond. Small pond solar electric systems can cost from $350 to $400. For large ponds, prices range from about $4,000 to $10,000 for a one to ten acre-area pond, respectively.
Photovoltaic electricity can be used to power lighting at remote sites, or to supply light during power outages. These independent units store power in low-voltage batteries. Typically, each light has its own photovoltaic panel and battery storage. To maximize a solar lighting system, use lights that are the most energy efficient to optimize use of the stored solar energy. Also, consider lighting that is powered by direct current (DC), so an inverter is not needed. Solar lighting costs range from $50 to $500 per fixture depending on the lamp type and fixture wattage.
Automatic gate openers can be used in remote locations with isolated solar electric systems. Power can be collected by a solar electric module and stored in a battery at the gate site until needed. Gates can be operated remotely, allowing management of livestock fields from a remote location. Solar electric-powered gate systems may cost from $700 to $2000 depending on the number of gates controlled.
Solar Battery Charger:
Solar battery chargers can be used to charge batteries in cell phones, cameras, MP3 players and as a trickle charger for 12-volt car or tractor batteries. For a car or tractor the solar charger is placed on the dashboard, oriented towards the sun and plugged into the cigarette lighter or DC power outlet in the vehicle. (The lighter or DC outlet must be wired so the ignition key does not need to be on to be powered). When the sun strikes the solar panel, direct current (DC) is generated to charge the battery. The maximum output of this type of solar panel is about 15 volts DC so it will not over charge a car battery. Solar chargers are useful on seldom-used vehicles and cost between $40 and $200 depending on the wattage output.
Typical solar electric systems that run off grid, using battery storage, cost about $12,000 per installed kilowatt of rated power. Battery storage increases system cost significantly.
Systems that track the sun, or have batteries will require little additional maintenance.
Step 1. Determine areas where you can conserve electricity use. Visit the energy assessment tools link to calculate potential energy savings for energy efficiency measures you could employ on your farm.
Step 2. Educate yourself. It is important to understand how a solar electric system operates, its limitations, and how to manage the system.
Step 3. Determine the availability of tax and grant assistance programs, including their application and payment processes. Some incentive programs may require paperwork or other steps to take before the system is purchased or installed.
Step 4. Get a site assessment - Consult a professional solar electric site assessor or installer to estimate the electrical generating potential at your site. Optimizing a solar PV system requires site-specific information and calculations including specific latitude and longitude of the location, weather and climate data, and area obstructions. A listing of certified solar assessors can be found at the Midwest Renewable Energy Association. A certified solar installation professional can be found at the National American Board of Certified Energy Practitioners. A professional solar site assessor should investigate the type of permitting that may be required for your area.
Step 5. Get quotes from at least three companies. Compare quotes to ensure they are quoting similar size and type of equipment. Ask questions. If possible, request recommendations for other systems they have installed and question prior customers on their experiences with the installer, system maintenance, and issues they have found.
Step 6. Consult with your tax preparer to ensure that you can take advantage of any state and federal tax credit available.
Step 7. Check with your insurance carrier to see if your proposed solar electric system is insurable.
Step 8. Order the system to be installed by a professional and reputable solar electric installer.
Step 9. Once the system is installed, complete all applicable grant or tax incentive forms.