What is depth of discharge?

We’ve all experienced that dread: Watching our cell phone’s battery percentage drop to single digits without a charger nearby. We dim our screen brightness, close unessential apps, take any step possible to buy time and avoid running out of battery power. 

Although fully draining our cell phone battery can be highly inconvenient, doing the same for your home power storage battery could actually damage the system, shorten its lifespan, and harm your home electrification investment.

This is why many home batteries come with a critical specification: Depth of Discharge, or how far down you can safely drain the battery without potentially causing a problem.    

Many batteries today feature depths of discharge, or DODs, of 100%, meaning it’s OK to use the battery’s entire energy capacity — but not all do. Let’s dive deeper into what affects battery lifespan and explore the DoDs of some of EnergySage’s most popular batteries.

Whether you want to boost your off-grid capabilities, keep the lights on during a grid outage, or save on electricity costs under less-than-ideal net metering policies or time-of-use (TOU) rates, adding batteries to your solar setup can be a game-changer. 

Depending on the size of the battery and your home’s needs, most solar battery systems can store enough extra electricity to fully power your home for hours. A recent EnergySage analysis found that homeowners are increasingly adding solar batteries to their rooftop solar projects as a way to make their systems more resilient, rely less on their utility companies, and maximize the long-term return on their investment.

When shopping for batteries, some of the important details to take into consideration include their cyclic life and whether its a lithium ion or lead acid battery. But depth of discharge (DoD) is another crucial factor. This detail can make a big difference when deciding which brand to choose for your home backup power. 

Depth of discharge of popular home batteries

It’s generally not recommended to discharge your battery entirely, as doing so could harm the system. To protect against this, many manufacturers specify a maximum depth of discharge, or DoD, which measures the amount of electricity you can safely pull from the battery without damaging it, relative to its overall capacity. 

For example, if a 10 kWh battery has a DoD of 80%, you shouldn't use more than 8 kWh from the battery without recharging. A higher DoD means you can use more energy stored in your battery. Many modern lithium-ion batteries now advertise a DoD of 100%, meaning you can discharge all the stored electricity before recharging.

On the flip side, a battery’s state of charge (SoC) is the percentage of battery capacity still stored in the battery. So, that 10 kWh battery with a DoD of 80% we mentioned above should be recharged when its SoC falls to 20%, or when it has 2 kWh of stored capacity remaining.

When you charge and then use your battery’s stored energy, you put it through a cycle. Your battery's cyclic life represents the number of charge/discharge cycles in its useful life. 

Battery manufacturers often define a battery’s cyclic life in the warranty’s cycles clause. But it can also depend on how much of the battery's capacity you normally use. If you regularly drain your battery just a little bit, 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% but only 3,000 cycles when the  DoD is 80%.

Not only are lithium-ion batteries more energy-dense than lead-acid batteries (meaning they can store more electricity), but they’re also more efficient, have longer lifespans, and offer a better depth of discharge. Unsurprisingly, most of the top batteries on the EnergySage Marketplace use lithium-ion chemistry. 

In terms of DoD, lithium-ion batteries are head and shoulders above lead-acid batteries: whereas lead-acid batteries generally shouldn’t discharge more than 50% of their total capacity, lithium-ion batteries can generally be drawn down to at least 85%. 

Let’s say you’re deciding between two batteries. One is lead-acid with 50% DoD, and the other is lithium-ion with 100% depth of discharge. Both are capable of storing 5 kWh (although lithium-ion batteries are usually capable of holding much more electricity than lead-acid batteries). You would only be able to use 2.5 kWh of the lead-acid battery before needing to recharge it, whereas you can pull double the electricity with the lithium-ion battery, expanding the resiliency of your system and reducing your dependency on utility companies.

Solar batteries are like Goldilocks – they don’t like getting too hot or cold. When exposed to hotter temperatures (over 90 F), batteries can overheat, which shortens their lifetime. Similarly, very cold temperatures also hurt batteries because they have to work harder and at a higher voltage to charge. 

To maximize your battery's useful life, keep it in a relatively mild environment like a garage or a basement.

That depends on your unique needs and setup. If you’re just looking to avoid costly time-of-use rates or provide backup power to your home during power outages, a smaller battery should do the trick. But if you’re looking to go off-the-grid, you’ll need a bigger battery system that can meet all your electricity needs and then some. 

That said, your best bet is to talk to an installer to understand which battery will best fit your needs.

Pairing your battery system with solar panels enhances the value of your investment and your impact on the environment. There are plenty of options to choose from, and you’ll want to ensure you’re getting quality equipment at the right price. 

The EnergySage Marketplace delivers personalized quotes from vetted installers to help you go solar (and add storage) with confidence. Sign up for free to compare your options apples-to-apples and consult with our team of unbiased experts.