How to Determine Battery Bank Sizing

March 24, 2021

Battery banks are another component of your solar system that provide a valuable resource for solar energy consumers. Similar to net metering, a battery bank can calculate and store excess energy produced by your solar system. Then, when your solar system goes offline at night or underperforms on cloudy days, you can use the energy stored in your battery bank. A correctly sized and properly-fitted battery bank can maximize your system’s efficiency and savings. 

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. This is especially important to consider during the rainy/winter seasons when batteries start with a partial charge. Eventually, this will shorten the length of time the battery can support the electrical load. 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. Another factor to note is that 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?

It may seem simple to buy the largest battery bank on the market. However, installing a battery bank too large for your system also has adverse side effects. When your battery bank is too large, you can run into issues such as chronic undercharging and shortened lifespan. Suppose your battery does not have the total charge because your system cannot produce enough. Over time, this will reduce the support you could see during lower production periods. If your battery consistently remains partially charged by your system, the unused battery cells will degrade. This degradation limits the amount of charge your battery can hold and decreases the overall performance of your battery bank.

How much energy do we use during lower production periods?

The next step is determining how much energy you use during lower production periods, like at night. Critical loads can range from lights to kitchen appliances. You should also consider other smaller items plugged into outlets and water pumps. To maximize the expected battery support during low production periods, you must determine how long each vital load will run and its watt usage. 

For example, suppose you have a light that uses 20 watts and keep it plugged in for 3 hours. Take the 20 watts and multiply it by 3 hours. This equals 60 watts. Add this amount to the rest of your critical loads, and you will have your total Watts-Hours. You can now determine your needed energy output and how long you want the battery bank to support you during lower production periods.

What type of batteries could we be using?

Lastly, we need to consider the batteries we could use. 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 experience?

Moreover, some have better life expectancies than others. For instance, Absorbed Glass Mat batteries perform better in cold or sweltering temperatures. Lithium batteries, on the other hand, should constantly be reviewed before exposing them to extreme temperatures.

There are many factors to consider when purchasing a battery bank for your solar system. At Everlight Solar, our trained professionals will analyze your home and energy needs and outfit you with the best battery bank for your needs. Contact us today