Depth of Discharge (DoD) & Cycle Life – Solar Battery Specifications Explained

If you’re exploring the possibility of adding battery storage to your current solar PV system or purchasing a combined solar PV and battery system all in one go, you’ll want to understand the terms Depth Of Discharge (DoD) and Cycle Life when it comes to choosing a battery. These two specifications are important factors when considering battery and could impact your savings, the lifespan of the battery and the way you use your stored energy.

Depth of Discharge

Solar Batteries in Australia

In simple terms the depth a battery is discharged is the percentage a battery has been emptied to its total capacity. The DoD is usually referred to in a percent, so a battery that has had a DoD of 100% means it has discharged to its full capacity. For example, if a 15-kWh battery was fully charged and had a DoD of 100% it has discharged 15kW. If it had a DoD of 80% it would have only discharged 12kW.

Most solar batteries require to preserve some charge at all times, as discharging at 100% or too deeply can considerably shorten the batteries lifespan.

Generally speaking, batteries these days have a lifespan between 10-15 years however this can be heavily affected by the number of discharge cycles it has been through and the depth of those discharges. For example, a discharge of 10% will make less of an impact or degradation to a battery’s lifespan, than a discharge of 40%.

When researching into different batteries the manufacturers will usually specify a maximum DoD for best performance. For example, if it’s specified that a 10-kWh battery recommends a maximum DoD of 70%, you shouldn’t use more than 7-kWh from the battery without allowing it to recharge.

Cycle Life

Solar Panels Power Cycle

A battery’s cycle life is based on the number of times a battery can be charged and discharged before the battery reaches the end of its functional life. The depth of each discharge will be a major impact on the cycle life of a battery. For example, a manufacturer may state that a battery has 1,200 cycles at a 80% DoD which would mean the same battery would have 4,800 cycles if your typical DoD was at 20%.

Is Adding Battery For You?

For a battery to be beneficial, a solar system needs to be generating excess energy during the day for the battery to store. This excess energy is there to run appliances at night or when the sun is not out. If you have a solar system without battery your home will run off the grid during these times and any excess energy will be fed to the grid in return for a feed-in tariff.

Adding a battery to your solar system is the next step for energy independence, however the benefits of batteries are still secondary to the benefits of installing a solar system. For some households adding a battery system can be of great benefit and minimise a home’s reliance to the grid or allow you to become 100% independent from the grid as part of a standalone (off-grid) system. For others the cost of a battery can outweigh the benefits, especially if you have a solar system under 5kW or your system is not generating enough excess energy.

Learn more about batteries and whether they’ll benefit your household!

About Battery Systems

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Understanding Batteries

Off-Grid Systems

For some households a battery system can be of great benefit and minimise a home’s reliance on the grid. However, it’s important to understand for a battery to be useful your solar system needs to be generating excess energy for the battery to store, which you can then use at night or when the sun is not out.

When selecting a battery, you’ll want to invest in a system that is most suited to your home and can drive the best return on investment (ROI). Despite a larger upfront cost, a higher quality battery may significantly increase your ROI.

    Battery systems start from $6,000 and costs can vary greatly based on the following factors:

  1. Cycle Life-Time

    The number of times a battery can fully charge and discharge.

  2. Battery Power (kW)

    How fast it can be charged or discharged.

  3. Storage Capacity (kWh)

    The maximum amount of energy a battery system can store.

  4. Battery Management System (BMS)

    An electronic ‘smart’ system that gathers data and manages the battery ensuring it does not overload or operate outside of its safe functioning zone..

  5. Inverter

    Battery systems require their own inverter if your solar system does not have a hybrid inverter.

  6. 'All-In-One Unit’

    A system which includes the battery, BMS and an inverter all in one unit.

  7. Warranty

    Length of time or cycles the battery system is under guarantee.

  8. Blackout Protection/Backup

    It’s important to note this is not a common feature of a battery system and could cost thousands of dollars to include. Blackout protection not only requires additional components but also a specialised installation and rewiring. For grid-connected homes, the cost for blackout protection can outweigh the benefit.

Additionally, if your purpose for adding battery is to go Off-Grid and become completely independent from the grid you will need to ensure your solar system can generate enough energy to power your home and your battery system is large enough to store this energy. For homes in metro areas going Off-grid is not cost effective and is only recommended for those in remote areas with limited access to the grid. Off-grid solar systems with battery start at approximately $30,000.