At the risk of stating the obvious, solar lights rely on the sun to generate power. When the sun shines on a solar panel, energy from the sunlight is absorbed and converted into electricity thanks to some magic science called the photovoltaic effect. This electricity can then be used to power your light—if you need light during the day.
Thing is, we don’t generally need light during the day when the sun is out. We need it at night (when it’s not). This mismatch is part of the challenge known as “intermittency,” and it would be a significant one were it not for batteries.
Rechargeable batteries like the ones Sol uses let solar lighting systems store energy collected during the day for use at night. They also keep the system running through periods of low solar irradiation when electricity generation is limited.
But not all batteries are created equal, and not every battery is right for every project. While some manufacturers offer a single battery and insist it works everywhere, Sol uses three—and understands when and where each works best. Here, we’ll cover some useful battery concepts and explain the ins and outs of the battery technologies we use.
Even if you don’t know much about batteries, you’re probably familiar with capacity: the total amount of energy a battery can store. It’s typically measured in watt-hours (Wh). For example, a 100 Wh battery could power a 100-watt light for one hour, or a 10-watt light for 10 hours.
Depth of discharge (DOD) is the percentage of a battery’s capacity that is consumed relative to its total capacity. For example, if you were to discharge 500Wh from a 1,500Wh battery, the DOD would be 33% (500Wh/1,500Wh); if you were to discharge 1,200 Wh from the same battery, the DOD would be 80%.
Different battery chemistries have different recommended DODs. Some are relatively shallow (~25%), and some are very deep (90% or more). You can discharge a battery beyond its recommended DOD, but doing so repeatedly will greatly reduce its lifespan.
That also means a battery’s rated capacity and its usable capacity generally aren’t the same. A battery may be rated for 1,500 Wh, but only a portion of that energy is intended for regular use before recharging.
Along with recommended DOD, manufacturers also estimate cycle life: how many discharge/charge cycles a battery is expected to deliver. Every time a battery goes from 100% to a specified DOD and back again is a cycle.
Cycle life varies greatly between manufacturers, chemistries, and how and where the battery is used. In general, deeper discharges put more stress on a battery, reducing the number of cycles it can deliver. Higher operating temperatures can also speed degradation and shorten service life, although the effect varies by battery chemistry and operating conditions.
It’s important to note that batteries don’t simply die on the last day of their rated cycle life. That rating commonly refers to the number of cycles a battery can experience before its capacity falls below a defined threshold, often 80% of its initial value. Most batteries will continue to operate after that point, although with progressively less available capacity.
Backup power describes how long a system can function when solar collection is limited. We sometimes call it “autonomy,” and it’s important because even if a system is sized correctly, weather is never 100% predictable. What if you get an unusually dark and stormy December? What if it snows… in Arizona?
Backup power essentially gives the system breathing room for these outlier conditions. How much you need depends on your location, operating requirements, and comfort level, but we typically recommend at least two days.
You can calculate days of autonomy by dividing a battery’s usable capacity—that’s rated capacity multiplied by the recommended DOD—by the system’s daily load. This assumes a fully charged battery and no solar recharge during the backup period (though even on cloudy days, solar panels still collect some energy).
Usable battery capacity ÷ daily system load = days of autonomy
Let’s go back to the 1,500 Wh battery from earlier. For the sake of simplicity, assume a DOD of 50% and a load of 550 Wh.
1,500 Wh × 50% = 750 Wh of usable capacity
750 Wh ÷ 550 Wh = 1.36 days of autonomy
That falls short of the two days we recommend. To provide two full days at the same load, you’d need:
550 Wh × 2 days = 1,100 Wh of usable capacity
At a 50% recommended DOD, that means a rated battery capacity of:
1,100 Wh ÷ 50% = 2,200 Wh
More backup power generally means more battery capacity, which can add cost, size, and weight. You can also extend autonomy by reducing the load with an operating profile that dims the fixture during lower-activity periods. In other words: store more energy or use less of it.
Now that you understand the key metrics that can be used to compare batteries, let’s… compare batteries! Sol offers three battery types: lead-acid, nickel-metal hydride (NiMH), and lithium-iron phosphate (LiFePO4).
Lead-acid batteries are a proven technology that’s been used across a wide range of applications for decades. They have a relatively shallow recommended DOD (~25%), so more total capacity is required to support a given load. However, that larger battery bank has an upside: substantial backup power. Our EverGen-M system, for example, comes with at least 4 days.
Compared to lead-acid, both NiMH and LiFePO4 batteries have a much higher recommended DOD (70 and 90%, respectively). They also have higher energy density, so they can store the same amount of energy in a smaller, lighter package, translating to more compact systems that can use lighter, more affordable poles.
Not all batteries perform well in all climates, and temperature ratings can be more complicated than they first appear. Batteries typically have different limits on charging and discharging, which is an important distinction in solar lighting, where the battery needs to both power the light at night and recharge during the day.
Our NiMH batteries tolerate a particularly wide range of temperatures: those used in our iSSL and UP systems can charge from -40°F to 158°F. Lead-acid batteries are also highly tolerant of cold, making them a good fit for projects in northern climates.
LiFePO4 batteries require a little more consideration. While lithium batteries can discharge below freezing, charging at low temperatures is more restrictive. The LiFePO4 batteries we use have a lower charging limit of -4°F, making them less suitable for regions that routinely experience colder temps but a good fit for more temperate zones.
Looking at upfront purchase price, lead-acid batteries are generally the most affordable. NiMH batteries cost considerably more, while LiFePO4 typically falls somewhere in between, although actual costs vary by battery size, design, and application.
Lead-acid batteries also have an edge when it comes to recyclability. According to Battery Council International, about 98% of lead-acid batteries in the U.S. are recycled since lead can be infinitely recycled without notable losses in quality or performance.
Recycling for newer chemistries is much less established. LiFePO4 batteries have historically been harder to recycle because they contain fewer high-value metals than other lithium-ion chemistries. A 2024 study estimated that the overall lithium recovery rate from spent LiFePO4 batteries was below 1%.
When batteries fail prematurely, the battery itself isn’t necessarily the problem. It may be undersized for the load, discharged too deeply, inadequately recharged, or simply the wrong chemistry for the climate.
Understanding these factors—and working with a manufacturer that understands how they interact—helps ensure that you get a battery capable of supporting your system now and 10 years from now.
Have a question about batteries or solar technology in general? Need some advice on a project? Drop us a line! We’d love to hear from you.
