How to Size a Battery Bank

Many renewable energy (RE) systems incorporate batteries. Photovoltaic panels, wind and hydroelectric generators and other DC power sources can be used to charge batteries. The energy stored in the batteries can then be used directly to power DC loads or it can be inverted to power AC loads. The batteries recommended for RE systems are deep-cycle batteries. To ensure you have enough reserve capacity to provide the electricity you need (without running additional generators), invest the time to size your battery bank properly. Because of the various conditions affecting battery bank sizing, this process may be one of the more challenging calculations you’ll have to do when planning your RE system.

Before tackling the calculations, start by identifying a few key pieces of information:

• Watt-hours of electricity usage per day
• Number of Days of Autonomy
• Depth of Discharge limit
• Ambient temperature at battery bank

Electrical Usage

The first thing you’ll need to know is the amount of energy you’ll be consuming per day. It’s worth the time to do a careful evaluation of exactly what loads (appliances, electronics, etc.) you plan to use and for what lengths of time. Keep track of this information on a loads list; you’ll refer to this list often for sizing other components as well. Your final tally should be expressed in Watt-hours (Wh) per day. If you know the kilowatt hours (kWh) per day just multiply that number by 1,000 to determine the Watt-hours per day. (Example: 1.2 kWh = 1,200 Wh)

Days of Autonomy

Next, you must determine the number of days of battery back-up that you want to have on hand. In other words, if you are unable to charge your batteries by any means, and you still need to draw power, you must provide this additional storage by increasing the size of your battery bank. For PV systems, Days of Autonomy represents the number of cloudy days in a row that might occur and for which you intend to store energy. Consult a weather website, local meteorologist or even long-term area residents. If you conclude that you need more then five days of battery backup, you may want to explore multiple sources of electricity generation or backup generator options (like a fossil-fueled generator). If your primary electricity source is wind power, determine the number of days when there is little or no wind. This information can be found in the data you’ve collected using your data-logging anemometer. Hydroelectric turbine systems are unique because they usually operate continuously, and therefore do not require extensive storage. If you’re sizing a battery bank to be used in conjunction with an on-demand fuel-powered generator, the number of days of backup will represent the number of days you wish to go without using your generator.

Depth of Discharge

Another factor to consider is the planned Depth of Discharge (DoD) of your battery bank. Flooded lead acid (FLA), sealed AGM and sealed gel batteries are all rated in terms of charge cycles. A single cycle takes a battery from its fully charged state, through discharge (use), then back to full charge via recharging. The depth of discharge is the limit of energy withdrawal to which you will subject the battery (or battery bank). DoD is expressed as a percent of total capacity. The further you discharge a battery, the fewer cycles that battery will be capable of completing. Simply stated, deeper discharge shortens battery life.

It’s recommended that you never discharge a deep-cycle battery below 50% of its capacity; however, many battery manufacturers recommend even shallower DoDs. For off-grid applications, a 25% DoD will extend battery life significantly. On the other hand, if you’re only using the batteries occasionally, as a backup system, you can factor in a DoD of 50% or perhaps more.