Battery Size Calculator Tool - Video Part 1
This video will walk you through the process to calculate how to size a battery bank. First, we’re looking at the load calculator, days of autonomy and the depth of discharge.
How does one correctly size a battery bank in an off grid solar application?
A good first step would be to watch this video and use CivicSolar’s battery sizing tool. This video will guide you through the various factors involved: load calculations, days of autonomy, depth of discharge, system voltage, battery types, and more. With a basic understanding of these concepts, you’ll be able to apply them in sizing your own battery bank.
Hello, and welcome to the CivicSolar’s guide to the CivicSolar Battery Sizing Tool.
First, we’re looking at the Load Calculator. I have, as an example system, an air conditioner running 10 hours a day, a refrigerator running 12 hours a day, 4 incandescent lights running 12 hours a day, and a TV running 2 hours a day. All of this adds up.
Press the tab button, and these fields will populate where it says “average energy used per day”—that’s the number that we are going to use in the battery sizing tool. You can extrapolate that out, too. This is in watt hours, and the “average energy used per month” would be measured in kilowatt hours, as it would be if it was per year. But for this particular exercise, we will just be using the “daily watt hours” which is 25,620; so 25.6kW/h.
In the next field, put in 25620. We’ll go with 2 days of autonomy. A common range is anywhere from 1 to 3 days. You want at least 1 day of autonomy; preferably 2 days is a good amount; and 3 or more may be necessary for some applications.
We can reduce the number of days of autonomy needed by incorporating a generator or some other source of power such as a fuel cell. This will cut it down because you will only need the full capacity of the battery bank in the worst weather conditions, in which there are multiple days of limited to no solar insulation leading to limited to no powering of the batteries. That will depend on location, but generally those extreme cases which test the limits of the days of autonomy are fairly infrequent.
So an easy way to reduce the days of autonomy needed and therefore the size of the battery and the cost of that is to incorporate a generator, which will run fairly infrequently. So the battery discharge rate: the most commonly used discharge rate is 50%. In this situation, the battery bank capacity is required to be 117,000 watt (117 kilowatt hours). That is determined by these three factors, and there is also a buffer we put on due to the various factors that will limit a battery bank’s capacity. A battery bank will discharge, or lose capacity, over time. A battery is considered to be at the end of its life when it is at 80% of its original capacity, though it will continue to operate. Just like panels, their warranty and minimum lifespan is 25 years, but the life expectancy is generally much longer than that.
Set the system voltage as 48. 24 and 48 are the more common system voltages; 12 is generally only for rather small off grid applications.
Let’s look at our available products. These are all DEKA. DEKA is our preferred battery vendor. You’ll see most of these are AGM and Gels, and there’s only one wet battery. This is an off-grid installation, so we’ll go with this Gel battery. If look at the different battery sizes you’ll notice the larger the battery, generally the lowest the cost per amp hour (amp hour being what it is rated in).
You should also pay attention to the voltage. Another way to judge the capacity of the battery is to multiply the amp hours by the voltage and that will give you the watt hour capacity.
So for this 225 amp hour Gel battery—with these 3 criteria of the 25 kW hour daily usage, the 2 days of autonomy, and 50% depth of discharge, and with this battery bank 48 volts—you only need 44 of these batteries. So let’s change the system voltage: right now we have 4 batteries in series, and 11 batteries in parallel, which is way too many to have in parallel. We definitely don’t want to have more than five. Four is the ideal maximum. In that case, we need to look at a different battery type, and one option would be a 6-volt battery. This is a much larger battery. The 6-volt 370 amp hour has the largest amp hour and/or watt hour capacity.
Going with a lower voltage is going to require and also allow more batteries to be on a string. All things being equal, if you have a lower voltage on a battery you can fit more on a string adding up to 48 volts, or 24, or 12, or whatever the voltage on the system is. That will allow you to have fewer strings in parallel.
Seven is lower than 11, but it is still pretty high. In this case, we would want to look at our other options. There are 2-volt which are generally for larger systems. You can fit 24 2-volt batteries on a 48-volt bank compared to fitting just 8 of the 6-volt. That will allow you to fit more batteries on a string. For larger battery banks, you might want to look into getting lower voltage batteries.
Let’s look also at the depth of discharge. This is 50% depth of discharge. We mean that the battery is discharged by 50% to a remaining amount of charge of 50%. So let’s see if go all the way to 100%, what is the number of batteries required? It’s 32, compared to 56, what it had been with a DoD of 50%. So going to 100% reduces it but for lead acid batteries you never want to discharge the batteries 100%, or even past 50%.
Greater depths of discharge than 50% (and certainly 80%) will lead to a very shortened battery life, and this is not very cost-effective. Although you’d have a much smaller battery bank, it would last for a much shorter of time. If you want to have the system going for any great length of time, it is more cost-effective to invest in a higher battery bank allowing for a lower depth of discharge.
What happens if you go to only 20% depth of discharge? 56 batteries are required for 50% depth of discharge, and 136 batteries are required for 20% depth of discharge. Going to 10%, it would be double that, 272. A depth of discharge of 10% to 20%, a rather shallow cycling, would require a much larger battery bank for the same application but it would also last dramatically longer. But 10 to 20% does require such a much larger battery bank that generally 50% depth of discharge makes the most sense as a balance between initial cost and size of the battery bank versus longevity.