Lead Acid vs. Lithium-ion Battery: Which One Is Better for Solar?

Lead acid and lithium-ion are two main battery technologies solar systems are often paired with. But which one should you go with?

It can be a tough choice, as both types of batteries have their own unique benefits and drawbacks. For one homeowner, lead acid would make perfect sense. But not for another homeowner—a lithium-ion technology will be more suitable for them.

In this article, we will compare lead acid batteries and lithium-ion batteries in terms of their performance, cost, lifespan, efficiency, and other metrics to help you determine which one is the better option for your solar energy needs.

Lead acid vs. lithium-ion batteries brief overview

Before going to discuss their pros and cons, let’s first take a closer at look at batteries, how do they work, and where are they most commonly used.

Lead-acid battery

Lead acid battery is the oldest type of rechargeable battery that have been around for over 150 years. Since its invention in 1859, the technology has been tested-and-tried and constantly improved, making it the most readily available type of battery anywhere. They work by using lead and lead oxide plates as the electrodes and a sulfuric acid electrolyte solution to facilitate the chemical reactions that produce electricity.

Lead-acid batteries are relatively simple and inexpensive to manufacture, which has contributed to their widespread use. They are widely used in applications such as backup power systems, vehicles, solar energy storage, and uninterruptible power supplies (UPS), to name a few.

Lithium-ion battery

Lithium-ion batteries, on the other hand, is a newer technology and uses an altogether different chemistry than their lead-acid counterparts. They were developed first in the 1970s, but they gained popularity among solar owners only in the early 2010s, thanks to their high energy density, improved efficiency, and longer lifespan. Besides solar, they are also commonly used in portable electronic devices, electric vehicles, and grid-scale energy storage systems.

Li-ion batteries work through a process called lithium-ion battery electrochemistry, where lithium ions move between the cathode (positive electrode) and the anode (negative electrode) to produce electricity.

Lead acid vs. lithium-ion batteries: how do they compare?

Cost

First of all, costs. Lead acid batteries are simpler and easier to manufacture, hence they cost less. Lithium-ion energy storage, conversely, cost more.

For example, if you want to pair a 6 kW off-grid solar system with batteries, the cost you would incur for lead-acid battery bank would be in the vicinity of $4,000. But if you decide to go with lithium-ion, the upfront cost immediately shoots up to around $13,000, 3x more than lead acid batteries. These costs can vary depending on your goals and requirements. The type of chemistry and design also impact the upfront and installation costs.

If you go with flooded lead-acid batteries (FLA), they are comparatively cheaper than sealed lead-acid (SLA) batteries. Similarly, lithium-ion batteries come in various chemistries, the primary ones being nickel manganese cobalt (NMC) and lithium iron phosphate (LFP). NMC batteries are 30-40% cheaper than their LFP counterparts.

In any case, it appears that lead-acid would make more sense, right? It is not that simple. Head on to the next section—lifespan—to get a better idea of how much would the energy storage system cost you in the long run. That will keep you in a better position to make a decision based on costs.

Lifespan

Lead-acid batteries have a lifetime of 2-5 years, and can work for up 10 years, if properly maintained. FLA batteries require regular maintenance—you will have to periodically add water to maintain the balance. SLA do not require regular maintenance, so the lifetime costs would be lower than FLA batteries.

Lithium-ion batteries, however, come with a warranty of 10 years and can work for up to 15 years. Both top li-ion battery manufacturers, Tesla and Generac, provide a 10-year warranty for their solar batteries. Warranty means that the battery would work efficient, sometimes up to 70% of its original performance at the time of expiry date, but it doesn’t mean that they get expired upon the completion of warranty expiry.

Now, the costs comparison would be more apt to consider here.

If you go with lead-acid batteries, you will need to replace your battery bank sometimes after 3 years. Or, 5. But it will not last for 10 years. It means that the total cost of installing a lead-acid battery bank would become thrice or twice, i.e., $8,000-12,000. Add maintenance costs and the lifetime cost of lead-acid batteries almost equal those of lithium-ion batteries.

So, while lithium-ion energy storage does cost more upfront, if we consider the total costs of ownership, the difference may not be much. Lastly, after the federal income tax credit, the cost of a battery system further decreases by 30%.  

Capacity

Lead acid batteries and lithium ion batteries have different capacity rating. The capacity rating of a battery refers to the amount of energy that the battery can store.

Lead acid batteries have a capacity rating of around 30-200 Ah (amp-hours), or 0.36-2.4 kilowatt-hour (kWh). This means that a lead acid battery with a capacity of 100 Ah can deliver a current of 1 A for 100 hours, or a current of 10 A for 10 hours. Or, 360 watts for one hour to 2400 watts per hour. Some types of lead acid batteries may have a capacity of 5kWh. 

Lithium ion batteries, on the other hand, have a higher capacity rating than lead acid batteries. They have a capacity rating of around 200-500 Ah, or 2.4-6kWh. This means that a lithium ion battery with a capacity of 200 Ah can deliver a current of 1 A for 200 hours, or a current of 10 A for 20 hours. Some li-ion batteries, such as Powerwall and PWRcell have capacities of 14kWh and up to 18kWh, respectively. 

Clearly, lithium-ion battery is a winner here.

Energy density

This is related to capacity of a battery but it adds a condition, i.e., physical space. It means the amount of energy a battery can store in a specific physical space. For example, how much energy can a battery store in one liter?

According to Cummins, lithium-ion batteries achieve an energy density 125-600+ watt-hour per liter (Wh/L). For lead-acid chemistry, the energy density stands around 50-90 Wh/L. This implies that lead-acid would require more space to produce the same amount of energy a lithium-ion battery can produce in that given space.

For solar homeowners with space issues, lithium-ion would be more suitable.

Depth of discharge

Depth of discharge (DoD) refers to the percentage of power capacity that a battery can safely provide. For example, if a 5kWh battery has 60% DoD, it means you should not take more than 3kWh (60%) from your battery.   

The depth of discharge of a lead-acid battery depends on the type of lead-acid battery. A flooded lead-acid battery can typically be discharged to a DOD of 50% without causing any damage. For sealed lead-acid batteries, the DoD is slightly higher, around 60-75%.

But lithium-ion solar batteries typically have a much higher depth of discharge than lead-acid batteries, and can typically be discharged to a DoD of 90% or even 95% without any problems.

If you regularly discharge a battery more than the recommended DoD, this not only affects its performance but also shortens its lifespan.

Roundtrip efficiency

Often simply called the battery efficiency, it means how much energy stored is actually usable. For example, if the efficiency is 70%, it means the remaining 30% is lost or consumed by the battery’s operating system.

Lead acid batteries have a round-trip efficiency of about 65-80%. This means that when you discharge a lead acid battery and then recharge it, only 65-80% of the energy you put back into the battery is available for use.

Whereas lithium-ion batteries have a round-trip efficiency of about 80-95%. Tesla’s Powerwall comes with an efficiency of 90%, while Generac’s PWRcell comes with 96.5% of roundtrip efficiency. That is, only 3.5-10% of energy is consumed by the battery itself, while the rest is available for use.

That said, it’s important to note that these figures can vary depending on the specific type of lead acid or lithium-ion battery being used, as well as how well the battery is maintained and how it is used. Factors such as temperature, depth of discharge, and charging and discharge rates can also affect a battery’s round-trip efficiency.

Charging rate

Charging rate means the amount of time taken by a battery to completely recharge. The higher the charging rate, the quicker the battery gets charged.

Lead acid batteries can take around 10 hours to get charged; lithium-ion technologies, on the other hand, take 3 hours, or even less time, to charge. For many homeowners, charging rate won’t be a concern, as with grid-tied systems, batteries are rarely used. But if you go completely off-grid, you will have to consider a battery bank that can be charged in less than 5 hours. As when the sun sets, you will have to rely on your energy storage to keep you powered.

Which solar battery is best?

It is obvious—lithium-ion is excellent for solar. They have a longer lifespan, takes up less space, and have better efficiency. But you can consider lead-acid if you want to reduce your upfront costs. Lead-acid may also make more sense if you want to install the system somewhere, such a boat or RV, where it is used less often. Lead-acid batteries when not regularly used can have longer lifespans.