How to Choose the Best Battery Storage Systems for Solar

Power Rating

Your battery’s power rating measures the amount of power it can put out at any one time. A power rating will normally be measured in kilowatts (kW) but sometimes appears in amps (A).

By knowing a battery’s power rating and checking it against the power usage of the various appliances you hope to power with the battery, you can determine if the battery is equipped for the job.

For example, suppose you checked the specs of a handful of appliances in your home and added up their power usage to around 4.5 kW. In that scenario, you could keep that group of appliances powered by a single battery with a continuous power rating of 5 kW.

With that said, it’s also important to note that some solar batteries will have two power ratings that measure different aspects of their power output capabilities. A continuous power rating measures how many kilowatts the battery can consistently provide, while an instantaneous power rating measures how much a battery can provide in a quick burst. Having a good instantaneous power rating ensures that you can use appliances that need a fair amount of startup power but run at a lower power level while in operation.

Battery Size

A battery’s size (or usable storage capacity) is the full amount of energy the battery can store and dispense to your home. Your solar battery’s size is measured in kilowatt-hours (kWh), multiplying the amount of power (kW) used by the amount of time it was being drawn(h). To put that into context, if an appliance of yours is drawing 1 kW of power over 1 hour, then the appliance has used 1 kWh.

While the average size of a solar battery is about 10 kWh, the best battery size (and amount) for your use case will depend on a variety of factors, including

Roundtrip Efficiency

The laws of physics can be a pain to combat sometimes, especially when it comes to electrical engineering. No method of power generation is 100% efficient. When your solar panels produce energy and send it to the battery, not all that energy will make it to the battery and back out again, even if you have the best battery for solar power storage in your specific use case. Some energy is lost to heat along the way, more is lost when the electrical current is inverted from DC to AC, and due to the electrochemical processes of a battery, you don’t get quite as much power out of a battery as what you put into it.

Some solar batteries are better at this process than others holding onto more of the electricity that was put into it than other batteries can do. How well a solar battery preserves the charge put into it is measured by the battery’s roundtrip efficiency. A higher roundtrip efficiency means that a battery loses less charge, giving a closer amount of kWh to what was put into it in the first place.

Battery Lifetime: Throughput and Cycles

While a good solar battery is a much more impressive technology than the pair of AAAs in your TV remote, they have one thing in common: a limited lifespan. Nothing lasts forever.

However, while most AAAs are single-use, your solar battery can be charged and recharged over several years before they need to be replaced.

A battery’s expected lifetime is measured in throughput and cycles. Throughput is the number of kWh you can reasonably expect the battery to give off in its lifetime. Meanwhile, every time the battery discharges its electricity and recharges back to full, this is considered a charge cycle.

The expected throughput and cycles of a battery are directly related. If a solar battery’s expected lifetime throughput is 25,000 kWh, and a full charge is about 10 kWh, then the battery in question can be expected to last around 2,500 full charge cycles. If that hypothetical battery charge cycled once every day, that’s an estimated battery life of just under eight years.

Safety

For a solar battery to be approved for residential and commercial use in the United States, it must undergo careful scrutiny and rigorous testing. Government mandates require solar batteries to be engineered in a way that ensures your safety. While some manufacturers go to greater lengths to guarantee a safe product, you can rest assured that all solar batteries available in the United States are safe for use in your home.

Chemistry

Different types of solar batteries are made with different chemicals and materials. Though these different types of batteries will be explored in more detail later in the article, each style of battery (lithium-ion, lead-acid, flow, etc.) has its pros and cons to consider.

Brand and Warranty

As with many other products, the brand you choose is big for determining the price, availability, and overall quality of a solar battery. Certain brands are more likely to deliver the quality you’re looking for, so pay attention to reviews.

Additionally, not all warranties are made the same. Before making a big-ticket purchase like a solar battery, carefully review the terms of the warranty to ensure you aren’t left on the hook for defective equipment that breaks down prematurely.

1. Lead-Acid

One of the oldest battery types, lead-acid batteries use a sulfuric acid electrolyte fluid to submerge lead plates creating a chemical reaction conducive to storing and discharging electrical currents. One of the main advantages of lead-acid batteries is that they are the most common type of battery available, making them very easy to find. Because they are common, they tend to be the most affordable option as well. Additionally, lead-acid batteries provide consistent and stable power output for the long term. They will generally be easy to replace or upgrade if you need to switch them out or upgrade them.

Some drawbacks of lead-acid batteries include their size and weight. They are often cumbersome and take up a fair amount of space. Their ambient working temperature is also lower than room temperature, so using them will necessitate you having an environment available to keep them cool.

Lead-acid batteries are a popular option for creating off-grid solar setups and emergency backup power systems.

2. Lithium-Ion

Lithium-ion batteries are made up of various metal sheets, separated from each other by polyolefin, and submerged in a lithium salt solution. Similar to lead-acid batteries, the arrangement of these materials within a lithium-ion battery creates a chemical reaction that directs electricity in and out of the battery for storage and discharge.

Lithium-ion batteries have many advantages, starting with the fact that they require little to no maintenance. They are also more energy-dense than lead-acid batteries, which allows lithium-ion batteries to come in smaller sizes while still packing the same energy capacity. Additionally, they can work reliably in environments with more temperature fluctuations. One of the biggest benefits of a lithium-ion battery is its higher depth of discharge compared to other batteries, meaning that it can use more of its full charge in any cycle, a fact that also contributes to lithium-ions lasting longer (up to 15 years).

Downsides of lithium-ion batteries include their cost and potential fragility. The materials involved can make lithium-ion batteries more expensive than others, sometimes double what a similar capacity lead-acid battery might cost. When not used properly, a lithium-ion battery can also be fragile, so they will need a stabilizing circuit if you want to use them safely.

Most commonly, lithium-ion batteries are found in electric vehicles, but in home solar energy use cases, they are also popular as space-saving options or battery options for climates and homes with more extreme temperatures.

3. Flow

In a flow battery, electrolyte chemicals are housed in two separate tanks (one with zinc and the other with bromine) and simultaneously pumped past opposite sides of a graphite membrane. This interaction allows the chemicals to exchange electrons and produce a current.

Flow batteries can offer several advantages for a solar energy system. First, flow batteries are very scalable, meaning that their potential energy output scales proportionately with their physical size. Flow batteries also don’t tend to overheat during use. Finally, a flow battery has a very high depth of discharge in comparison to most other battery types, so you’ll get a longer lifespan and more power per charge from each battery.

On the other hand, there are some drawbacks to consider with flow batteries. For one, they tend to be more expensive because of the costly materials used to make them. This can also make them a challenging item to acquire since very few companies actually produce flow batteries for commercial use. Their scalability is a double-edged sword. If you have a higher energy usage, you will likely need a larger flow battery to make it work. One other thing to watch out for is corrosive and toxic materials. Zinc and bromine can be dangerous, so flow batteries should be very carefully handled to avoid exposure to these materials.

Flow batteries are best used in cases with high energy demand. If you have a large-scale system and plan on doing multiple charge cycles daily, a flow battery might be suitable for your solar setup.

4. Sodium-Nickel Chloride

Sodium-nickel chloride batteries use sodium, nickel, and sodium chloride electrodes immersed in a complex mixture of ceramic and sodium-based electrolytes. These mechanics work similarly to lead-acid or lithium-ion batteries but with different chemicals causing the reaction.

A few benefits of sodium-nickel chloride batteries include their sustainability, lack of toxic chemicals, and a broad ambient working temperature range. A sodium-nickel chloride battery is fully recyclable and does not produce any toxic chemical emissions or create any fire risks. Additionally, their ambient working temperature range spans anywhere from -4ºF and 140ºF, allowing them to function efficiently in some truly extreme temperatures.

Looking at the negatives, sodium-nickel chloride batteries usually have a shorter lifespan than other types of batteries, so long-term reliability is an issue. They also have a shallow depth of discharge, which means that a significant amount of the energy it stores is functionally unusable.

Sodium-nickel chloride solar batteries are most often used in off-grid solar installations requiring a large power load. They can also be used as emergency backup power sources.