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What is the best battery type for a residential solar system?

Updated: Jan 28


Learn about products like the Tesla Powerwall 2 to identify what is the right home solar battery for you.
What is the best home solar battery for a residential solar system?

Introduction:

As energy storage has become more common for residential homeowners, the number of home solar battery products has increased dramatically. With new products and exciting rebates, like California's Self Generation Incentive Program, home solar batteries are a hot topic of conversation. This is excellent news for homeowners but also means that they have more variables to consider when comparing battery options.

Below we will outline the benchmarks we believe are the most important for homeowners to compare when looking at different solar battery options.

How to compare solar storage criteria?

Like other high-end electronics, before deciding on one product, it is crucial to compare and contrast the features and abilities of different products. Unlike a laptop or smartphone, most people have never looked into residential solar batteries.

We understand that this process can be overwhelming. We believe understanding the following product details is essential when comparing batteries: the battery's capacity & power ratings, specifics of the warranty, depth of discharge, battery efficiency, and product manufacturer.

Battery Capacity & power rating


When discussing what a battery can do, the first factor to consider is the solar home battery capacity. The capacity is the total amount of electricity that is held in the battery. Home batteries are measured using kilowatt-hours (kWh). Because every home has different needs, most residential solar batteries can be "stacked." In other words, multiple batteries can be connected to provide extra capacity to a solar battery system.

Just as important as how much electricity a battery can hold is how much electricity a battery can release at one time. To measure this, a homeowner should look at the battery's power rating. The power capacity of solar battery systems is measured using kilowatts (kW).

It is vital to understand these two factors and what type of function the battery system is expected to serve. Suppose a battery has larger capacity but has a lower power rating. In that case, it will push out electricity for a long time but not power as much electrical load (critical loads in a home, for example). If a battery has a lower capacity but higher power rating, it will have a shorter life but can power a more significant electrical load (an entire home, for example)

Depth of discharge (DoD)


In order to extend the life of a battery, some types of batteries require that some amount of charge is held in a battery at all times. As a result, many batteries do not allow for the entire battery capacity to be used at any given time.

Depth of discharge (DoD) is the term that is used when discussing what percentage of a battery's capacity can be utilized. To ensure efficiency, the number of cycles, and warranty, most battery manufacturers will cap the DoD for a battery. If the energy storage is 10 kWh and has a Depth of discharge of 95 percent, the battery will not use more than 9.5 kWh before it starts the process of recharging.

Round-trip efficiency

As electricity moves, some of it gets lost. For a battery, the amount of electricity that can be utilized as a percentage of the power required to store it is known as a battery's round-trip efficiency.

Suppose a solar system feeds ten kWh of energy into a battery, and eight kWh of usable electricity can be used. In that case, the round trip efficiency for the battery is eighty percent.

In most situations, the higher the round trip efficiency is, the more efficient the battery is. As a result, the return on investment will be better.

Battery life & warranty


Every time a battery charges and discharges, it is called a "cycle." Most residential battery systems will cycle once a day as the system charges during the day and discharges when it is most cost-effective.

Like most other rechargeable batteries, as a battery experiences more cycles, its ability to hold a charge will slowly decrease.

Because the lifespan and efficiency of a solar battery are directly connected to cycles, most battery warranties are based on the number of cycles it experiences or the percentage of the battery's original capacity.

Most of the battery warranties are currently designed to ensure a specific battery capacity level for ten years. When deciding on what battery to use, a homeowner should focus more on "what will my battery capacity be in BLANK years" instead of "how long will a residential battery last?"

Manufacturer


When deciding on what solar-battery to install at home, it is crucial to consider what company manufactures the battery. As the solar battery space has matured, everyone from startups to automakers and consumer electronic companies has stepped into the energy storage space.

Different manufacturers have different advantages; some offer revolutionary technology, while others bring manufacturing experience while others might have specific solar expertise.

The energy storage space is developing rapidly, and as the demand for electric vehicles has increased, so has the funding for new products. Tesla, for example, has leveraged its experience with electric vehicles to produce the Powerwall Battery.

These all can provide value, but the product warranty and company stability must be considered when deciding on the energy storage manufacturer. As the saying goes, the warranty is only as good as the company providing it.

How long do residential solar batteries last?


When this question is asked, it can refer to two very different things. The first is, "how long will a solar battery last on one charge?" The other is, "how long is the lifespan of a solar battery?" We touched on the second when discussing the warranty, but let's dive into both in more detail.

How long will a solar battery last on one charge?


A fully charged battery will be set up to power a portion of a home overnight in most cases. Solar batteries can have multiple functions time of use offset, rate arbitrage, or battery backup. Regardless of the function, most people want to make sure that their battery can provide some backup power if an electrical outage happens.

To get a ballpark understanding of how long a battery will backup a home during an electrical outage, a homeowner needs to complete the average electrical usage in a day against the battery's capacity and power rating.

For example, in the perfect world, if a home utilized 20 kilowatt-hours (kWh) in a day, it could be powered using two batteries with a ten kWh capacity.

In the real world, the calculation is a bit more complicated. If the sun is out, the battery will utilize the solar system’s power to charge and power the home simultaneously, lowering the demand in the home and extending the battery’s life. As discussed in the Depth of discharge section, most batteries cannot utilize all of its capacity. As a result, it is a crucial factor to consider when calculating how many batteries home needs. It is also possible to only back up critical loads in the home, something that many manufactures recommend. This extends the battery life by limiting the draw on the battery to essential electrical needs.

Other factors also come into play, but for most homeowners combining energy storage with a grid-tied solar system, we find that one or two 10kWh batteries cover an average home's needs.

How long is the lifespan of a solar battery?


The most frequently utilized solar batteries are warrantied for ten years. As a result, life expectancy is somewhere in the ten to the fifteen-year range. Because the average life expectancy of a PV system is between twenty-five and thirty years, if a homeowner installs a battery and solar energy system, they will most likely have to replace their battery at least once during the solar system life.

In an effort to extend the life and performance of a solar battery, everything must be done to maintain the best conditions for its performance. If the temperature gets too low, it will require more voltage to reach its maximum charge, and if it gets too hot, it may overheat and need a reduced charge. Both can put an extreme strain on the battery that can impact its long term performance and life expectancy. It is usually recommended to keep the battery from below 30° F or above 90° F.

Many batteries can control temperature on its own, but if a home is an area with extreme weather, it is usually recommended that the battery be located in a temperature-controlled enclosure of some sort.




What is the best type of solar battery?


Many different kinds of battery chemistries exist for home energy storage. For most homeowners, lithium-ion batteries are the best solution, but lead-acid and saltwater can also make sense.

  1. Lithium-Ion- Lithium-ion is by far the most common home solar battery technology. Lithium-ion batteries are lighter in weight, making them easier to transport and install. They also usually have longer lifespans and Depth of discharge rates compared to other technologies. They can be a bit more expensive, but because of the advantages, Lithium-Ion batteries make the most sense for homeowners.

  2. Lead-acid- Lead-acid is a battery technology that has been around for decades. As a result, the technology is proven and has been the most common product used for off-grid solar systems. Lead-acid batteries do have a hot life, lower Dod compared to Lithium Ion, and can be very heavy. Because they are a bit less expensive on a per-unit basis. They might make sense for some off-grid applications that require much storage but do not make sense for a grid-tied residential solar system.

  3. Saltwater- Because of issues in the past with heavy metal-based battery systems, people have been focused on developing new energy storage technology. Saltwater batteries have stepped into this space using saltwater electrolytes. Because it is a new technology that has not been tested at scale, we do not recommend it currently, but it might be an exciting technology in the future.

If you are interested in learning more about the best battery solution for your needs, schedule a free consultation with Alchemy Solar today.

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