Energy 101: Solar PV

People invest in solar electric systems for a variety of reasons: they want a clean, reliable source of electricity, they want independence from a utility company and price increases, and they want to leave a smaller environmental footprint. Investing in a system also helps support local renewable energy companies and their employees. Maryland has and continues to build an infrastructure of qualified and certified companies and independent contractors who install and service solar electric systems.

Maryland also has an excellent solar resource that makes solar electric systems worth considering. You do not need to be an electrician or PV installer to understand how solar electricity systems work. This article provides basic information that can help you decide if a solar electric system will work for you and help you discuss the topic knowledgeably with an installer.

Solar PV has many benefits, but investing in a system may not be for everyone. While the many benefits and challenges of solar PV will need to be weighed on an individual and local basis, the continued growth of on-farm solar in Maryland is expected; particularly considering the declining installation costs and potential volatility in future electric rates.

Solar PV systems are generally compatible with many agricultural operations, either with rooftop installations on top of farm structures or with open land for ground-mounted systems. Many Maryland farmers support solar PV because it reduces the volatility of future energy costs, has low maintenance costs, and uses a free fuel resource. These attributes are particularly relevant for farmers with high electricity demands and utility rates.

The environmental impacts of solar PV are also substantial, particularly considering the average Maryland home consumes almost 12,000 kilowatt hours of electricity annually (EIA, 2019). Producing 50% of that demand with a solar PV system would offset the equivalent carbon dioxide emissions from over 85,000 pounds of coal burned, or about 26 tons of landfill waste, over a 20-year period based on calculations performed with the EPA’s Greenhouse Gas Equivalencies Calculator (EPA, 2021). The total amount of water needed to generate solar electricity is also dramatically less than the manufacturing processes of more traditional electricity sources such as nuclear, natural gas, and coal-fired facilities.

Answering these questions will help you determine if a solar electric system will work for your building or site.

1. Do you have a south-facing roof? 
   Because Maryland is in the northern hemisphere, PV panels (modules) need to face south for maximum performance. This placement allows panels to take full advantage of the sun’s path in the sky. The sun shines longest on a building’s south side. Southeast- and southwest-facing panels will perform about 5 percent less efficiently. PV panels can also be used as structure such as a porch cover or window awnings. They can also be ground-mounted or pole-mounted. If you cannot place PV panels to face south, a solar electric system will likely not be an efficient investment. Panels can be mounted on east- or west-facing roofs to face south, but they stick up, are highly visible, and can be unattractive. Architects and builders can address this by designing “solar ready” buildings and integrating solar technology components into their designs.
2. Does your roof have enough space for PV panels?
   The rule of thumb for PV panels is 100 square feet of space is needed for every kilowatt (kW) of electricity produced. For thin-film PV materials (such as solar shingles), about 175 square feet of space per kW is needed. If your roof does not have enough space, review the options section under the previous question.
3. Is your roof unshaded?
   Photovoltaic panels are very sensitive to shading. Any shading will dramatically reduce electricity generation. Installers use a Solar Pathfinder device to determine if there are shading concerns from trees (consider mature height), chimneys, nearby buildings, etc. Keep in mind the sun’s path changes throughout the year. For maximum electricity production, make sure panels will be unshaded year-round (especially from 9 or 10 am until 3 pm). If the shade is from landscaping, consider removing the plants. Check local and state codes regarding “solar access” rights if a neighbor might produce shade on any solar system you are considering. If some shade is inevitable, ask the installer about microinverters.
4. What is the angle of your roof?
   Installers typically mount panels directly (flush) on an existing south-facing roof for aesthetics. To maximize electricity generated year-round, mount modules at an angle equal to or close to your site’s latitude (39 degrees for northernmost Maryland; 38 degrees for southernmost Maryland). Installers can tilt at an angle best for your site, system type, and electricity needs. For more summer electricity production, tilt at latitude minus 10 degrees to 15 degrees; for more winter production, tilt at latitude plus 15 degrees. Panels can be angled on flat roofs often found on commercial, industrial, and institutional buildings, but should not be placed flat (horizontal) because of snow buildup that will block the sun.
5. Is your roof in good condition?
   Most roofs can safely support PV panels and mounting system weight. The rule of thumb is 2 to 5 pounds per square foot depending on the panel type and installation method. For example, a 230 watt crystalline panel (3.5 feet x 5.5 feet) weighs about 50 pounds. An installer should determine if the roof/structure can handle the added weight. Innovative mounting systems can make panel removal easy, but because panels can last 30+ years, it may be less expensive and labor intensive to make needed roof repairs before installing panels. Complete any needed repairs first. If considering a new roof, contact a PV system installer/contractor for roof options/recommendations that might make panel installation easier or less expensive.

This section will provide information on the four most common solar electric system options. Each system is designed based on whether it includes batteries, whether it is connected to the utility grid, and the electrical load. The term “grid” refers to a utility company’s system of transmission and distribution lines that carry and deliver power plant-generated electricity to your home or business.

• Grid-Connected Systems: Grid-connected systems are connected to the utility power grid. They are also called grid-tied systems. Grid-connected systems without batteries are currently the most common/popular system type. PV panels produce electricity when the sun is shining during the day. At night, electricity comes from the utility grid. If during the day the building or equipment needs more electricity than the PV panels are producing, electricity is provided by the utility grid. If the PV panels are producing more electricity than needed, the extra electricity is fed into the utility grid. If there is a daytime power outage, the PV system automatically shuts down (does not supply electricity) for utility worker safety.
• Grid-Connected Systems with Batteries: Grid-connected systems with batteries work exactly the same way as grid-connected systems except electricity is stored in batteries for use during a utility power outage. Homes/buildings can have dedicated “critical” electrical loads powered by the batteries. These loads might include a refrigerator, water- and heat-related pumps, furnace fans, medical equipment, or a computer for a home-based business.
• Off-Grid Systems: Off-grid systems are not connected to the utility grid. They are also called stand-alone systems. PV-generated electricity is stored and used from batteries. These systems are typically installed in remote areas where connecting to the utility grid costs more than an off-grid system. Off-grid solar electric systems typically have supplemental and back-up power from a small wind turbine and/or a fossil-fueled generator.
• PV-Direct Systems: PV-direct systems do not entail batteries and are not tied to the utility grid. Thus, they only power the load when the sun shines. They can have moving parts such as pumps. These systems have the fewest components and are used with DC-powered appliances or equipment. Applications include water pumping, building ventilation, etc.

Simple, DC-powered systems can have batteries for applications such as electric fences that need to be powered at night. Whether from the ground or a river, water can be pumped for crops or livestock using photovoltaics.

Whether installing a solar electric system to power a building or pump water, make sure to purchase quality, certified components.

Photovoltaic (PV) Materials
Photovoltaic (PV) materials are the electricity producing component of a solar electric system. PV materials are made of solar “cells.” When the sun’s light energy (not heat energy) hits and is absorbed by the cells, electrons are released and flow as electricity. The greater the amount and intensity of the sunlight, the more electricity generated. PV materials generate direct current (DC) electricity. Commercially available PV materials include crystalline silicon panels and thin-film materials that are both made in various sizes with various wattages of electrical output. On partly cloudy days, PV materials will produce about 80 percent of their capacity. Extremely overcast days may reduce electricity output to 30 percent of capacity. PV materials are relatively unaffected by severe weather and temperatures, although like most electronic devices, they operate more efficiently at cooler temperatures. Because they are typically a dark color and face the sun at an angle, snow slides off or melts quickly. PV materials are designed to resist hail damage (one name brand panel is tested to withstand one-inch hail at 51 mph). They typically come with a 25-year power output warranty, but
most will produce electricity 30-plus years.

• Crystalline Silicon: Crystalline silicon flat-plate panels range in size and electrical output. They can be used for a variety of applications. Those typically placed on home rooftops range from about two-to-three feet wide by four-to-five feet long with a three-inch thickness. Electrical output typically ranges from 200 to 450 watts.
• Thin-Film PV: Thin-film PV materials are flexible and versatile for a variety of applications. They are made by spreading silicon and other materials in a very thin layer (human hair thickness) directly onto base materials. This makes them ideal for building-integrated products such as roof shingles, tiles, building facades, windows, and skylight glazing.
• Other: A third generation of new solar materials includes lightweight foil-based panels, solar inks and dyes, and
  conductive plastics. Researchers continue to investigate how to make all PV materials more efficient at converting sunlight into electricity.

Balance-of-System (BOS)
Balance-of-System (BOS) is a term that refers to the remaining components that accompany PV panels. BOS includes an inverter, meter(s), safety equipment (disconnect switches, etc.), batteries, and a charge controller. It also includes conduit, cables, and combiner boxes.

• Inverter: An Inverter converts and conditions electricity. All PV materials/panels produce DC electricity, which can be used for DC-powered appliances, and camping and boating-related equipment, etc. Most appliances, electronics and machinery require alternating current (AC) electricity, and an inverter converts the PV-generated DC into AC electricity. Inverters also “condition” the PV-generated electricity to match the qualities of the utility grid-produced electricity in order to properly power the electrical load. Contact your utility company to ask if it requires a specific Underwriter’s Laboratories (UL)-certified inverter. All solar electric system components must be matched to work together as a system. If you plan on adding more PV panels at a later date, size your inverter for the future system. It will be less expensive than upgrading to a larger inverter and the accompanying equipment changes that would also be required. Inverters should be accessible, weather-protected, and kept out of direct sun. Inverters can be up to 98 percent efficient and last up to 20 years. Warranties are typically for 10 years. Some installers will connect microinverters to each individual PV panel instead of installing one larger inverter. Microinverters work well where there might be potential panel shading. They can make system expansion easier and less expensive.
• Inverter/Charger: For systems with batteries, a combined inverter/charger is used. It converts PV-generated DC (stored in the batteries) to AC and it also allows batteries to be charged by the utility grid or an off-grid system’s back-up generator. It converts the utility’s or generator’s AC electricity to DC for battery storage.
• Meters: Meters track the amount and “direction” of electrical flow in grid-tied systems (off-grid systems often have meters to track battery charge levels, etc.). When the sun is shining, the PV system generates electricity. If your building or machinery does not use all of the electricity being generated at any one time, it is fed into the utility’s grid. When this occurs, you are credited at either a retail or wholesale rate from the utility. The retail rate is the rate you pay for electricity from the utility. The wholesale rate is a lower rate the utility pays for electricity it buys on the market. Typically, a special Net Meter is provided and installed by the utility company once a grid-tied system installation is completed. This meter spins forward (clockwise) when you are using electricity from the grid and spins backward (counterclockwise) when you are generating excess PV-generated electricity that is fed into the grid. If at the end of a year’s billing period you used more electricity than your PV system generated, you pay the utility company. If your PV system generated more than you used, you receive a utility credit. Contact your utility company to determine if it allows connection to the grid. If it does, ask for current interconnection and net metering requirements.
• Safety Equipment: Safety equipment protects owners, utility workers, and system equipment. Safety equipment includes AC and DC disconnect switches, grounding equipment, and surge protection. This equipment is very important for protecting people and system components from power surges, lightning strikes, ground faults, and equipment malfunctions. Automatic and manual disconnect switches are recommended. Disconnect switches shut down the system so it can be worked on safely whether for routine maintenance or repairs. Switches also prevent the system from sending power to the grid and endangering utility workers while they conduct repairs.
• Charge Controller: A charge controller regulates battery charging. When batteries are part of a solar electric system, a charge controller, also called a regulator, is required. It is connected between the PV panels and the batteries. A charge controller regulates and optimizes electrical flow from the panels to the batteries, keeps batteries fully charged, and prevents battery overcharging. It also prevents batteries from being excessively discharged, which can damage or ruin them. Charge controllers must be properly matched to the overall solar electric system for proper function. Charge controllers can be up to 98 percent efficient and are typically warrantied for up to five years. Inverters and charge controllers can be combined into one piece of equipment.
• Batteries: Batteries store electricity. Off-grid buildings require batteries as part of the solar electric system. Electricity is stored and used from the battery bank, which is sized to provide electricity for the full electrical load for two or three days. Grid-tied buildings with battery back-up typically have a small battery bank used to store electricity for use during utility power outages. Batteries can lower the overall efficiency of a solar electric system because they only release a percentage (80-95 percent) of the electricity that is fed into them. Batteries need periodic maintenance, and have safety considerations. They may last from seven to ten-plus years before requiring replacement. Lifespan depends on factors such as number of discharges and the temperature where they are stored.

If your building does not have a south-facing roof or surface (or you cannot use PV materials as structure), panels can be ground-mounted or pole-mounted in a yard or field. Pole-mounted panels can be in a fixed south-facing position or placed on tracking devices. Like sunflowers, tracking devices follow the sun’s sky path. A single-axis tracker follows the sun from east to west. A dual-axis tracker follows the sun from east to west and adjusts for seasonal sun angles. Trackers increase system cost, but can increase power production by 20 to 30 percent. For the more hands-on homeowner or building manager, adjustable rooftop mounting structures are available for making seasonal sun angle adjustments.

PV Material/Panel Performance
Manufacturers will provide a minimum warranted power rating (in watts) that may be called peak power or peak tolerance rating, etc. Many panels are tested under either Standard Test Conditions (STC) or PVUSA Test Conditions (PTC). The main difference is the testing temperatures. A PTC rating is deemed a more realistic rating. If the panels you are considering have a STC rating, actual performance may be 85-90 percent of stated wattage output. Be sure to also compare efficiency ratings.

System Performance
If you are sizing your own complete system, you can use the rated wattage output (referred to as nameplate DC rating) to estimate the number of panels you will need to meet your targeted electrical load. Actual output of electricity will depend on factors such as roof orientation, tilt angle, and
overall system efficiencies. Because there are inefficiencies in the remaining components, multiply the PV panel nameplate DC rating by 77 percent (a conservative de-rate factor used in NREL’s PV Watts on-line tool) for an estimate of the amount of electricity that will actually reach your electrical load. For example: a 230 watt DC Nameplate rating x .77 = approximately 177 watts of actual electrical power will reach your electrical load.

The size of your solar electric system depends on:

• How much electricity is used and the percentage of solar electricity to be generated.
• Type of PV material used (crystalline silicon or thin-film).
• Roof or other PV material mounting surface orientation, tilt, area, and condition.
• Local solar resource (solar radiation) and peak (direct) hours of sunlight.
• Budget.

You or a system installer can review your utility bills to determine how much electricity you use–typically shown in kilowatt-hours (kWh). If the information is not on your bill, contact your electricity provider and ask for your average monthly use in kWh. One method for approximating system size is using Daily Peak Sun Hours:

1. Determine your average monthly electricity use in kWh.
2. Divide by 30 for average use per day.
3. Find the Peak Sun Hours for your location on the map. Peak sun hours are the hours of direct sunlight that fall on a PV panel (not total hours of daylight).
4. Divide the answer calculated in #2 by your Peak Sun Hours.

#4’s answer is a rough estimate of the solar electric system size you will need (in kW) for 100% of your electricity. See the following example:

1. A Baltimore home’s monthly average electricity use is 900 kWh/month.
2. 900 kWh ÷ 30 = 30 kWh average per day
3. Baltimore is near the diagram’s “4” line = 4 annual average peak sun hours per day.
4. 30 ÷ 4 = 7.5 kW PV system would be needed to produce 100 percent of this home’s electricity.

See "Working on Solar Design and System Sizing" [FS-2023-0655] for more information on system sizing.

System costs depend on a variety of factors. As a general rule of thumb, an installed, grid-tied residential solar electric system without batteries costs approximately $5,000 to $7,000 per kilowatt (kW). Using watt units, $5 to $7 per watt. Larger systems typically cost less per installed kilowatt. An “installed kW” price includes the purchase and installation costs. Using the Baltimore home system sizing example, a 7.5 kW system that provides 100 percent of the home’s electricity would cost about $45,000. (7.5 kW x $6,000 = $45,000). Utility rebates and government tax incentives can significantly reduce the final system cost, but be prepared to pay or finance the full purchase price because some incentives that lower the final cost are received after the system is installed.

There are a variety of federal, state, and local government and utility incentives for energy efficiency and renewable energy. These incentives vary by state and in the length of time they are available. The Department of Energy’s Database of State Incentives for Renewables and Efficiency (DSIRE) — http://dsireusa.org — keeps track of tax credits, rebates and other incentives available to reduce your system’s final cost.

To estimate cost savings and simple payback for a net-metered system, first, calculate the yearly cost savings of your PV system using the formula:

• (PV system size) x (Energy Production Factor) x (Electricity Rate) = $/year saved

For the Baltimore example:

• PV system size: 7.5 kW
• Energy Production Factor: 4 kWh/m2/Day x 365 days/year = 1,460 kWh/kW-year
• Electricity (utility) Rate: $0.16 per kWh
   
• 7.5 kW x 1,460 kWh/kW-year x $0.16/kWh = $1,752 saved per year
•  

Simple Payback is calculated by dividing the system price by the amount saved per year. Examples below use the Baltimore home numbers.

• System Cost, without incentives: $45,000 ÷ $1,752 saved per year = 25.7-year simple payback
• System Cost, with 30% Federal Income Tax Credit: $45,000 x 70 percent = $31,500

There are a variety of financing options for solar electric systems, including:

• Alternative Energy Low-Interest Loans
• Bank Loans
• Home refinance — roll into a mortgage payment
• Construction loans
• Home equity loans