How to Size a Solar Street Light Correctly
We at Sol by Sunna Design are pleased to be able to provide communities dependable solar street lighting so that they may accomplish sustainability goals while also illuminating their parks and public spaces. Our lights have been field-tested to consistently reach industry-standard light levels for years with no maintenance. What is the process? We spend a lot of time making sure the solar and batteries in our systems are the right size, in addition to having an innovative system design and purpose-built, efficient energy management.
A properly sized solar light system will have just the right amount of solar power, battery storage, and LED fixture efficiency to run at the project's required light levels every night for several years, while also providing backup power to keep things running in case of inclement weather and avoiding the need for extra solar panels or batteries. It is the ideal solution—not too many solar components, which would make the system too costly, nor too few, which would cause the system to fail early.
Three essential components—a healthy array-to-load ratio, enough battery capacity and backup power, and an effective LED fixture and operating profile—are necessary for a properly scaled, dependable solar street light.
Download our Ultimate Solar Lighting Guide to learn more about optimum sizing. This comprehensive reference explores product details and comparisons, as well as how solar lighting works and why customers select it.
Ratio of arrays to loads
Sizing a functional solar light correctly necessitates balancing a variety of inputs and outputs. These include examining the project's location, defining the correct battery chemistry and capacity, choosing an effective LED fixture and operation schedule, keeping enough battery backup power on hand in case of inclement weather, and studying the project's location.
The array-to-load ratio (ALR), a straightforward, unbreakable criterion for designing solar lighting systems, should be taken into account initially. It is the ratio of the energy produced by the solar panels (referred to as the "array," or energy-in), to the energy used by the light fixture (referred to as the "load," or energy-out). A lighting system has a healthy ALR if it captures more solar energy during the day than it uses when the light comes on at night.
Any solar lighting installation should always start with the area in mind. The quantity of solar energy that reaches various latitudes varies; this is known as solar insolation and is expressed in kWh/m2/day. The average annual daily sun energy for the Americas is shown in the graph below. As you can see, California and other southern states get far more solar energy each day than Alaska and other northern states. This implies that to achieve the same light levels, northern sites will often need a bigger solar array and extra batteries than their counterparts in the south.
Direct Normal Radiation from Solar America
The location of a project may be used to estimate the solar power and battery capacity of a potential system. Failure to consider location might result in a system that can't handle the modest demand and fails early or in a more expensive system with redundant solar capacity. As a result, location should always be taken into account initially.
In order to hide ineffective energy management or an inadequately designed system, manufacturers may install more or bigger solar panels. Unfortunately, there may be too much solar energy. It costs extra to transport and install an excessively big machine. Depending on the aesthetics of the local urban architecture, it appears heavy and unattractive and increases the wind stress on the panels, necessitating larger, more expensive poles to compensate.
For additional information, see our article on the best practices for solar panel sizing.
2. Backup power and batteries
A solar street light's batteries determine whether it will work or not, therefore a potential buyer could be worried about a battery that fails too soon. A battery or solar technology's inherently flawed design is virtually never the cause of premature battery demise. This problem is the result of faulty system scaling, poor energy control, and incorrect design. This solar light will operate dependably for many years when a manufacturer has carefully constructed a system, worked on effective energy management, and scaled it with an adequate solar array power and battery capacity.
The primary battery types are used by solar lighting producers.
Lead-acid: Reliable and inexpensive, lead-acid batteries have been in use for many years. They are often used in autos and in larger industrial applications, including as hospital equipment and uninterruptible power supply (UPS) systems, where having access to dependable power in an emergency is essential. The most common battery technology for solar illumination applications is this one.
One of the most popular rechargeable battery types for consumer usage is the nickel-metal hydride (NiMH) battery type. NiMH batteries, like the All-in-One (iSSL) and All-in-Two from SOL by Sunna Design, are ideal for solar lighting systems when you don't require extra-large battery banks due to their high energy density, deep cycle capabilities, and broad working temperature range (UP)
Lithium-ion (Li-ion) batteries have the best energy density while being the priciest of the three. Li-ion batteries are often found in laptops and mobile phones, but they are also being employed in a growing number of new products, including aerospace and military hardware. One drawback of lithium-ion batteries is their inability to withstand very cold temperatures (they cease charging below 32°F), as well as their limited capacity for recycling. Less than 5% of lithium-ion batteries are thought to be recycled in the US.
The advantages and disadvantages of each battery chemistry vary based on the application and project requirements. Their distinctive depth of discharge patterns are one of the three groups' main differences.
The proportion of a battery's capacity that is used up while it is in operation is referred to as the depth of discharge (sometimes referred to as DOD). The DOD would be 25%, for instance, if a solar lamp ran all night and used up a quarter of its battery capacity.
Understanding depth of discharge is important for solar applications since it greatly affects a battery's cycle life, or how many times it can be depleted and then recharged. Some battery chemistries, such as NiMH and Li-ion, may safely sustain being nearly completely discharged before having to be recharged. This amount of discharge would significantly shorten the battery's cycle life for other chemistries, such lead-acid. The capacity that may be safely drained for each of the three battery types is shown in the chart below as an example.
While NiMH and Li-ion batteries may safely drain more each night, the lead-acid battery has the extra advantage of having greater built-in backup power because to its shorter DOD. More batteries would be needed and the system cost would rise significantly if a NiMH or Li-ion-based system could provide backup power on par with a lead-acid-based solution. When extended periods of bad weather are frequent, making sure a system has enough backup battery power may assist enhance the operation and endurance of the light.
Here is an illustration of how to size solar batteries. Consider for the sake of this example that our solar light is powering a 40-watt LED fixture for a 14-hour winter night in Los Angeles at 100% brightness. The overall load on our system each night would be 560 watt-hours (40 watts x 14 hours = 560 watt-hours). What is the minimum capacity for each battery type, assuming ideal conditions and a fully charged battery at the beginning of the night?
Here are some samples of healthy and low system battery size utilizing the battery kinds listed above so that we may have a better understanding of what our minimum battery capacity should be.
For additional details on battery size, see our page on backup power for solar illumination.
3. The size and operational profile of LED fixtures
LED technologies and solar gadgets go along well. The most energy-efficient lighting fixtures on the market, LED luminaires, have made solar-equipped light systems reliable and affordable substitutes for conventional commercial lighting. Additionally, LEDs' efficiency is growing, allowing them to produce more lumens (also known as units of light) while using less energy than in the past. For instance, at warm color temperatures like 3000K, modern LED lighting may provide 160 lumens per watt. In the area of solar system size, this is a welcome breakthrough since it allows smaller systems to get the same outcomes as bigger installations that employ lower effectiveness fixtures.
Selecting an acceptable operational profile is another element in the solar sizing process. A schedule known as an operational profile governs when a light fixture is turned on and off as well as if (and when) it needs to decrease its output. These profiles enable manufacturers to adjust their systems to specific power management requirements.
Here are a few illustrations of typical operational profiles:
Dusk to dawn (all-night operation): the light will remain on throughout the night at the same output level.
Dim at non-peak times; for instance, the light may remain on for five hours after sunset at the necessary output level before being dimmed to 30% of that level. The output level goes back to 100% till sunrise two hours before dawn.
At a certain time, the light will be dimmed or turned off. For instance, it may remain on until 11 p.m. at the appropriate output level.
The operational profile, together with fixture power draw, aids in calculating overnight energy usage and is crucial for choosing the right system size.
The most crucial phase in developing a solar street light to guarantee long-term dependability is proper size. Check out our infographic here to understand more about the science of solar scaling, or download our comprehensive reference to solar lighting specifications.
