Is solar energy storage worth it?

All solar batteries have the same basic function, but each type is suited for different applications. Your solar battery will offer higher reliability and return on investment when its chemistry is suitable for the application at hand.

For example, some electricity consumers are subject to higher kWh prices at certain times of the day, or additional charges for sudden peaks in consumption. In this case, you need a battery bank capable of delivering large amounts of electricity in a short time. Lithium-ion batteries are suitable for this task, but not redox flow batteries.

Regardless of the battery type, you also need to consider the depth of discharge (DoD), which indicates a battery’s usable capacity. The service life of a battery can be drastically shortened if you exceed the DoD, or you can even cause permanent damage. For example, using 70% of the stored energy is acceptable with a solar battery rated for 80% DoD, but not a 50% DoD battery.

Lead-Acid

Lead-acid batteries are an established technology, commonly used by off-grid solar energy systems in remote locations. Lead-acid batteries are affordable and have a well-established supply chain due to their popularity, so you can easily find vendors and technical support.

In spite of their low cost, lead-acid batteries have some technical limitations you should consider:

• Lead-acid batteries typically have a charging life of 500 to 1000 cycles.
• Batteries should be fully recharged after each use, or the lifespan is drastically reduced.

An absorbed glass mat or AGM battery is an improved version of the traditional lead-acid battery. They can charge faster while having a spill-proof design and more durability. You can also find AGM deep cycle batteries that are designed for 80% DoD.

Using lead-acid batteries along with solar panels requires charge controllers to sustain a suitable charging current. These batteries should not be wired directly to your solar array, or your system may be damaged by excessive current.

Lithium-Ion

Lithium-ion batteries have become very popular in recent years since they can achieve synergy with solar panels and wind turbines. For example, the Tesla Powerwall and Enphase IQ are two types of lithium-ion batteries commonly used in home solar applications. You can also find smaller lithium batteries from brands like Renogy and WindyNation, which are portable and better suited for DIY solar projects.

Lithium iron phosphate or LFP batteries are a subtype of lithium batteries, characterized by a superior service life. The best LFP batteries offer a service life of over 4,000 cycles at 80% DoD, which means they can last for over 10 years on a daily charging cycle. This makes LFP batteries the ideal complement to solar installations. Unlike lead-acid batteries, which need separate charge controllers, many of the lithium battery models that are commercially available come with built-in chargers and controls.

The main drawback of lithium-ion batteries is the high price, but this could change in the near future — the U.S. Department of Energy is targeting a 90% energy storage cost reduction by 2030. Lithium batteries can also suffer a phenomenon called thermal runaway when used at high temperatures, which causes them to catch fire. You can prevent thermal runaway by making sure your batteries are high-quality and installed by qualified electricians.

Nickel-Cadmium

Nickel-cadmium batteries are characterized by their durability, tolerance to high temperatures and simple maintenance needs. Thanks to these performance features, nickel-cadmium batteries are popular in industrial and utility applications. Unfortunately, cadmium is highly toxic to humans, so nickel-cadmium batteries are not recommended in homes.

Flow

Flow batteries store energy by separating positive and negative electrical charges in chemical solutions, which are stored in separate tanks. When these two solutions interact, they undergo a reduction-oxidation reaction (redox) and the battery releases energy. This battery technology is also referred to as “redox flow” for this reason.

The main disadvantage of redox flow batteries is their space requirement, and they are not cost-effective for small-scale projects. Even a small redox flow battery system can be the size of a shipping container, so using flow batteries in home solar systems is not viable.

Imagine you’re home on a stormy night, watching TV with the washing machine running, and all of a sudden the power goes out. Now imagine the same scenario, except you have a rooftop solar energy system with battery storage. When the power goes out in your neighborhood, you’d be blissfully unaware.  

A common myth about solar power is that you can count on it only when the sun is shining. You do need sunshine to generate electricity with solar, but what about the times when the sun isn’t shining? Most people rely on electricity from the power grid to supplement their solar-generated power. But residential solar energy systems paired with battery storage—generally called solar-plus-storage systems—provide power regardless of the weather or the time of day without having to rely on backup power from the grid. Here are the benefits of a solar-plus-storage system:

• Around-the-clock power. If you use the utility billing mechanism known as time-of-use, and don’t have a solar energy system, your electricity in the evening is likely more expensive because of the higher demand on the system. With battery storage, however, you can use electricity generated during the day later on, rather than relying on the utility for power. This is also useful if you live in an area with frequent power outages.
• Pocketbook protection. If your utility raises electricity rates, you can avoid that with a battery. Battery storage lets you leverage low-cost energy that has already been generated and stored, ensuring your rates stay low and don’t affect your monthly budget. In some cases, you can even sell the energy you’re storing back to the grid when the rates are higher and bank the profit.
• Better monitoring. A solar-plus-storage system can help you to better track the energy your system is generating through monitoring capabilities, providing an enhanced level of transparency and precision. These systems allow you to track the energy your home is producing and using in real time.
• More energy self-sufficiency. While most jurisdictions require homes to be connected to their local utility even if they don't use any electricity from the utility, a solar-plus-storage system takes you closer to “off the grid” status. Battery storage means you don’t have to rely on your utility to deliver electricity to your home most days of the year. And you can always keep some battery capacity in reserve, so that if the power goes out in your neighborhood, your house will be the one with the lights on.

You may be sold on the merits but curious about the costs. A solar-plus-storage system costs about $25,000–$35,000, depending on the size of the battery and other factors. It is easier and cheaper to install the panels and battery at the same time. But if you’ve already installed solar panels and want to add storage, you can: The battery will cost anywhere from $12,000 to $22,000. Ask your solar installer if they can add a battery to your system. If you purchase a battery on its own or a solar-plus-storage system, you will be eligible for federal tax credits. Some states provide additional solar battery incentives. 

To learn more about resources available to consumers, including guidance on demystifying the solar installation process, check out the Homeowner’s Guide to Going Solar.