Your Guide to Monthly Snow Loss Values from NREL
In addition to delivering a quality solar installation, providing an accurate estimate of how much energy the PV system will produce is one of the most critical ways solar contractors can ensure their customers are satisfied. But sometimes environmental factors–like snow–can complicate your efforts.
That’s where loss values come in. As discussed in Aurora’s PV System Losses Series, these kinds of solar PV system losses are accounted for with linear loss factors. These are applied as percentage reductions to the energy production calculated by the simulation tools of your solar design software.
Determining the appropriate loss factor to account for energy production lost as a result of snow on the customer’s solar panels is particularly tricky, however. Not only does the amount of snow vary widely between different locations, the design of the solar installation–particularly the tilt of the panels–can play an important role as well.
Thankfully, a number of models have been developed that help solar contractors determine the appropriate snow loss factor to use when estimating solar customers’ energy production in order to get the most accurate result. In 2017, the National Renewable Energy Laboratory (NREL) published research that verifies the accuracy of a leading snow loss model and provides recommended snow loss values for different types of solar designs in different areas.
Following up on Part 3 of our System Loss series, which introduced snow losses, today’s article discusses this snow loss model and shares NREL’s recommended snow loss percentages for PV systems at different tilts and different locations in an easily accessible format. With this local data, solar contractors can ensure their solar production estimates capture the effects of snow as accurately as possible.
NREL’s Snow Loss Research
In order to address the solar industry’s need for a reliable model that estimates solar production losses as a result of snow coverage, NREL integrated the methodology of a leading snow model (developed in the paper by Marion et al. in 2013) into its System Advisor Model (SAM) software.
NREL then conducted a validation study demonstrating that the model reduced error in estimated annual energy by over 45% compared to measured values. Finally, using historic weather data, the agency applied the snow model to a wide variety of locations in order to determine snow loss values for different locations and tilts that reflect typical trends. These findings are published the 2017 paper, Integration, Validation, and Application of a PV Snow Coverage Model in SAM.
About the Model
The snow loss model assumes that the sliding of snow off of a PV Array is the primary method of removing snow, and other works support this conclusion. The model can be used in NREL’s SAM software, another tool for simulating the production from PV systems. 1
The model is used during the PV simulation step, and acts to reduce the irradiance on modules similar to how shading and soiling are accounted for; Aurora applies snow losses in the same step. (For a quick summary of system losses, and how to configure your account settings in Aurora, see the Aurora Help Center.)
Find the Right Snow Loss Values for Your System
In order to provide solar contractors an accessible way to determine a representative snow loss factor to enter in their solar design software of choice, we ran the model in SAM for a range of locations in the U.S. across a range of tilts and compiled the appropriate percentage loss factors for each month according to NREL’s model.
The map linked below has icons for each of the locations modeled, presenting monthly snow loss values for each respective tilt.
(Click here to open the map in it’s own tab.)
Each of the 64 locations modeled had a south-facing system oriented at a tilt between 20 and 60 degrees. On the map, each different tilt is represented by a different color icon for accessibility; users who want to look up snow loss for a given tilt near an available city can simply click the appropriate icon to see the monthly snow loss values. (You may need to zoom in to see all of the icons.)
The example plot demonstrates the snow losses for varying tilts at a given location in each month. For the town of Rockford, IL, snow losses can range from 20% to 50% in a typical January, depending on the tilt, and that snow loss drops down as spring encroaches.
In general, rooftops with modules at a greater tilt will see less snow blockage, but the exact values will still depend on whether there’s enough space for snow to slide off and if the system is oriented away from south, resulting in less direct sunlight and less chance for the snow to slip away.
You strive to provide a high level of accuracy to your solar customers when estimating how much energy the system will produce. If your customer lives in a U.S. location that experiences snow, using NREL’s monthly snow loss percentages in Aurora will help you ensure that your estimates more accurately represent reduced energy production from snow coverage.
1 This model can only be applied if snowfall data is available; the National Solar Radiation Database (NSRDB) has this data available for a “Typical Year” in the older TMY2 weather data sets, but not TMY3, so we only ran the model for those locations across a range of tilts.
Georgia’s Solar Energy
The potential for solar energy use in Georgia is dependent upon the amount of sun shining on the earth’s surface called solar insolation. Several factors such as weather patterns, humidity and haze can affect local insolation levels.
As can be seen on this solar map, insolation values in Georgia are significant enough to support solar energy systems in our state, with the southern two-thirds of Georgia having solar insolation values equivalent to most of the state of Florida.
Calculate Your Solar Power
Get an estimate of the amount of energy your solar panels will produce in an annual period.
An uncapped 26% federal income tax credit is available to homeowners for solar equipment placed in service before December 31, 2020. Find information about tax incentives at the Database of State Incentives for Renewable Energy. Please consult your tax advisor to determine how this federal incentive may apply to your particular circumstances.
Georgians may qualify to be eligible for the Federal Solar Tax Credit.
Government agencies and utilities offer a variety of tax credits and rebates. See if you qualify for nationally available rebates or find savings that may be available to you at Energy.gov Tax Credits, Rebates, and Savings.
Georgia Solar Potential
The potential for solar energy use in Georgia is dependent upon the amount of sun shining on the earth’s surface called solar insolation. Several factors such as weather patterns, humidity and haze can affect local insolation levels. However, insolation values in Georgia are significant enough to support solar energy systems in our state, with the southern two-thirds of Georgia having solar insolation values equivalent to most of the state of Florida.
Solar Basics: Learn more about the basics of how solar works from the U.S. Department of Energy.
Solar Research: The National Renewable Energy Lab (NREL) offers research data and information for photovoltaics for homes and solar water heating for homes.
Solar Electric Power Association: The Solar Electric Power Association provides solar info from the utility company perspective.
Cost and Production
PVWatts Georgia: Use the NREL PV watts calculator to estimate the amount of energy your solar panels will produce in an annual period.
Georgia Incentives Rebates
An uncapped 30% federal income tax credit is available to homeowners for solar equipment placed in service before December 31, 2019. Find information about tax incentives at the Database of State Incentives for Renewable Energy. Please consult your tax advisor to determine how this federal incentive may apply to your particular circumstances.
Energy.gov Tax Credits, Rebates, and Savings
Solar Demonstration Project
Housed on the roof of our Georgia Power’s headquarters, the energy generated by the PV panels is partially offsetting the electricity needs of our corporate headquarters.
REC Disclaimer: A Renewable Energy Certificate (REC), sometimes called a renewable energy credit, represents the renewable attributes of energy produced from a renewable energy facility. RECs are considered a commodity and can be sold or traded separately from the actual energy. Georgia Power purchases only the null energy output from the renewable generating facilities that have contracted to sell energy from their solar facilities through the Large Scale Solar (LSS) program and the initial (2013/2014) Advance Solar Initiative (ASI). The sole ownership of RECs belongs to each generating facility, as specifically stated in each respective power purchase agreement (PPA). The original intent of these programs was to grow renewable resources in Georgia, while allowing the generating facilities to retain the benefits of the RECs. Georgia Power does not report emission reductions from the null energy purchased through PPAs that do not bundle the RECs for sale to Georgia Power.
Identifying the Best Solar Panel Calculator
There are plenty of solar panel calculators out there for understanding your solar energy potential and cost savings, as we discussed in a previous article. The criteria for selecting the best calculator can be condensed into two for simplicity. First, does the solar calculator cover your region? As noted, PVGIS developed by the Joint Research Center in Italy and funded by the European Commission is excellent for Europe and North Africa but no good for North America since its database is regional. Second, is the tool simple and intuitive to use? Many tools such as Solar Prospector or System Advisor Model (SAM) are too complicated for most homeowners to use and make sense of the data. Both Solar Prospector and SAM are developed by the trusted US-based National Renewable Energy Laboratory (NREL). RETScreen is another example of a powerful but complex tool. Developed by National Resources Canada, it can compare between many renewable energy sources and generate both power and financial charts. However, it requires the user to download a desktop software which then requires the user to input a variety of data before generating any results.
So for a primarily North American audience, let’s compare the two main contenders: PVWatts vs. WhatNextNow Solar Discover.
Presenting PVWatts: Free, Simple and Intuitive
For the United States, one of the best available tools is PVWatts. Developed by NREL, PVWatts is a free, simple and intuitive tool which allows you to put in a home or business address. The tool finds the closest location to you which has a weather data file in its database. If there are multiple options, you can choose which datsource to go with. Then, it gives you several boxes to change the solar panel system info, as well as the utility cost. Economic incentives for solar power such as tax credits and utility cost reductions are also shown. Under the solar panel system, you can click on Draw Your System to draw a bounded figure within an interactive satellite map to designate the size of your solar panels.
This leads to a chart detailing the amount of incoming solar radiation, the solar electrical output, and the value of this output in dollars for every month, and overall during the year. In addition to the chart, a dollar per kilowatt-hour (USD / kWh) comparison between the grid system cost and the solar panel system cost is available. This gives you a baseline to decide whether solar panels will be worth it. Data tables can be downloaded for each month or even each hour.
WhatNextNow Solar Discover: An even simpler alternative with some added benefits
Another tool that is useful for both Canadian and American residents is our very own WhatNextNow Solar Discover. The user interface for WhatNextNow Solar Discover is even simpler than that of PVWatts. Simply, go to WhatNextNow Solar Discover and input an address or postal code. Unlike PVWatts which uses localized ground stations in the United States and its territories, limiting its analysis to the United States, WhatNextNow Solar bases its data analysis on a global data set allowing the tool to be used anywhere in the world.
After you select your site, WhatNextNow Solar Discover provides the monthly average solar radiation based on the last 20 years of data. Next, users can check out the Energy page which shows the total energy produced in kWh for each corresponding month. You can also change the specifications of your solar PV system, such as the slope, power capacity, orientation, PV efficiency, and system losses. The energy graph updates automatically as you change each parameter. At the bottom of the screen is an interactive satellite map where you can draw a bounded figure to calculate the size of your solar panels. The power capacity is updated automatically to reflect the size of the solar panels you draw.
Next, the Finance page yields a graph of revenues and expenses for the next 25 years based on financial data including the cost of electricity in USD / kWh, system cost in USD, project lifetime, and inflation. This tool gives you a comprehensive look at the cost of your solar panel project and how much money it will generate for you over the years. Once again, the graph updates automatically with each adjustment.
A bonus feature of WhatNextNow Solar Discover is the ability to save your session with the key information on your site, the solar PV system you configured and the resulting financial analysis.
Battle of the solar panel calculators: Lancaster, CA
Now let’s look at a specific example to compare WhatNextNow Solar Discover and PVWatts data for a home in Lancaster, California. Lancaster has recently href=http://www.forbes.com/sites/peterdetwiler/2014/05/13/can-a-city-get-to-net-zero-lancaster-california-mayor-thinks-so/ target=_blank rel=noopener passed a law requiring that starting from January 1, 2014, all single-family homes must have a solar PV system generating 1. 1.5 kilowatts (kWs). Presently, one in five newly-built homes in California is solar powered and this trend is projected to grow.
First, we will set up the same site using both online solar panel calculators to estimate solar radiation and electricity output using identical solar panel configurations.
Let’s start with WhatNextNow Solar Discover:
- Slope: 35 degrees
- PV Efficiency: 15%
- Power Capacity: 4 kW
- System Losses: 0%
- Orientation: 0 degrees (south)
Clicking the right arrow again leads to the Finance page where the levelized cost is given as 0.14 USD / kWh with a capacity factor of 24%. href=/home/solar/what-is-the-levelized-cost-of-solar-energy/ target=_blank rel=noopenerLevelized cost refers to the cost of building and operating this system over a period of time. The capacity factor is the percentage of time that the system is operational at full capacity, based on the availability of the sun.
Now, let’s use PVWatts for the year are also displayed. Here, you see that 8,568 kWh with an energy value of 1,351 is generated by the solar panels by the end of the year. The average cost of electricity purchased from the utility is 0.16 USD / kWh while the cost of electricity generated by the system is 0.14 USD / kWh. PVWatts unfortunately does not have a more detailed financial analysis tool like WhatNextNow Solar and presents no information on levelized cost, capacity factor, annual revenues and expenses, etc. Furthermore, only the data tables can be downloaded.
A side-by-side screenshot comparison of the results is shown above (please note that there are two screenshots for WhatNextNow Solar since unlike PVWatts, WhatNextNow Solar combines the inputs and outputs on one page but separates each section).
Our verdict: Both are free, both are fun to use. but WhatNextNow Solar Discover offers some added benefits
WhatNextNow Solar Discover has two different pages for energy and finance to allow the user to play around with either the energy parameters or the finance parameters for instantaneous feedback on either page. The final results show that WhatNextNow Solar predicts 8,457 kWh generated in one year, whereas PVWatts predicts 8,568 kWh generated. These two energy values are close enough to warrant a look at both tools. However, those who are interested in a more detailed look at the revenues and expenses for their solar investment may want to choose WhatNextNow Solar Discover.
PV Watts Calculator
The PV watts calculator we’re going to be using in this article comes from the National Energy Renewable Energy Laboratory.
The calculator itself serves as a tool that’s used to do photovoltaic analysis on buildings or residential homes around the states.
The calculator is like an AC watts calculator. or any solar calculator for that matter, as you can use the values you obtain to determine if solar is feasible.
So, let’s dive in and see how this bad boy works.
But first, what do people use this calculator for?
A Bit About the Calculator
The PV Watts calculator gives accurate estimates of the cost of energy as well as energy being produced on grid-tired photovoltaic systems.
In simpler terms, it allows users to size and determine how their system will perform before its built.
The calculator belongs to NREL, a company that has been at the forefront of transforming energy through research and deployment of solar technology.
Their calculator is easy to use and doesn’t require you to do any tedious mathematics.
How to Use the PV Watts Calculator
Let’s cover each step so that you can be on your way to building your dream solar system.
Enter Your Location and Select the Weather file
The first step to using a PV watts calculator is to enter your address so that the calculator can get an understanding of your location’s exposure to the sun.
You’re then greeted with a geographical representation of your house along with weather files surrounding the area.
If you choose the weather file, it will show up alongside your location, allowing you to proceed to the next screen.
The system information refers to the rating of your solar system, like how an appliance kWh calculator works.
You’ll first need to enter the DC system size in kilowatts followed by the module and array type.
The module refers to the type of panels your solar system uses. In other words, either polycrystalline, monocrystalline, or a thin film panel.
Array type describes how your solar system is set up. You’d pick the fixed system if your solar panels are stuck to your roof, or a tracking system allowing the sun to come in from all angles.
The latter is used more in farming usage over residential but it’s not uncommon.
The next two values needing to be inputted relate to the orientation of the solar panels. The first is the tilt angle, or the angle at which your solar panels are tilted, and the azimuth.
No, the latter isn’t a spell cast by wizards. It’s the arc between the North and the sun or moon.
How do we obtain this angle?
Well, 180 degrees is due South, 90 degrees is due east, and 270 degrees is due west.
Finally, we need to enter the system losses, or how much energy is lost within the system before converting the solar to electric energy.
NREL knows that certain elements affect this rating and it’s not the easiest variable to calculate manually.
Factors like soiling, shading, snow, and wiring can make the system perform less than optimal.
Luckily for you, NREL has a built-in calculator to help you get a more accurate reading while compensating for these factors.
This isn’t a completely new idea in the solar game as mah watts calculators have built-in calculators to assist in the conversion process of other non-related units of power.
Advanced System Information
The NREL’s PV Watts solar calculator has a set of advanced settings that aren’t necessary to change. These include the dc to ac size ratio, the inverter efficiency, and the ground coverage ratio.
This data isn’t as simple as looking at a voltage-to-watts chart and converting one unit of measurement to another.
Size ratios and efficiencies require calculators of their own or a real solar techy to know the mathematics.
Leaving these values at their default settings won’t hurt a fly.
This section lets the calculator know what kind of economic setup your solar system falls under.
There’s a drop-down menu where you can either pick residential or commercial and chances you’re picking the former if you’re reading this.
However, if you’re a farmer with a far-stretching solar array, the latter is your best bet.
The next value that needs to be entered is the average cost of electricity purchased from the utility.
Depending on your location from the beginning, this data should automatically be entered and updated. If it’s incorrect or slightly off, you can edit it and use the value on your power bill.
If you’re a remote user and you plan on using a solar-powered generator or battery to store the incoming power, then this value won’t be relevant.
Once all the variables have been entered correctly, the calculator gives you a monthly breakdown of the solar radiation, the AC energy, and the value of that energy.
It also gives you a yearly value of how many kilowatt hours the system can output each year.
These statistics can be downloaded, edited, and exported to an excel sheet, allowing you to organize the data with ease.
Knowing these values, along with your home solar needs, can help you size the perfect solar system.
It’s important to note to obtain the correct values that for your solar needs using a mA to watts conversion calculator. the variables are the correct unit of power.
Why should you use a PV Watts Calculator?
The main reason that incoming solar users make use of this calculator is to get an understanding of the size and scale of their system.
Without this, you could be installing a solar system that’s underpowered.
Also, since you’re given annual kWh, you can compare this with your solar needs to see if it will match your daily kWh usage.
The NREL’s PV Watts calculator is a good way to ensure that your solar system can handle anything you throw at it.
Where this differs from the off-grid solar calculators is that the PV Watts is designed specifically for grid-tied solar systems.
It doesn’t stop there though. There are plenty more electrical calculations you need to familiarise yourself with to keep your solar panel system running optimally. Check out our VA to Watts converter.
Did You Find Our Blog Helpful? Then Consider Checking:
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Using NREL’s PVWatts calculator for solar
If you’re starting to think about installing solar at your home or business, you may have come across PVWatts, a calculator developed by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).
PVWatts estimates the energy production and cost of energy for solar photovoltaic (PV) systems around the world, making easy for homeowners, small building owners, installers and manufacturers who want to estimate the performance of a potential PV installation.
Using PVWatts to design a solar panel system
PVWatts is super-customizable – you can use it to can get solar production estimates based on your specific location, the type of equipment you’re using, and your installation site. Here’s how it works.
Step 1: Enter your location
When you land on the home page for PVWatts, you’re prompted to “get started” by entering an address. You can choose to be as specific as you want – enter a city or town, zip code, or the exact address where you’re thinking about installing solar.
This location information is the foundation of the calculator, and a big part of what makes it so customized. Instead of looking at national averages, PVWatts uses your location to tap into local weather data and sunlight data from NREL’s National Solar Radiation Database.
Step 2: Enter your system information
PVWatts comes preloaded with assumptions about the size of your system, the equipment you’re using, and what you currently pay for electricity for your utility. You can use the assumptions in the calculator, or input your own information. The specific inputs for the simulation are:
DC system size: This is the power rating for your system. The default assumption is 4 kilowatts (kW), equivalent to 16 standard-sized 250 watt panels or 13 high-efficiency 300 watt panels.
Many homeowners will need a larger system to cover all of their electricity use – the average system size for EnergySage Solar Marketplace users is 7.8 kW. If you’ve input your full address, you can also use the calculator’s “Draw Your System” tool to see what size system will fit on your roof.
Module type: Some solar panels are more efficient than others. If you have limited space, or are prepared to pay a little more, you can change the default setting from “standard” (which is about 15 percent efficient) to “premium” (19 percent efficient).
Array type: Most homeowners will install a fixed-mount system on their roof. However, you also have the option of installing a ground-mounted system that has tracking capabilities to follow the sun across the sky throughout the day. A system with trackers will cost more, but will improve your electricity production.
System losses: No system performs at 100 percent efficiency. PVWatts will make standardized assumptions about the performance losses you can expect in real-life conditions. If you have more specific information for your location, you can use the “loss calculator” to adjust for things like dirt, shading, and snow on your panels.
Tilt: Most solar panels aren’t installed perfectly flat – there is some degree of tilt to the system. If you are installing a rooftop system and know the pitch of the roof, you can input that here. If you’re installing a ground mounted system, you’ll want to install the panels at the latitude of your location to maximize production.
Azimuth: The azimuth refers to the direction that the panels face. For maximum electricity production, your panels would face perfectly south – an azimuth angle of 180°. You can click the info button on the calculator to get angles for other compass headings.
Retail electricity rate: This field is used to tell you how much the electricity you produce is worth. It’s auto-populated based on a dataset from 2012, but electricity rates have changed significantly over the past few years. If you know what you’re paying for electricity now, you can input it here to get more accurate savings information on the results page.
There are also some advanced parameters that you can adjust if you want to get really technical. For all of these inputs, you can leave the defaults in place, or adjust based on information that you have for your specific property.
Step 3: Results
The results page shows you monthly and annual solar radiation, electricity production, and the dollar value for electricity produced by the system you just designed. It also offers a system output range that factors in year-to-year variations in weather.
The example results below show that the system will produce just over 5,000 kWh annually, with an annual value of slightly under 1,000.
Also on this page, you can download monthly or hourly results, which give detailed electricity production information that is based on a “typical year weather file.”
What about the cost of installing solar, or long-term savings associated with it?
PVWatts is a flexible tool that can give you customized estimates of electricity production based on your specific weather patterns and system design, but it doesn’t have everything you need to get a complete picture of your solar options.
In addition, EnergySage’s calculator factors in current electricity rates, tax credits and local rebates to tell you what you can expect in terms of net savings over 20 years when you install solar. Costs and savings will depend on whether you pay for your system in cash, take out a solar loan, or sign a solar lease or power purchase agreement with an installer. All of these options are presented in the calculator.
Calculators and other methods of estimating solar production are helpful, but they will never be as accurate as a custom-designed quote. If you’re ready to take it a step further, you can use the EnergySage Solar Marketplace to get actual quotes from local pre-screened installers that are specifically designed for your home at no cost to you. Installers pay EnergySage for the opportunity to work with Solar Marketplace shoppers, so you never have to pay to use the Marketplace to compare quotes for your home.
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