So, you’ve decided to use solar lighting on your next project. Exciting! Now comes the hard part: deciding which product and manufacturer to use.
Unfortunately, shopping for solar lighting isn’t like shopping for an air fryer, fitness tracker, or home security system. There is no Wirecutter or CNET for solar lighting. Instead, there are… spec sheets.
What’s a spec sheet, you ask? It’s the document that contains all the information you need to compare multiple products and determine which is best for your project. It can also help you identify features that set manufacturers apart from one another, like warranties, certifications, and testing standards.
Spec sheets are indispensable for solar lighting customers, but they can be confusing if you don’t ‘speak the language.’ What do all the numbers and acronyms mean? Which ones matter most? How can you see through the hype and verify that a product will live up to its claims? We’ve got the answers, so let’s get to it!
Here’s the good news: You don’t need to understand everything on a spec sheet to make an informed purchasing decision. Spec sheets contain loads of technical information, and not all of it will be relevant to you.
Your first priority should be determining whether the product you’re considering will be sustainable for your project (i.e., it has enough solar collection and battery capacity to power the fixture at the required output for the desired duration). To do that, you need just four pieces of data:
Here’s an example of a spec sheet that does include the necessary info (and where to find it):
With these inputs, you can do some simple math that will lead you to a yes/no answer about whether a system will fit your needs.
How big does the solar panel need to be? The first step is to find the average number of peak sun hours (hours of strong sunlight) your location receives during the worst month of the year for solar insolation. The NREL’s PVWatts Calculator is great for this.
Let’s say your project is in Charlotte, NC, where the peak sun hours in December are 3.73 (assuming a 30° panel tilt to maximize insolation). To find the maximum solar charging capacity you’ll get each day, multiply the peak sun hours by the panel size in watt-peak (Wp).
3.73 x 110Wp = 410 Wh
Using this value would be fine if peak sun hours never dipped below 3.73. But we know that 3.73 is the average, not the low, and there will certainly be days when less (or even no) energy is collected.
That’s why it’s critical to factor in an adequate array-to-load ratio (ALR)—to ensure the system can recover from the previous night’s usage, even when solar collection is subpar. We use an ALR of 1.2:1 to calculate the maximum available daily load.
410 / 1.2 = 341 Wh
If you skip this step and assume an ALR of 1:1, your system will operate fine during summer but will almost certainly fail when collection drops in winter.
Pro tip: Reputable solar lighting manufacturers will also apply derating factors to their solar panel and battery sizing calculations to account for losses due to dust, snow, shade, temperature, and other factors that can cause the power to deviate from the expected value. The derate factor isn’t something you’ll find on a spec sheet, but it is something to be aware of and ask about. If a company doesn’t factor in deratings, the system autonomy will be less than what is advertised.
You now know how much energy is available, but you still need to calculate how much is needed to run the fixture for the duration you need it. To do that, you need the length of the longest night—another readily available data point you can get using NOAA’s Solar Calculator.
For our Charlotte example, it’s 14 hours (5:18 p.m. to 7:18 a.m. on December 21). Next, you can return to your spec sheet and locate the fixture output (in watts). Note that there is almost always a range (e.g. 4-60W). If your project has a lumen requirement, you can use this and the fixture’s efficacy to guide your selection.
For example, if you need 4,500 lumens, and your fixture has an efficacy of 150LPW, you will need at least a 30W fixture (4,500 / 150 = 30W). Then, multiply the fixture output by the longest night to get the system load.
30W x 14 = 420Wh
This is the daily load of the fixture and the amount of energy that the solar panels will need to be able to recharge each day. In our example, the 110Wp panel would not be enough for this operation (because the maximum available load is 341Wh, and 420Wh exceeds that).
This doesn’t mean the system is a no-go. It just means you need to reduce the load, either by using a lower power fixture or changing the operating profile. Rather than dusk to dawn, you might reduce the output in the middle of the night.
A 4D1 profile would deliver 100% output for the first four hours, then dim to 50%, before returning to 100% one hour before dawn. This is how the math looks:
30W x 5 = 150Wh
30W x 50% x 9 = 135 Wh
150 + 135Wh = 285Wh
Since 285 Wh is less than the max load of 341 Wh, the 110Wp panel and 30W fixture with a 4D1 schedule would work.
When it comes to solar lighting, batteries mean the difference between a system lasting six months before fizzling out or lasting five years (or more!) without issue, so it’s important to get them right.
The first thing to know is that there are different types of batteries. This might seem obvious, but since most manufacturers offer just one, it’s worth pointing out. Sol offers three chemistries, and we have a guide to help you understand their differences.
The best battery for your project depends on a slew of factors—read the guide!—but one of the main things you need to pay attention to is autonomy, which is how long a battery can function without recharging. Without adequate autonomy, a system will shut down when it can’t collect enough solar to power it.
Solar lights should always have enough battery capacity to last two whole nights. Returning to our example, we can multiply the load by two to (start to) calculate the minimum battery capacity.
285Wh x 2 = 570 Wh
However, you also need to consider that no battery should ever be discharged completely. Different batteries have different recommended depths of discharge (DOD), the percentage of a battery’s capacity that can safely be consumed relative to its total capacity.
Lead acid batteries can be discharged to 25%. Lithium can be discharged to 70%. Repeatedly discharging a battery beyond this threshold will drastically reduce its lifespan.
As we did earlier with the array-to-load ratio, we need to factor DOD into our calculations. Here’s what you do (assuming the system has a lithium battery):
570Wh / .7 = 814Wh
Looking again at the example spec sheet, a single battery has a capacity of 460.8 Wh. To meet the two-day autonomy requirement, the system would need two batteries.
A word of caution: Some manufacturers overlook the recommended depth of discharge, resulting in systems that are, in some cases, drastically undersized. Smaller batteries can make for cheaper systems, but the savings are hardly worth it if the system fails on the first day without solar.
Now that you understand solar sizing, you should be able to identify when a system will and won’t work for a project. That’s worth celebrating! But before we leave you—and before you decide on a product—we wanted to draw your attention to a few other things worth scanning your spec sheets for.
Even if federal funding and other grants can offset a significant portion of the cost, solar lighting remains a significant investment. Warranties can help ensure peace of mind, allowing you to return, replace, or repair a system if it doesn’t work as expected.
Most spec sheets include warranty information, usually something like a “5-year limited warranty” or “10-year warranty, excluding battery.” Checking whether a product’s warranty is full or limited is a very good idea, as is finding out what exactly it covers. A common practice is to offer a full warranty on the system but pro-rate coverage on the battery.
Another thing to be aware of is that “lifespan” and “warranty” are not the same. Spec sheets frequently include lifespan estimates for various components (solar panels especially have a very long lifespan—like 25 years), but that does not mean the manufacturer is obligated to fix or replace them. For that, you need a warranty.
>>Sol proudly offers a 10-year warranty on all its solar lighting systems, including the battery.
Certifications are one of the best ways to independently verify manufacturers’ claims about their products (as well as overall safety and regulatory compliance). Certificates are granted by independent entities, like the International Electrotechnical Commission (IEC and Institute of Electrical and Electronics Engineers (IEEE).
Here are a few others you might encounter:
Additionally, you can see if a product has undergone independent testing—specifically, LM-79, a standardized IES-approved testing method that measures LED products’ optical and electrical characteristics. LM-79 is the only test that provides verifiable data on the output and efficacy of the complete fixture (versus the bare LED chip), making it an important metric and comparison point.
>>Read more about LM-79 and efficacy
We’ve covered fixture output as it relates to sizing, but you should also investigate the fixture more broadly, starting with, ‘What is it?’ and ‘Where does it come from?’
There are two approaches manufacturers take to fixtures—building their own or using someone else’s. Which is better depends on who you ask, but neither guarantees a high-quality product.
Building it in-house is difficult. LED fixtures comprise numerous intricate components, including chips, drivers, heat sinks, and optics. When a company simultaneously develops solar engines and other components, keeping up with rapidly evolving LED technology can be tough. Think “Jack of all trades, master of none.”
Using off-the-shelf components has a whole other set of issues. While the output of cheap, imported LEDs can be impressive, they typically have poor light quality, thermal management, and optical controls—not to mention limited warranties and support.
Our solution: Leave the lights to the experts. Tier-1 manufacturers like Acuity Brands have a long-standing reputation for delivering high-performance fixtures for various applications, including solar. Fixtures spread light evenly, avoid glare and light pollution, and maintain brightness—up to 100,000 hours (more than 11 years)!
Understanding and comparing solar lighting spec sheets might seem daunting, but it’s not impossible if you have the right information and know how to use it. Hopefully, this article has given you the tools to navigate the complexities of solar lighting more confidently and ultimately leads you to choose the best product for your needs.
Have a question about spec sheets—or anything else? Our team of solar lighting experts would be happy to help.
