Batteries are becoming a popular add-on to solar systems thanks to the extra benefits they can offer for solar system buyers. Batteries offer backup power benefits when the grid goes down, increases the usefulness of off-grid systems, and improves solar economics if you have less than ideal net metering policies or time-of-use (TOU) rates.
A growing number of states and utilities are offering incentives for installing battery storage. (Some examples of this include the Self-Generation Incentive Program (SGIP) in California, Maryland’s newly announced Home Energy Storage Tax Credit, and the Massachusetts SMART program that offers a tariff adder for batteries.) If you live in one of these states, you might be looking at some of the popular battery options for residential applications like the Tesla Powerwall, the LG Chem RESU, or the sonnen eco.
With an increasing amount of options comes more questions about how to compare the batteries: which is the best, or which one is right for you? One of the most important factors to consider when shopping for battery backup for your home is the depth of discharge (DoD).
What does the depth of discharge (DoD) of a battery represent?
A battery’s depth of discharge (DoD) indicates the percentage of the battery that has been discharged relative to the overall capacity of the battery. For example, if you have a LG Chem RESU that holds 9.3 kilowatt-hours (kWh) of electricity, and you discharge 8.8 kWh, the DoD is approximately 95 percent.
The more frequently a battery is charged and discharged, the shorter its lifespan will be. It’s generally not recommend to discharge a battery entirely, as that dramatically shortens the useful life of the battery. Many battery manufacturers specify a maximum recommended DoD for optimal performance of the battery.
For example, if the manufacturer of a 10 kWh battery recommends a maximum DoD of 80 percent, you shouldn’t use more than 8 kWh from the battery without recharging. You can see why DoD is an important factor to consider: a higher DoD means you can use more of the energy being stored in your battery. Many modern lithium ion batteries these days advertise a DoD of 100 percent.
Your battery’s “cyclic life,” or the number of charge/discharge cycles in its useful life, depends on how much of the battery’s capacity you normally use. If you regularly discharge the batteries at a lower percentage amount, it will have more useful cycles than if you frequently drain the battery to its maximum DoD. For example, a battery may have 15,000 cycles at a DoD of 10 percent, but only 3,000 cycles at 80 percent DoD.
Below is a table of some of the more popular battery options, as well as the suggested maximum DoD as given on the product’s detailed specification sheets.
DoD of popular home battery options
* These manufacturers produce batteries of different sizing/capacity, only one of which is listed in this chart.
Another factor that affects your battery’s lifespan
Most solar batteries these days are guaranteed to last somewhere between five to 15 years. This partially depends on how often the battery is discharged and the number of cycles it’s gone through, but there are other things that have a big impact.
Another factor affecting the lifetime of your battery is how well you maintain it, and more particularly the temperature it’s kept in. Batteries in a hot atmosphere (over 90 degrees F) may overheat, which shortens the lifetime of the battery. Very cold temperatures also have a negative impact on the battery, because it has to work harder and at a higher voltage to charge. To maximize your battery’s useful life, try to keep it in a relatively mild environment – not too hot and not too cold.
Which battery is the right option for you?
Depth of discharge is important when comparing battery options, but it isn’t the only factor to evaluate.
Outside of DoD and costs, one of the most important things to consider is whether the battery will fit your particular situation. A small battery will be less expensive, but works best if you are just trying to reduce demand charges or avoid costly time of use rates. If you’re looking to go off the grid, you’ll need batteries that can store as much energy as possible (or, multiple smaller batteries stacked on top of one another). Homeowners who experience frequent (and long) power outages may also consider getting a large battery for peace of mind.
With any large purchase like solar and batteries (paired or separately), you want to consider your options. You can sign up on the EnergySage Marketplace to receive turnkey quotes for solar installation from pre-screened local solar installers. If battery storage is something you’re interested in pairing with your system, we recommend adding a note in your account preferences specifying you’re interested in pricing and information about batteries. Even if a solar installer doesn’t install batteries themselves, they can design a solar panel system so that you can add a battery later down the line.