What batteries are good for Solar Storage?

When considering your solar-plus-storage options, you are going to meet a lot of complicated product specifications. The most important ones to use during your evaluation are the battery’s capacity & power ratings, depth of discharge (DoD), round-trip efficiency, warranty, and manufacturer.

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Solar Capacity and Power

A solar battery system for your home will store surplus solar power for later use. The solar battery will extend your use of a PV-system’s generated energy and will be provided for free. Allowing you to have a sustainable power source even when the sun’s not shining and the panels aren't producing energy. Therefore, with a solar battery little to no electricity is fed into the grid while also avoiding having to buy expensive electricity from the grid later. Due to increased self-consumption, electricity costs decrease significantly. You will pay less perused kWh.

Measured in kilowatt-hours (kWh), the Capacity is the total amount of electricity that a solar battery can store, measured. Most home solar batteries are designed to be “stackable,” which means that you can include multiple batteries with your solar-plus-storage system to get extra capacity. 

The capacity will tell you how big your battery is, it is unable to tell you how much electricity a battery can provide at a given moment. You must also consider the battery’s power rating, to get the full picture. When you think about the context of solar batteries, the power rating is the amount of electricity that a battery will give at one time. This will be measured in kilowatts (kW). A battery with a high capacity and a lower power rate will deliver a lower amount of electricity (enough to run a few crucial appliances) for a long time.

Round-Trip Efficiency

So the battery’s round-trip efficiency will represent the volume of energy that can be used as a percentage of the volume of energy that it took to reserve it. So if you feed for example 5 kWh of electricity into your battery and can only get 4 kWh of useful electricity back, your battery has an 80% round-trip efficiency (4 kWh / 5 kWh = 80%). Altogether a higher round-trip efficiency means you will get more economic value from your battery.

Conversion losses will occur during the storage process, a small amount of the energy will be converted into heat. The total efficiency gives you an indication of what percentage of the energy stored in the system can then actually be used. Fluctuations in the market are between 70 and 95%. You would be best choosing a storage system with a total efficiency of over 90%.

Now, total efficiency and battery efficiency are different specifications. Total efficiency will be depending not just on the battery but on the complete storage system, all the way from your sockets to your roof. Due to conversion losses, total efficiency is normally lower than the battery’s own efficiency and won't be disclosed by most dealers/vendors, etc.

Depth of Discharge (DoD)

Now, the depth of discharge (DoD) of a battery is concerning the amount of a battery’s capacity that has been used. The majority of manufacturers will specify a maximum DoD for optimal performance. Most solar batteries have to preserve an amount of charge all the time due to their chemical composition. If you have used all of a battery’s charge, its useful life will be significantly shortened. To avoid damage, the majority of the battery needs a residual charge. The conventional depth of discharge lies in and around 50% for lead batteries and up to 100% for lithium-ion batteries such as the SOLARWATT MyReserve storage system.

If a 10 kWh battery contains a DoD of say 90 percent, you wouldn't need to use more than 9 kWh of the battery before having to recharge it. Generally speaking, the higher the DoD means you will be able to make the most of your battery’s capacity.

According to the German Federal Association of the Solar Industry, the battery system should never be discharged below its specified depth of discharge. This will shorten the life of the battery.

Solar Battery Life and Warranty

When you decide that you would like to invest in a solar system, it is normal to inquire about longevity. There are several components to a solar power system, so it is important that you consider everything when evaluating the life expectancy of the overall system you want to purchase.

The first and most obvious part of a solar power system is the solar panels. Solar panels can last longer than 30 years depending on the model, but most panels can be expected to perform at their highest capacity for up to 25 years. The majority of the top-tier solar panel manufacturers warranty their solar panels for up to 20-25 years. Solar panels are exceptionally efficient during their lifespan, less than 1% of their efficiency each year.

For most uses of energy storage, your battery will “cycle” (charge and drain) on a daily basis. The battery’s ability to hold a charge will slowly decrease depending on how much you use it. Solar batteries could be viewed like the battery in your cell phone, you charge your phone each night to use it the next day. As your phone ages, you start to notice that it isn’t holding as much of a charge as it did when you first bought it. 

It is a must that batteries are considered when measuring the lifespan of a solar system. There are two main types of chemistries that are used for solar power systems – lithium iron phosphate and lead-acid. With lead-acid batteries, you should expect a much shorter lifespan than with lithium batteries. Around 1500 charge cycles can be expected from a lead-acid battery and in some cases 3000 from high-end lead-acid batteries assuming they are kept in the best condition. The majority of lithium battery companies boast that the lifespan is double compared to their lead-acid counterparts which fall in a range of 6000-1000 charge cycles.

The application in which batteries are used largely influences the lifespan also. Excessive use and deep discharging continuously will obviously affect the life of your battery bank. Deep discharging of lead-acid batteries will drastically reduce the lifespan because the recommended depth of discharge is 50% compared to the lithium batteries which is 80%.

Solar power inverters are another component that needs to be considered in terms of the overall lifespan of your solar power system. It's not uncommon to see 10+ year old inverters being used in solar applications. Pushing a system through heavy use all the time shortens the life of an inverter. Just like your car, the lifespan is indicated by how hard you drive it. It is important that the inverter is appropriately compatible with the system in which it is being used so overworking it, affecting the life span.

What are the best batteries for solar panels?

Batteries used in residential solar energy storage are generally made with one of these three chemical compositions: lithium-ion, lead-acid, and saltwater. In general, we would say, lithium-ion batteries would be the best option for a solar panel system. But saying that other battery types can be more affordable.

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What are Lead-Acid Solar Batteries?

With Lead-acid batteries being tested technology that has been used in off-grid energy systems for decades. Lead-acid batteries are cheaper, but they do have more of a shorter lifespan and require routine maintenance to keep them running to their full potential. They have a shorter life span and lower DoD than other battery types, they are also one of the least expensive options on the market at the moment in the home energy storage sector. For homeowners who want to go off the grid and need to install lots of energy storage, lead-acid would be a good option to consider.

What are Lithium-ion Solar Batteries?

Lithium-ion Batteries are a premium battery technology with a higher efficiency rate and a  longer lifespan, but you’ll pay more money for the boost in performance. The best lithium battery chemistry for solar applications is Lithium Iron Phosphate, shorted to LiFePO4 or LFP batteries. The majority of new home energy storage technologies use some form of lithium chemical composition. Lithium-ion batteries are more compact and lighter than lead-acid batteries. When compared to lead-acid batteries they also have a longer lifespan, higher DoD can be put through deeper cycles. They also require no maintenance or venting, unlike lead-acid batteries. However, lithium batteries are more expensive than their lead-acid counterparts.

What are Saltwater Solar Batteries?

The saltwater battery is based on the sodium-ion battery technology and is a sodium ion storage battery. Sodium-ion battery technology and lithium-ion battery technology were invented at about the same time. Saltwater batteries don’t contain heavy metals, relying instead on saltwater electrolytes. Unlike other home energy storage options. A different design used an electrolyte that has a salt to water ratio of six to one, nearly saturated, such that it could also be called a water in salt battery. Batteries that do use heavy metals, including lead-acid and lithium-ion batteries, need to be disposed of in a professional manner. Saltwater batteries are easily recycled. Saltwater batteries and lead-acid batteries are similar in design, barring the materials used for the batteries are all non-toxic and environmentally friendly.

In closing, the life of a solar system is ultimately determined by how hard it is being pushed, the operating environment of the system, and how it is designed to meet the demand of the application.