To properly size a backup battery bank, compute your critical load profile to determine daily watt-hour consumption during power outages. That number can often be your guide for the correct battery size. Most grid outages are less than one day, and a battery bank sized to be discharged to 50% of capacity by the critical load profile will meet most needs nicely.
If you’re off grid and rely on your batteries to meet all your electrical loads, buy a long-lived battery and be prepared to maintain it well. These systems—which cycle the batteries daily—use batteries with a lead-antimony alloy, which performs better under conditions of regular cycling.
Typically, off-grid battery banks are sized by considering the required “autonomy”—the number of days that the battery will provide for the loads before reaching 50% depth of discharge (DOD). Off-grid systems usually size a bank to provide two to four days of autonomy. For example, if your load profile requires 5,000 WH per day, you’ll want a battery that stores 10,000 WH to achieve one day of autonomy. Four days of autonomy would require a 40,000 WH battery capacity.
Off-grid system designer opinions on maximum DOD vary widely. Some prefer to keep the depth no greater than 20%, while others have no fear of going below 50%. The deeper the regular discharge, the fewer cycles a battery will give you before needing replacement. So if you do not mind swapping your battery bank more often, go with a deeper discharge—it will save you money up front. But if swapping batteries into and out of your system is a royal pain, you might prefer maximizing battery life by buying a higher-capacity battery. For the design choice that will save you money in the long run, calculate the savings from buying fewer batteries up front, plus the cost of more frequent battery replacement (higher DOD)—versus more batteries up front, with fewer replacements (lower DOD).
To maximize battery life, batteries need to be properly maintained by:
• Making sure the batteries get completely recharged at least once a week by RE generation and/or supplemented with backup generator or grid charging
Manufacturer. Battery manufacturers build batteries for many different applications. Historically, RE systems used batteries originally designed for other applications, such as powering electric golf carts. Today, many battery manufacturers list which of their batteries are appropriate for RE systems. All battery manufacturer Web sites listed in this guide, with the exception of FullRiver Battery, list batteries specifically for use in RE systems.
Model name. These letters and numbers are used by the battery manufacturer to “name” a group of batteries that have similar characteristics, and distinguish them from the company’s other battery lines. It is important to not use batteries with differing model numbers within the same battery bank, as mixing different battery types can create an imbalance within the pack which leads to poor system performance and may cause premature battery bank failure.