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How to Properly Size Solar for Lighting and Power Projects

SEPCO 12/4/23 6:30 AM
How to Properly Size Solar for Lighting and Power Projects

When embarking on a solar project, it is crucial to determine the specific solar system needed to power the light or component you are working with adequately. Cutting corners is not an option when it comes to off-grid solar, as there is no backup grid power to compensate for any deficiencies. This process consists of seven essential steps, with six focusing solely on the solar aspect and the final step dedicated to determining the size of the battery assembly.


In this blog post, we will delve into these steps and provide you with valuable information to ensure that your system is accurately sized for long-lasting performance.

Solar Power Assembly Sizing

Step 1 - Determine The Load

The very first step is to determine the load. This is the most crucial step and, if not calculated correctly, will set the entire system up to fail. The load is not an average but the total load that will be used per 24 hours. 


For example, a light fixture drawing 40 Watts that is used from dusk to dawn year-round would have a load of 40 times the longest night of the year. Using the longest night will ensure that no matter what, the light will operate from dusk to dawn. If you use an average of 12 hours, your system will fail in the winter.


If you are powering a camera 24/7 and the camera and all other equipment has a draw of 15 Watts, then the load is 25 times 24 hours per day.


If your equipment uses motion sensors, you need to calculate the worst-case scenario for operating the lights. If the camera has a heater, you need to calculate that into the setup to ensure it has plenty of power to operate.


Step 2 - Calculate the Amps

Once you have the total load, divide it by 12 (for 12VDC; if it is 24VDC, this formula will still work just fine), providing you the amps required from the solar panel daily to operate the load adequately. Let's use 14 hours for the fixture and 24 hours for the camera:


40 x 14 = 560 / 12 = 46.667 Amps

15 x 24 = 360 / 12 = 30 Amps


This is the total required amps per day that the solar system must provide to the battery system to ensure that there will be enough charge to operate the load. 


Step 3 - Multiply Out the Load

If you use one fixture or other piece of equipment per system, skip this step. If you are using multiple fixtures, cameras, or something else, multiply the amps by the number of fixtures or cameras installed on one solar system. This is only required when a single solar source powers more than one item.


We are going to assume one item for this exercise. 


Step 4 - Add for Solar Degradation

This is important if you want your solar system to last. Solar panels degrade during their lifetime and typically have a warranty of no more than 20% over the course of 25 years. By adding 20% to the final amps at this point, you ensure that over the course of 20+ years, your solar panel will still produce a charge that can charge the batteries enough to operate the load. 


40 x 14 = 560 / 12 = 46.667 Amps + 20% = 56 Amps

15 x 24 = 360 / 12 = 30 Amps+ 20% = 36 Amps


Now, the total load required by the solar is increased enough to handle this degradation over the system's life, ensuring your system is built to last.


Step 5 - Find the Available Sun

Now you need to find the available sun hours in the winter for your system. Many online calculators are available. We use one that is not really available to the public, but some great ones to reference include Solar Irradiance Calculator by Footprint Hero and the Solar Electricity Handbook's Solar Irradiance Calculator.


Both of these allow you to calculate the available sun in your location in the winter. We will use Greenville, SC, for this exercise because it is one of my favorite places. You can use your location for this if you want to see how it can vary for your location. 


No matter the location, your solar panel must face the equator at around a 45-degree angle for the best winter performance. If you live anywhere it snows, flat panel installation is not recommended since the snow will collect and not allow the panel to collect sunlight during the day. Plus, a flat panel collects the least amount of sun in the winter months because the sun is so low on the horizon. 


Greenville, SC, has a total of ~3.89 sun hours in December. You divide the number of amps required by the solar panel per day by the number of sun hours the location receives per day:


40 x 14 = 560 / 12 = 46.667 Amps + 20% = 56 Amps / 3.89 = 14.40 Amps

15 x 24 = 360 / 12 = 30 Amps+ 20% = 36 Amps / 3.89 = 9.25 Amps


Now you can see that for a 40 Watt fixture to operate all night, from dusk to dawn in Greenville, SC, you will need a solar panel assembly that produces 14.4 Amps, and the 15 Watt camera will need a solar panel assembly that produces only 9.25 Amps. As you can see, these steps are critical to ensure the solar panel assembly produces plenty of energy.


Step 6 - Determine the Solar Panel

Now that you know what the solar panel needs to produce every day to ensure that the system will operate correctly, you can find out what size solar panel system is needed. When looking at a solar panel specification, you look at the Max Power Current (Imp) to see how many amps it will produce at full capacity. Depending on the panel type, this can be shown in 12 or 24 VDC and also depends on the controller used. 


Our 200-watt solar panel produces 10.72 Amps with a standard controller and 15 Amps using an MPPT controller. Therefore, our 200-watt panel will work for both of these applications. This varies for every manufacturer; you should always consult the manufacturer's specifications. 


Battery Assembly Sizing

The final step is determining the battery assembly sizing. Now that you have all the information required to size up the solar, you need to work on sizing up the appropriate battery assembly to operate the load and have plenty of backup for those times when the sun isn't at its brightest. 

Step 7 - Size up the Battery Assembly

Take the load in amps and multiply that by the number of nights of backup you need. The number of nights of backup is determined by your location and the type of battery you will be using. 


Lithium batteries should have ~3 nights backup to ensure you can get through a couple of days of bad weather, but should only be used in locations where the temperature is moderate year-round. Lithium batteries do not perform in extremely hot or cold climates


So, for this example that we have been using - you want a battery that provides:

46.667 Amps x 3 = 140 Amps or 30 Amps x 3 = 90 Amps at 12VDC


GEL and AGM batteries should have closer to 5 nights of backup for moderate-temperature areas. If installed in a colder climate, the backup should be closer to 7-10 nights, depending on how cold it gets. Most of these types of batteries will operate to -40° in the winter. 


So, for this example that we have been using - you want a battery that provides:

46.667 Amps x 5 = 234 Amps or 30 Amps x 5 = 150 Amps at 12VDC


Not only does having the backup provide you with the security of knowing your system will work even when conditions are not perfect, but it will also increase the solar battery lifespan because the less depth of discharge you have, the more cycles a battery has. This is especially true with GEL and AGM batteries; however, Lithium isn't affected quite the same.


Lithium batteries can still perform with higher depth of discharges, which is why you see so many companies with tiny batteries; however, they typically provide little to no backup. One cloudy, overcast day, and everything fails because the solar can't put back all the energy used at night.


Comprehending these crucial factors can determine the success or failure of a solar system, which is why relying on "off-the-shelf" solutions is not advisable when aiming for a reliable system design. Each system must be tailor-made to meet the specific requirements of the project. By fully understanding and implementing the abovementioned steps, you can ensure a dependable system design that will operate efficiently for over 25 years.