Whether you’re looking for a way to keep the lights on when the grid goes down or you want to help offset demand charges, continued advances in the solar storage industry are making batteries cheaper and more efficient. While still an expensive option for home energy backup, batteries run on a free energy source and are more environmentally friendly compared to a gasoline generator.
If you’re wondering what batteries are best for solar storage, there’s no one-size-fits-all answer. It’s important to keep in mind your specific goals and budget.
In this blog, we’ll go over the common battery types of lead-acid, lithium-ion, and saltwater. We’ll share the advantages and disadvantages of each to help you determine what the best type of battery is for your solar system.
Solar Battery Basics
Before we dive into what battery is best for your solar system, we want to go over a few common battery-related terms.
Capacity is how much electricity your solar battery can store. Just like your energy loads, capacity is measured in kilowatt-hours (kWh). The greater the battery’s capacity, the more energy it holds.
Power, on the other hand, is the amount of energy the battery can provide at a single moment and is measured in kilowatts (kW). These two factors go hand-in-hand when it comes to storing your solar panels’ energy and keeping your loads running.
A battery with high capacity stores a lot of energy, whereas a high-power battery can power equipment that uses a lot of electricity.
Depth of Discharge (DoD)
Depth of Discharge, or DoD, is the percentage of the battery’s charge you use. For example, say you have a battery with a capacity of 13.5 kWh. You use 10 kWh of its charge, which is about 74%. The depth of discharge is 74%.
Battery manufacturers have a recommended maximum DoD for most batteries, meaning they don’t recommend you use 100% of your battery’s power. Doing so could shorten your battery’s life significantly. The DoD of batteries varies, but in most cases, batteries with a higher DoD mean you can use a greater percentage of the battery’s power.
If you’ve ever owned the same laptop or smartphone for a few years, you’ve likely noticed the battery doesn’t last as long in year two as it did fresh out of the box. This is because the battery’s ability to hold a charge decreases with each charging and discharging cycle.
Most batteries have warranties that will guarantee a battery for a certain number of cycles or a specified time period. And just as with other solar system equipment like panels and inverters, picking a well-established manufacturer with a long history makes it more likely they’ll be around to honor those warranties.
Best Types of Batteries for Solar Storage
Lead-acid batteries have been commercially available for over 100 years and have been used for off-grid solar systems for decades.
Lead-acid batteries come in a few different types, including wet-cell or flooded lead acid batteries, gel cell, and absorbed glass mat or AGM batteries. For decades, wet-cell deep-cycle batteries were the go-to for off-grid systems, providing dependable storage for a relatively low cost.
However, flooded batteries need to be refilled regularly with water to prevent the electrolytes from evaporating during charging. While gel and AGM don’t require this added maintenance, they are typically more expensive and have a shorter battery life.
For all types of lead-acid batteries, their popularity is in part to the ratio of watt-hour capacity and cost. However, as one of the cheapest options for battery backups, they also tend to have a shorter life and lower DoD (about 60%) than lithium-ion batteries.
With a relatively low battery life, you may find yourself replacing them every few years. This can be shortened if you don’t care for and use your battery properly. Additionally, lead-acid batteries tend to be heavy, large, and can produce harmful gasses if overcharged.
Another caveat is their need for regular use, maintenance, and ventilation. You should keep this in mind if you are not able to regularly use and maintain the battery.
While lead-acid batteries are an affordable, tried-and-true method of solar energy storage, their low DoD and their need for regular maintenance might make them an impractical backup system for your grid-tied home or business.
Lithium-ion batteries have grown to become the most popular method of solar storage, and are rapidly improving and becoming more affordable as electric vehicle companies like Tesla spearhead their continued development and improvement.
As a smaller, sleeker, and longer-lasting option, lithium-ion batteries are a great option for solar energy backup, but they do come with a higher price tag compared to lead-acid batteries.
Lithium-ion batteries offer a much higher DoD over lead-acid batteries (about 80% - 90%), meaning you can use more of the battery’s charge to power your loads. They also have a much longer battery life, meaning it will last longer than a lead-acid battery, which can help offset the initial cost.
If you are looking to provide power to equipment that runs on a lot of electricity - like heavy electrical equipment, lithium-ion batteries tend to be a better bet over less-powerful lead-acid batteries.
Though there are some safety concerns regarding fire, lithium-ion batteries do not produce the toxic gasses that lead-acid batteries do. They also require little maintenance and their lighter body means easier installation.
Saltwater batteries or sodium batteries use salt to produce power. Their ability to produce non-toxic, long-duration power make them a safer option. Unlike lithium-ion batteries, saltwater batteries can be drained. If you try and do this with lithium-ion or lead-acid batteries, you will damage the batteries and they can fail.
Saltwater batteries are the best option if you’re concerned about safety. Not only do they lack the harmful chemicals included in lead-acid batteries, they can operate in a broader temperature range, and their chemical composition leads to essentially no risk of fire. They’re also fully recyclable.
Though not as expensive as lithium-ion batteries, saltwater batteries do cost more than lead-acid batteries. They also tend to offer less in terms of capacity and power, compared to lithium-ion batteries.
So whether you’re most concerned about initial investment, overall efficiency, or safety, the right battery for a solar system is based on your specific needs.
At Paradise Energy, we’re currently installing lithium-ion batteries for our customers and are actively researching other technologies to consider on a case-by-case basis.
Want to learn more? Sign up for our four-part video series to learn more about the installation process and cost.