The sun is a seemingly endless supply of energy, at least from a human perspective. It will one day run completely out of puff, but that point in time is billions of years ahead of us. Right now that big ball of nuclear fire is beaming down energy onto the surface of the Earth, whether we want it to or not.
We’ve figured out a few ways of generating electricity from solar energy and one of the most popular methods, photovoltaics, converts a portion of that solar energy into electricity. You can use this electricity to directly power all sorts of things. For instance, most of us have owned a solar-powered calculator at some point.
But what if you want to use the energy at a later point in time? At night, for example, or when the sun is hidden behind clouds. Most solar-power applications won’t work just running directly from a fluctuating photovoltaic cell. This means that various energy storage solutions have been created over the years to save up the power captured from the sun. These are the main options out there for the typical solar power consumer, each with their own unique pros and cons.
Before we look at the actual battery technologies in question, let’s first talk about the different characteristics that determine what a battery is good for in the solar context.
The capacity of a battery is a pretty straightforward concept. This is the measurement of how much electricity it can store; in effect, how much work can be done with the energy stored up in that particular battery.
Battery capacity in solar systems is usually measured in kilowatt hours, or kWh. This is the same unit that your electricity bill comes in. It’s also exactly what it sounds like. If a battery has a 1kWh capacity it means the total power is equal to drawing 1 kilowatt for an hour. Or half a kW for two hours, and so on. However, that does not mean that the battery can deliver that much in one go, which is a separate aspect to consider.
Regardless of how much capacity a battery has, each differs in how much of that power it can deliver all at once. The power delivery of a battery is measured in kW. So a full-capacity 1kW battery can run for an hour, assuming you are drawing that much power.
Power delivery figures matter when you are trying to figure out what sorts of appliances you can run from a given battery. Although a battery may have plenty of capacity, running high-draw devices such as an oven may be impossible because it can’t deliver the power levels that device needs. This means that when designing your solar power system you have to carefully calculate how much energy you’ll need at any one time.
When your phone says that you have 0% left in the tank, it’s actually lying a little. Batteries need to keep some charge in order to keep working or they can suffer irreparable damage.
The “depth of discharge” is the percentage of the batteries’ total capacity you can use before you risk something going wrong. Obviously the higher this number is, the more of the stored energy you can use. That doesn’t mean bigger is always better. Use the DoD number as well as the total capacity of the battery to figure out if it will serve your specific needs.
Like most things, batteries wear out thanks to use. This is as true for the batteries in a solar installation as it is for the battery in your phone or car. The big difference here is that replacing batteries in a off-grid home installation is both expensive and a pain in the neck. Since these installations are generally meant to be long-term, it’s important to know how long a given battery is designed to last.
The lifespan of a battery is measured in two ways – charge cycles and years. This is usually how warranties are expressed as well. You get this many cycles or this many years, whichever comes first. It’s actually pretty much like car warranty; manufacturers use both mileage and age to limit their warranties. All batteries lose some capacity over time and the companies that make them will usually guarantee a certain percentage by the end of the warranty period. If your batteries have lost more than that, you could be eligible for a replacement or repair.
Battery lifespan is also affected by how you use the batteries, along with the specific battery technology itself. So be sure to follow the manufacturer’s recommendations, or you could void the warranty and mess up your batteries. Different battery technologies will give you varying total lifespans. Traditionally, the battery parts of solar systems haven’t lasted nearly as long as the actual photovoltaic components, which can go up to 30 years before needing replacement. As a rule of thumb, you can expect to buy two battery sets for every one solar panel set. The technology in both camps is progressing so quickly however, that you should take these rough numbers as mere speedbumps on the road to much better performance.
Whenever you transfer energy from one form to another, some portion of it is lost and unable to perform useful work. This happens, for example, when your car’s engine turns the energy released from burning fuel toward actually turning your wheels. So while the engine produces a certain amount of power, by the time it has passed through the gearbox and final drive, this amount is smaller.
The more efficient an energy transfer system is, the less energy is lost overall. When you turn solar energy into stored electricity in a battery, you’ll also lose some of the overall power you put in. So let’s say you pour 10 kWh into a battery, but only get 7 back at the outlet. This means the “round trip” efficiency is about 70%. Since we’re getting the initial energy for free, this might not seem like a big deal. However, wasting the solar capture capacity you paid for costs you money. Preferably you want to store as much of the energy you capture as possible. So highly-efficient batteries are overall a good thing.
Just remember that battery efficiency isn’t the end of the efficiency chain. Your stored DC power has to be converted to AC power by using an inverter, and there’s a bit of loss there as well. The good news is that inverters are actually very efficient, with numbers well over 90% in general.
Lead Acid Batteries
The lead acid battery is one of the oldest and best-understood battery technologies in the world. It was invented in 1859 by Gaston Plante. Lead acid batteries are all over the place. Just about every motorized vehicle has one onboard to help it start and to run onboard systems when the engine is off.
The main drawback with these batteries is that they have very low energy densities. About 60-110 Wh per liter. These batteries can, however, deliver a strong surge of power on demand, which is why they are perfect to power starter motors, which need to produce a lot of crank force. The vehicle’s alternator will then quickly restore the power that’s been used up.
In the case of a home battery setup, the low energy density of these batteries isn’t that big of a deal. Storage space is generally easy to get, so you simple need to keep adding lead acid batteries in order reach the total amount of energy you need. Lead acid batteries are not particularly long-lived, but make up for this somewhat by being relatively inexpensive.
Just because your car battery uses the same basic battery technology as the lead acid batteries put to use in solar setups does not mean you can just wire a bunch of car batteries together and call it a day. Solar setups use a special kind of lead acid battery known as a “deep cycle” battery. This means that these batteries are meant to be discharged to a higher level than something like your car battery. They are meant to be discharged regularly, which makes them suitable for a solar setup where you will be using stored power on a daily basis. The recommended DoD on these batteries range from about 45% to as much as 75%, depending on the specific model. Going down to 20% if needed isn’t a major problem, but can have a negative effect on lifespan if you do it too much over time.
Flooded Lead Acid Batteries
These batteries are also known as “wet cell” batteries and are the closest to the basic lead acid battery you’d find in your car. Inside there is an electrolyte fluid. You get both sealed and open versions of this battery. The sealed ones are maintenance-free, which is another way of saying you can’t top them off yourself. These batteries carry the highest risk of hydrogen gas explosion compared to other lead acid battery types. So you need to make sure they operate in a well-ventilated area.
If you use the type that you can open, it means performing regular maintenance on the electrolyte level. Cheap, and still an effective choice, but newer technologies are addressing the shortcomings of flooded batteries.
Gel Cell Lead Acid Batteries
Gel cell lead acid batteries are quite a lot more expensive compared to flooded ones, but they make up for it with much better storage characteristics. For example, they don’t produce nearly as much hydrogen gas and have a very low chance of causing an explosion.
The reason it’s called a “gel cell” battery is because it doesn’t use a liquid electrolyte that can evaporate, leak, or slosh around. The liquid sulfuric acid is mixed in with silica to a gel. One advantage of this is that you don’t have to worry about storing these batteries upright or having them falling over. They are also better at handling shock and vibration, so if you live next to a railroad or in a place prone to tremors, that’s something to consider.
The Best Overall Lead Acid Choice: Absorbed Glass Mat Batteries
When it comes to safety and overall suitability, lead acid batteries have some drawbacks for solar power storage. They are filled with a corrosive liquid, release flammable gas when charging, can’t be stored long-term, and tend to self-discharge quite a bit – typically 5% a month!
Absorbed Glass Mat batteries solve many of the issues inherent to lead acid battery technology. They work by using an incredibly fine fiberglass mat to absorb the sulfuring acid. So unlike the battery in your car, there’s no acidic liquid sloshing around.
These batteries have a depth of discharge of 80%, and thanks to low internal resistance they can charge and discharge rapidly. They have a good lifespan and deep cycle with no issues. It should be no wonder that they are incredibly popular for solar installations.
Lithium ion batteries have revolutionized the world, and chances are you are surrounded by devices that use this technology. The main advantage of lithium batteries is that pound for pound they have the highest energy density of any commercial battery technology. Lithium batteries are the reason you can have such paper-thin smartphones that still last 24 hours. At least lately. They are also the reason that electric cars have become practical in recent years.
Lithium ion batteries have found a home in the renewable market as well. The most prominent example is the Tesla Powerwall, which is essentially a huge wall-mounted lithium battery with some very clever electronics built into it.
Lithium ion batteries are just one of a number of batteries that use lithium chemistry. Just because a solar battery uses lithium doesn’t mean it will be exactly the same type of battery that you have in your smartphone. There are two types of lithium ion battery that are popular for home energy storage today. The Tesla Powerwall uses the same batteries that Tesla cars do – Nickel Manganese Cobalt. These are currently an affordable type of battery and one which is pretty safe. After all, they put them in cars that are liable to crash every now and then. However, Lithium Iron Phosphate batteries are making inroads on that front and are much more resistant to exploding, thanks to thermal runaway.
The best thing about something like the Tesla Powerwall is that you can have it right there in your house, along with every other interior design and chintzy decoration. If it were lead acid or flow batteries, you would have to build a special little shed for them or put them in the basement – an installation involving lots of additional costs in preparing a suitable safe operating environment. Not so with these slick lithium systems.
Flow batteries are a very new addition to the battery options for solar installation. Inside the flow battery is a liquid mix of (usually) zinc-bromide. There are numerous variations on the flow battery, but all of them use a mainly water mix of chemicals inside. This liquid flows (hence the name) between two tanks. When you put electricity into the battery, it takes the zinc out and into a separate storage zone. When you take power out, the reverse happens.
What makes these batteries so interesting for solar use? The simple answer is that they have a whopping 100% depth of discharge. That’s right, you can empty these puppies out completely without fear of messing them up. This also means you can leave them in storage for as long as you want without any charge in them and they will be fine. They’re also incapable of exploding from thermal runaway like lithium batteries. In fact, the stuff inside is actually fire retardant, which sounds like the sort of thing you want from something you keep at your house.
You can also restore worn out flow batteries just by swapping out the electrode, which works out to a much lower expense than replacing the whole battery. They don’t last quite as long as lithiums though. You’ll get about 4000 cycles out of a flow battery. This is compounded by the fact that they often have to be 100% discharged to maintain them.
Finally, they are also more expensive than lithium batteries. So on balance and in general I’d still go for lithium-based batteries over these or even AGMs if cost is a major factor. If flow batteries were cheaper and a little more refined, they’d be serious competition for more established technologies. This is definitely a space to watch closely.
Various Nickel Batteries
The last main category of batteries that are often used with solar setups are ones that use the metal nickel in their makeup. This includes Nickel Cadmium (NICAD), Nickel Iron (NIFE), and sodium nickel chloride.
NICADS are a pretty old technology. I remember having an RC car back in the late 80s or early 90s that ran on these batteries and, to be honest, they were not great – inefficient, expensive, very toxic thanks to the cadmium, and not really suitable for daily power provision.
NIFE batteries are basically Edison cells – terrible efficiency, makes lots of gas and lots of internal resistance, and with an overall drag for any solar setup they are connected to.
Neither of these nickel-based batteries sound like good options and they are pretty expensive to boot, but there is one much newer nickel battery that holds a lot of promise for solar applications. It’s called the sodium nickel chloride battery and it’s not quite like anything else we’ve seen here. Compared to lithium batteries, SNC batteries can handle much colder and hotter temperatures without issue. They are very eco-friendly and 100% recyclable, which is not a feature of any other battery in this article.
Finding Your Type
Battery technology is confusing at the best of times. There seems to be a million different battery chemistries and they’re coming up with more all the time. The good news is that current solar storage technologies don’t have that many options. The ones I covered above are more or less the most viable solutions right now.
Obviously there is no such thing as a “best” battery technology for every occasion. You will have to take the various factors I mentioned above into account when deciding which way to go. The good news (and I guess sort of bad news) is that this decision isn’t permanent. Whatever battery technology you choose will undoubtedly be obsolete by the time your current system reaches the end of its life. It’s a good idea to get an opinion from a professional consultant before you pull the trigger on your initial battery setup, although in most cases choices are mainly driven by budget. So it’s a good thing a given battery setup is not forever, since you’ll probably have more money to spend the next time around on something better.
The last thing you should remember is that you can never do too much research. For example, just because the batteries you’re looking at are the right general technology, that doesn’t mean they’re good in particular. Two lithium batteries, for example, may have very different specifications. Some manufacturers skimp on safety circuitry or quality control. If the batteries seem cheaper than they should be, then you should find out why. If something is too good to be true, it often is. And for goodness sake, read the warranty terms and make sure it covers the battery for as much of its rated life span as is possible.
The devil is in the details!