Choosing the best solar battery: What to know

When comparing quotes for different home solar battery systems, it can be difficult to determine which characteristics and technical specifications matter most, and with good reason: the home energy storage industry is so new that you, as a homeowner, probably don't know anybody with a battery who you can ask about their experience. While every battery has to meet certain reliability and safety requirements to be sold and installed in the US, outside of those standards, there is very little standardization of specs and characteristics across the batteries available on the renewable energy market today. We've provided some tips on what to look for when comparing different battery quotes for your energy needs.

A solar battery is a device that stores electricity for later use, so you can keep appliances running during a power outage, use more of the solar power you produce at your home, and even save money on electricity in some cases. They are often called "deep cycle batteries" due to their ability to charge and discharge a significant amount of electricity compared to a car battery. They are also rechargeable.

Solar battery storage systems provide a number of different benefits, from emergency backup power to financial savings. But they also bring technical complexity and a new set of unfamiliar terminology. Here's what to focus on and look for in a deep cycle solar battery:

Power rating

The power rating of a battery refers to the kilowatts (kW) of power that the battery can provide at once. In other words, a battery's power rating tells you both how many appliances your battery can power at once and which appliances those are. 

Power is expressed either in kilowatts (thousands of Watts) or in Amps, and different appliances use different amounts of power. For instance, a typical compact fluorescent lightbulb will use 12 Watts (or 0.012 kW) of power, while a 3-ton AC unit will draw 20 Amps, which is equivalent to 4.8 kW. Most of the batteries available on the market today have a continuous power output of around 5 kW. 

Importantly, solar batteries often have two different power ratings–a continuous power rating and a 5-minute or instantaneous power rating–meaning they can provide more power in short bursts. This is important if you have an appliance like a sump pump that requires a large amount of power to turn on but then runs at lower power.

Battery size/usable storage capacity

A battery's capacity (or size) is the amount of electricity it is able to store and supply to your home in the battery bank. While power is expressed in kW, battery size is expressed in kilowatt-hours (kWh), which is power multiplied by time. As a result, a battery's storage capacity tells you how long your battery can power parts of your home. Be sure to look for the usable capacity of a battery, as that number represents the amount of stored electricity that you can actually access in a battery.

Since electricity usage is power multiplied by time, if you use more power, then you'll run out of stored electricity faster. Conversely, if you're only using your battery to backup a few appliances with relatively small power consumption, you can keep them running for a longer time. This makes the size of a battery slightly misleading because the length of time a battery's charge will last is directly influenced by how much power it's outputting.

Think about the example above of the difference between a light bulb and an AC unit. If you have a 5 kW, 10 kWh battery, you can only run your AC unit for two hours (4.8 kW 2 hours = 9.6 kWh). However, that same battery would be able to keep 20 lightbulbs on for two full days (0.012 kW 20 lightbulbs * 42 hours = 10 kWh).

Roundtrip efficiency

Roundtrip efficiency is a system-level metric that measures how well your solar power system  (battery + inverter) converts and stores electricity. There are losses associated with any electrical process, meaning you'll lose some kWh of electricity when you invert it from direct current (DC) electricity to alternating current (AC) electricity or when you put electricity into a battery and take it out again. A solar battery's roundtrip efficiency tells you how many units of electricity you'll get out of a battery for every unit of electricity you put into it.

Battery lifetime: How long do solar batteries last?

Battery lifetimes are measured with three different metrics: expected years of operation, expected throughput, and expected cycles. A battery's expected throughput and cycles are like a car's mileage warranty. Throughput lets you compare how much electricity you'll be able to move through your battery over its lifetime. Cycles measure how many times you can charge and discharge a battery. 

To convert a battery's expected or warranted throughput into an expected lifespan, divide the throughput (expressed in kWh) by the usable capacity of the battery to estimate how many full cycles you'll get from your battery and divide that number of full cycles by the number of days in the year: a 20,000 kWh throughput warranty on a 10 kWh battery means 2,000 expected cycles, or a cycle per day for 5.5 years. 

To convert a battery's expected or warranted number of cycles into an expected lifespan, divide the number of cycles by the number of days per year: a 4,000-cycle warranty equates to a cycle per day for 11 years.

Safety

All solar batteries have to meet certain safety requirements to be certified for installation in homes and businesses. Every battery that you receive a quote for on EnergySage is safe and meets these safety requirements! There are, however, some battery chemistries that have been tested for safety to different levels, going even beyond the government-mandated safety requirements for batteries, meaning some battery chemistries are slightly safer than others. But the most important thing to remember is that all batteries installed in the US are very safe!

Chemistry

A battery's chemistry refers to the primary compound used to store electricity inside the battery. Chemistry may be the most important characteristic to compare since it ultimately determines a lot of the characteristics of batteries listed above. For instance, different lithium ion chemistries may be more power dense–meaning they store more electricity in a smaller amount of space–or may do a better job of cycling–meaning they can perform at a higher level for more years. And those are just the differences within lithium-ion chemistries, to say nothing of the differences between lithium ion batteries and lead acid batteries, AGM batteries, or other experimental battery chemistries. As with most things, different solar battery chemistries come at (often significantly) different price points and upfront costs.

It's hard to say. The best battery for you will depend on a number of factors, from the size of your home to the characteristics of your solar installation and all the way to what you even want to get from an energy storage system. If you have a large home with many appliances, you'll want to look for a high-capacity battery that can keep pumping out electricity for hours. If you're price-conscious and care more about optimizing your solar panel system, including charge controllers, a smaller battery with great battery integration may be the best choice.

Curious how some of the top brands stack up against one another? Check out our article about the best solar batteries from popular brands like Enphase, LG Chem, and the Tesla Powerwall.

These are just a few of the most common decision criteria for installing a battery. Others include depth of discharge or DoD. If you're making a decision based on other criteria, let us know! Feel free to reach out to our Energy Advisor team, who are happy to answer any questions you may have about the differences between the batteries available on the market today.

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