How to Determine Battery Bank Sizing

March 24, 2021

Why is it essential to have the correct battery bank size for your system?

A battery bank too small might not handle your electrical needs for the time required, especially during the rainy/winter seasons when batteries might not start with a full charge. Eventually, this will shorten the length of time the battery can support the electrical load. Now, what happens when your battery hits a maximum amount, and the sun is shining? Systems that cannot sell their extra power back to their utility go into what they call a float mode, and inevitably the solar energy will end up wasted. One of the last effects is when a battery bank’s amp-hour capacity increases, the discharge time also increases. Smaller systems will have a higher discharge rate, leading to a more in-depth cycle and shortening your battery’s life expectancy. 

So, what if you have a battery bank too large for your system?

Imagine being required to drink 60oz of water daily, but you only have enough water for 32oz. Over time, will your body feel the same support of only drinking 32oz vs. 60oz? The same will happen with a battery size too large for your solar system. The battery might not hit the full charge if the system is unable to produce enough, which over time will reduce the support you could potentially see during lower production periods if the battery was able to hit a full charge. 

How much energy do we use during lower production periods?

The next step is to determine how much energy we use during lower production periods, like at night. Critical loads can range from lights to kitchen appliances and other smaller items plugged into outlets, water pumps. We need to determine how long each of these vital loads will be running and their watt usage to maximize the expected battery support during low production periods. For example, if you have a light that uses 20 watts and you keep it plugged in for 3 hours, you would take the 20 watts to multiply it by 3 hours which equals 60 watts. Add this amount together with the rest of your critical loads, and you will have your total Watts Hours! You can now determine your autonomy or how long you want the battery bank to support them during the lower production periods.

What type of batteries could we be using?

Lastly, we need to consider the type of batteries we could be using. There are many potential factors, but one of the key points to consider is the Depth of Discharge. Many recommend allowing a maximum of 50% cycled; however, this does not mean the battery can’t cycle them lower. If a battery has 1,000 cycles at 50%, we can assume that the battery has 2000 processes at 25% Depth of Discharge. It would help if you also kept in mind the temperature extremes. What kind of weather will your battery be going through? Some have better life expectancies than others. For instance, Absorbed Glass Mat batteries perform better in very cold or sweltering temperatures, whereas lithium batteries should always be reviewed before exposing them to extreme temperatures.