How to Size Your Battery Bank

How to Size Your Battery Bank

So how big should my battery bank be? There are two main considerations. Load size/duration, and physical space. Large amp-hour capacity batteries are heavy and take up space, so naturally you must take this into consideration when choosing how big of a battery bank you can fit in your well ventilated space.

Assuming you have the physical space, the calculations for load size/duration are the same regardless of which type of solar system you have, off-grid, or grid-tied with battery backup. In your calculation, you must consider how much power each device (light, refrigerator, television, etc.) will draw when on, and how long (duration) each device will operate every day. Keeping in mind that we don't want to drain our batteries down to less than 70% of their rated capacity to prolong their life, this shall also be taken into consideration when sizing our battery bank. Here are the steps for sizing a battery bank with three electrical devices (for simplicity, but can be followed for as many devices as needed):

1) Continuous power draw (in watts) of device one, times the number of hours of operation per day.
2) Continuous power draw (in watts) of device two, times the number of hours of operation per day.
3) Continuous power draw (in watts) of device three, times the number of hours of operation per day.
4) Add up the total number of watt-hours needed per day.
5) To not get below 70% battery capacity (30% drain), divide the number you got in line 4 by 0.3.
6) Take the number you got in line 5 and divide it by the voltage size of your battery bank to get amp-hours needed.

An example shall now be shown using the above steps. In this example, ten 15W compact florescent lights, a refrigerator that draws an average of 500kWh/year, and a television that uses 100W shall be used. The lights shall be needed for 7 hours/day, the refrigerator 24hours/day, and the television shall be used 5 hours/day. We are going to arrange our battery bank into a 48V configuration. Following the above steps:

1) 15W x 10 lights x 7 hours = 1,050Wh/day
2) Refrigerator uses 500kWh/year, divide this by 365days/year = 1,370Wh/day on average
3) 100W x 5 hours = 500Wh/day
4) 1,050Wh/day + 1,370Wh/day + 500Wh/day = 2,920Wh/day
5) 2,920Wh/day / 0.3 = 9,733Wh/day
6) 9,733Wh/48V = 203Ah

So your battery bank needs to have a 203Ah capacity in a 48V configuration. That's it! Keep in mind these numbers are conservative since your solar system will provide all needed power to your loads during the day once the batteries are fully charged, so in reality, the batteries only have to power your loads for a portion of the day, but it is always a good practice to calculate your battery bank size based on worse case conditions so that your loads will never be without power. Remember, the capacity you need changes if you change the voltage level of your battery bank!

How to Arrange the Batteries to Achieve the Desired Capacity and Voltage Levels

Depending on if you wire up your batteries in a series, parallel, or series-parallel combination, will determine the overall capacity and voltage level of your battery bank. Here are the general rules for wiring batteries:

When in series, battery voltages add, their capacities do not.

When in parallel, battery capacities add, their voltages do not.

We can combine these two types of configurations to achieve any capacity and voltage (as long as it is a multiple of an individual battery's voltage and/or capacity). The following graphic visually explains these relationships and shows how you can manipulate the voltage and capacity levels of your battery bank:

A good solar contractor will do all these calculations for you!

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