Choosing a battery: how to pick the best battery for you
Last updated 7/6/2021
When comparing quotes for different energy storage 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 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 market today. We’ve provided some tips on what to look for when comparing different battery quotes.
What to look for in a battery
Energy storage systems provide a number of different benefits, from emergency backup power to even financial savings! But they also bring technical complexity and a brand new set of unfamiliar terminology. Here’s what to focus on and look for in a battery:
There are a number of different potential decision criteria and comparison points to make when evaluating your energy storage 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 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 battery for a longer amount of time, look for a battery 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 roundtrip 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) 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 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, 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.
Usable storage capacity
A battery’s capacity refers to the amount of electricity that a battery is able to store and supply to your home. While power is expressed in kW, storage capacity is expressed in kilowatt-hours (kWh), 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.
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).
Roundtrip efficiency is a system-level metric that measures how well your energy storage 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 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: throughput and cycles
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 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 vanadium flow batteries, or other experimental battery chemistries. As is the case with most things, different battery chemistries come at (often significantly) different price points.
In conclusion: there's a lot to consider
These are just a few of the most common decision criteria for installing a battery. 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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