In order to understand how much power your solar panels can produce and how this might vary over time, we need to look at solar system performance and efficiency.
- Solar system performance measures the amount of energy your solar panels produce at any moment in time or across a period of time such as a day, week, or month. It is usually measured in watts, a unit of power, and reflects both the size and conditions of your solar energy system.
- Solar panel efficiency is closely related to performance and measures how well solar panels convert sunlight into usable electricity. Unlike performance, solar panel efficiency is calculated as a percentage: the amount of energy received from sunlight relative to the amount of energy produced.
Both performance and efficiency are variable, not fixed, measurements that can be used to map the long-term savings potential and health of your solar energy system. In this article, we’ll break down the key factors that impact system performance and offer tips to track and optimize your solar energy output.
Designing for Solar Performance
Many of the variables that impact solar system performance can (and should!) be addressed in the solar design process. The goal is to match your solar energy system to the energy needs of your household.
This requires a thorough understanding of site conditions that will affect system performance.
System Size
Most solar panels installed today have a power rating—also called panel wattage, capacity, or power output—of 200 to 400 Watts. This indicates the amount of direct current electricity each panel can produce under standard test conditions. Panels are also rated on efficiency, or power output relative to size, and currently average a 15-20% efficiency rating across manufacturers.
Because solar panels vary in both power output and efficiency, we cannot measure system size on the number of panels or square footage alone. Instead, system size is calculated by multiplying the number of panels in your solar energy system by the power rating of each panel. For example, a 10-panel system that uses 400 watt panels will have a system size of 4,000 watts or 4 kilowatts (kW). Under standard test conditions, a larger system will produce more power.
Offset
Solar energy offset measures the amount of electricity generated by your solar panels relative to the electricity consumed in your home, and is typically expressed as an annual percentage. Many homeowners target a 100% solar offset when selecting the right system size for their home. This requires a careful audit of your historical energy usage: We need to understand the energy needs and consumption patterns of your household in order to target a specific offset and map the right system size for your home.
As a starting point, we’ll look at your utility bill over a 12-month period. We can also choose to account for any new or upcoming changes in energy consumption. For example: Some of the changes that might increase your annual consumption include new appliances or electric vehicles, additional residents or family members, or working from home rather than working from an office.
Peak Sun Hours
The amount and intensity of sunlight in a given area has a direct relationship to solar panel performance. To evaluate solar potential by geographic location, you can use a measurement called peak sun hours.
Peak sun hours reflect the average daily amount of time that the sun’s rays are most powerful—or when they reach 1,000 watts of power per square meter—in a given area. The number is calculated as an annual average to account for seasonal changes in sunlight and intensity.
In the U.S., peak sun hours are highest in southwestern states like Utah and New Mexico, and lowest in the northernmost states. While solar remains a viable energy source regardless of location, homeowners who live in areas with low peak sun hours might need a larger system to achieve the same results.
Roof Pitch and Orientation
The layout of your roof—including pitch, orientation, and complexity—plays a significant role in the design and performance of your solar energy system. Solar panels perform best in direct sunlight, when the sun's rays hit your solar panels at a 90° angle or as close to perpendicular as possible.
In the U.S., solar panels generally perform best on south-facing roof surfaces where the pitch or angle of your roof matches the longitudinal coordinates of your home. For example:
- A homeowner in Miami, Florida, 25.7° north of the equator, will yield the best solar system performance by mounting panels on a south-facing roof surface with a 25° pitch.
- A homeowner in Portland, Oregon, 45.5° north of the equator, would benefit from a roof pitch closer to 45°.
However, the path of the sun changes throughout the year and, with that, the shadows that are cast on the various planes of your roof. These shifts become more dramatic the further you get from the equator.
In order to fully optimize your solar system performance, we need to calculate the amount and intensity of sunlight that hits every point on your roof over the course of a year and place solar panels accordingly. Depending on the pitch and orientation of your roof, your solar installer may add support structures to increase or decrease the angle of your solar panels.
Shading
Anything that blocks sunlight to or casts shadows on your solar energy system will reduce the amount of energy your solar panels produce. While shading from nearby structures or buildings may be difficult to address, shading from nearby trees can often be mitigated by trimming or, in some cases, removing select trees.
Irradiation
At Palmetto, we use proprietary 3D LiDAR data and mapping software to calculate the solar irradiation for every square meter of your roof across every hour of the year. Irradiation is the sum of solar energy falling on a surface over a given period of time.
Our data accounts for peak sun hours, roof pitch, and shading as well as local weather and seasonality. High irradiation areas:
- Receive the most aggregate sunlight throughout the year
- Will produce the most electricity per solar panel
- Are ideal for panel placement
Our design team will use this data as part of a comprehensive site survey to engineer your solar energy system before scheduling installation.
Variations in Solar Performance
Once installed, you'll see variations in production based largely on daylight, weather, and season. Clouds, shade, or extreme weather can block sunlight from reaching your solar panels and result in a slight decrease in production. These variations are factored into your solar energy system design and reflected in your solar savings estimates. Some of the key elements that impact solar performance are:
- Seasonality
As day length, sun angle, and weather patterns change throughout the year, you will see differences in your solar system performance. For most of the U.S., solar performance is highest in late summer and early fall, and lowest in the winter. - Temperature
While solar panels benefit from direct sunlight, extreme heat can actually decrease performance. Solar panels perform best at lower temperatures when electrons are less excited and can move with greater ease. - Weather
Solar panels are designed to withstand wind, snow, hail, and heat, and will continue to capture and generate power even in indirect light. While cloudy or inclement weather can block sunlight from reaching your solar panels and thus temporarily decrease performance, rain can remove dust and debris—helping to increase performance when the sun returns. - Snowfall
Snowfall can actually increase production as light is reflected back to the solar panels from the surrounding landscape. However, any snow that accumulates on your roof and blocks sunlight from reaching your panels will reduce solar energy system performance. In most cases, this problem will take care of itself as snow melts and easily slides down the glass panels. - Age
While the average lifespan of solar panels is about 25 to 30 years, solar panel efficiency will decrease slightly with every passing year. This degradation rate is factored into the initial design of a solar power system, but it still impacts your solar system’s efficiency and performance.
How to Optimize Solar Performance
Solar panels installed by Palmetto are highly efficient and require little to no maintenance. That said, there are a few things you can do to ensure you’re getting the most out of your solar array.
- Maintenance
For peak performance and efficiency, make sure your solar panels are free of dust and debris. Regular weather patterns will do most of the work for you, but annual cleanings can be of benefit. - Performance Tracking
Once your system is active, you can track energy production by day, week, or month from your personal solar dashboard in your Palmetto solar account. This is the best way to see and understand normal variations in system performance. - Proactive Monitoring
Your purchase also includes three years of Palmetto Protect Essentials with proactive energy monitoring, so we can make sure your system is on track and alert you to any changes in performance. - Energy Management
You can further optimize your clean energy usage by scheduling high-energy chores—such as doing the laundry or running your dishwasher—for sunny hours/days when production levels are at their highest.
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