How to store solar energy? Learn how storing energy can augment your solar system

f you're reading this, you have probably thought about how to maximize the benefit you can get from your solar power system. Solar energy storage can become the best solution to fully tap into the potential of your solar PV. Let’s look at some of the factors that make batteries an excellent companion for your solar panels.

How beneficial is solar energy storage?

First of all, a battery can become a great way to reduce energy bills. This particularly applies to those who don’t have a one-to-one net metering program in their area, which means the energy generated by their solar panels is sold to the power company at a lower price than the energy bought from it. A solar battery can also make sense financially in case of time-of-use rates, then the energy cost changes depending on the time of day. With solar power storage, you will be able to save generated electricity for later and use it when it is most expensive.

But how can solar batteries help the homeowners who have net metering programs in their region? For example, they help reduce reliance on the electric grid. This advantage is especially relevant for those living in areas with frequent power outages or time-worn electricity infrastructure. During blackouts, solar panels must stop working to ensure safe power line repair. But if you are storing solar energy within a battery, you will be immune to any power grid failures.

One more argument for installing a solar battery is carbon footprint reduction. If your solar panels underproduce, you will have to draw electricity from the power grid, that is, to use energy generated by burning fossil fuels. With a solar battery, you will be able to make your power consumption 100% sustainable.

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As you can see, solar batteries offer lots of opportunities. But how is solar energy stored? Let’s take a look.

Battery chemistries for storing solar power

Battery chemistry is the combination of electrochemical cells that react with each other to create a flow of electricity in a circuit. The two most popular chemistries for storing energy generated by solar panels are lead-acid and lithium-ion.

Lead-acid solar battery

Lead-acid solar batteries are a time-tested and well-established technology for solar power storage. Invented back in 1860, they have been the primary way of storing solar energy for about 50 years. They consist of two lead plates serving as electrodes: a positive electrode made of lead dioxide and a negative electrode made of spongy or porous lead. Both of them are suspended in the electrolyte – a solution of sulfuric acid and water.

These solar batteries have a relatively lower energy density, meaning they need more space than lithium-ion to store the same amount of solar energy. They are also characterized by only 50% depth of discharge, i.e. you can use only half of its available energy storing capacity. On the other hand, these solar batteries are considerably cheaper than their lithium-ion alternatives.

Lithium-ion solar battery

Lithium-ion batteries work by transferring lithium ions between a cathode and an anode. This type of battery is widely used in portable electronics like laptops and mobile phones as well as in electric cars.

Li-ion batteries are generally considered a better way to store solar energy. They offer a much higher energy density compared to lead-acid batteries, and their efficiency reaches 95% as opposed to the 70% efficiency of lead-acid batteries. They also have a far better depth of discharge - 80% or even higher.

On top of that, li-ion batteries are considered a more durable power storing option, as they last for around 10 years, while lead-acid batteries last 3-5 years on average. The only drawback is the price, which is often two times higher than that of lead-acid batteries.

Battery coupling: two types of connection with solar panels

Solar panels generate direct current (DC) electricity and it must be converted into alternating current (AC) electricity to power most domestic appliances. However, solar batteries store only DC energy.

Solar battery coupling refers to how a battery is connected to solar panels. A solar storage system can be DC-coupled and AC-coupled. Let’s discuss how to store solar energy with each of these battery types.

AC-coupled solar battery

AC-coupled solar systems have been the most common option for electricity storage for a long time. In such a system, DC electricity travels from solar panels to an inverter, which transforms it into AC electricity and thus makes it suitable for powering your household. The unused energy flows into another inverter that converts it back into DC so that it can be stored in a battery. Then when you need to take some of it from your storage, it is transformed into AC again. As a result, electricity gets converted three times before you use it.

One of the key advantages of AC-coupled solar batteries is that they can be retrofitted into an existing solar energy system and expanded over time. In addition to this, AC coupling allows you to charge your batteries from both PV system and the electric grid.

There are, however, some disadvantages as well. As the electricity is converted three times, more of it is lost along the way. Besides, installing an AC-coupled system will require more expenses as you will have to buy two separate inverters.

DC-coupled solar battery

In a DC-coupled system, electricity generated by solar panels flows directly to a battery with the help of solar charge controllers. Then it is converted into AC energy by an inverter and goes further to your home appliances or the electric grid.

DC-coupled solar batteries are valued for how efficient they are. As the energy is only converted once, less of it is lost in the process. These energy storage systems are typically more affordable: you will only need one inverter, which means lower equipment costs.

However, a DC-coupled battery is more difficult to integrate into an existing solar power system, because you will have to purchase a new inverter compatible with your new battery solution.

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How much does solar energy storage cost?

The cost of solar power storage depends on how many batteries you need, as well as what battery chemistry and coupling type you choose.

The price of most lithium-ion solar batteries is between $7,000 and $14,000 including installation costs, while an average lead acid battery would cost around $7,500. But remember that the federal solar tax credit allows you to claim a 26% rebate on your solar power storage expenditures.