How to choose a solar battery
Last updated 3/30/2023
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 in order 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.
What is a solar battery and what are they used for?
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 referred to as "deep cycle batteries", due to their ability to charge and discharge a significant amount of electricity compared to something like a car battery. They are also rechargeable.
Should I get battery storage for my solar energy system?
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:
There are a number of different potential decision criteria and comparison points to make when evaluating your stored energy options. Here are a few of the most common decision criteria, as well as which battery specs matter most if these criteria match your situation:
- If you want to power more of your home at once, look for a solar battery with a high power rating
- If you want to be able to power a more energy-intensive appliance (like a sump pump), look for a battery with a high instantaneous power rating
- If you want to run your home with your solar battery for a longer amount of time, look for types of batteries with a higher usable capacity
- If you want to get the most out of every kilowatt-hour of electricity you put into your battery, look for batteries with a higher round trip efficiency
- If you are space constrained and want to get the most amount of storage out of the least amount of space, look for lithium-ion nickel manganese cobalt (NMC) solar batteries
- If you want a battery with the longest lifetime that you can cycle the most amount of times, look for lithium iron phosphate (LFP) batteries
- If you want a battery with the absolute highest safety rating possible (don’t worry, they’re all safe!), look to LFP solar batteries
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 are using 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 amount of 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 2 full days (0.012 kW * 20 lightbulbs * 42 hours = 10 kWh).
Depending on what you want to use your energy storage system for and the characteristics of your household appliances, the number of batteries you'll need can vary widely. There are several questions to answer: how long you'll want to stay battery powered, what kind of production your solar panels get, what appliances you absolutely need to keep on, and the list goes on and on.
While we can't say for certain how many batteries you'll need. This includes applications, such as if you are trying to use diy or off-grid solar system, or even as a backup in case of a blackout. We've outlined the steps you can take to start doing that math in our article about how much of your house you can power on batteries. You can also check out our article on using an off-grid system , where we do some example math on what it would take to keep your home completely solar and battery powered.
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.
All solar batteries have to meet certain safety requirements in order 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!
A battery’s chemistry refers to the primary compound that’s 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 are able to 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, or agm batteries , or other experimental battery chemistries. As is the case with most things, different solar battery chemistries come at (often significantly) different price points and upfront costs.
What is the best solar battery?
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 lots of appliances, you'll want to look for a high-capacity battery that can keep pumping out electricity for hours on end. 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 including Enphase, LG Chem, and the Tesla Powerwall.
How to choose the right solar battery
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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