Designing an Off Grid Solar Electric System, Step 4, Sizing the Battery Bank

Properly sizing the battery bank is important to the operation of the solar system. Often I will see designs were the battery bank is made too small and as a result the system will fail. The batteries store all the energy for the system without this storage your load loses the ability to operate when you want it to rather than when the sun is shining.

The basic calculation for this step is:

Calculation 1

(Load WH/day)  /  Base Battery Voltage = (Load AH/day)

Calculation 2

(Load AH/day)  x (Days of Autonomy) / (Max DOD) / (Cold Temp Factor) = (AH Battery Bank Size Needed)

Base Battery Voltage

The base battery voltage is the nominal battery bank voltage, 12V, 24V or 48V. While there are many more possibilities of battery voltages if you use a different voltage you lose the ability to use standard off the shelf charge controllers and inverters.

The higher the voltage the lower the current. Lower current and higher voltages will result in less voltage loss and typically a less expensive solar system.  For example if you are using a charge controller rated for 50 amps max output you can more more power through the same controller.

  • 50A x 12V = 600W
  • 50A x 24V = 1200W
  • 50A x 48V = 2400W

Here are some general rules to help you select the best base battery voltage for your off grid solar electric system.

If you are powering AC loads on large inverters 4000W and above, use a 48V battery bank to eliminate voltage loss.

Small AC Loads or just a few hundred watts, 12V might be best. As you design the system, start with 12V and if you find the result is a minimum of two batteries then you have the option of 24V. But if the results are for a single standard battery size, stay at 12V.

Mid size loads 1000W to 2500W, 24V is typical as you can easily find inverters in this size range at these wattage ratings.

In this example lets use 24V so I can also talk about series and parallel connections of the batteries.

Days of Autonomy

The days of autonomy is the number of days the load can operate without any charging from the sun.This is an important number because if you have bad weather today, you still need to make sure you have power for tomorrow stored in the battery. Consider a week of solid rain, will your system contine to operate? The number of days used is highly dependant on the location of the system. Also consider if you have an alternative way to charge the batteries. For example if you have a cabin equipped with a generator than 2 or 3 days of battery autonomy is plenty.  In general if you have a low critical load that can fail, 2 or 3 days of autonomy is ok. My default is 5 days of battery autonomy as I find at 5 days produces a very reliable system design and allows the batteries to have a nice long life as they are not cycled too deeply.  If you have a critical load that can’t fail increase the days of autonomy to 8 to 10 days.

Max DOD

Max DOD stands for Maximum Depth of discharge. As you discharge and recharge a battery this is referred to as 1 cycle. The deep you discharge a battery the less life it will have and the life is referred to as cycles. For example (based on a specific battery data sheet I’m looking at for this write up, your battery will be different) if you discharge a battery just 10%, leaving 90% of the power in the battery, it will last 5000 cycles. If this happens every day this equals (5000 cycles / 365 cycles/year) 13.7 years. But is you discharge the same battery 50% every day you will see a life of just 1250 cycles or 3.4 years.  For solar, without using computer simulation software it is hard to estimate the average cycle depth of discharge. In general what I’ve found is that a good quality battery with 5 days autonomy and an 80% max depth of discharge will last 5 to 8 years.  For this calculation 80% depth of discharge is considered dead for a battery and you will find the battery voltage approximately 10.5 volts. In this calculation I would use 80% Max DOD or 0.80 in the calculation. If you want a longer life from the battery use a smaller number.

Cold Temp Factor

Batteries do not like cold temperatures and their capacities are greatly reduced. Most battery manufacturers will publish a temperature vs capacity chart.

In general you can use the following if you can’t find a chart specific to your battery.

  • 80F = 1.0
  • 60F = 0.95
  • 40F = 0.88
  • 32F = 0.80
  • 20F = 0.77
  • 0F = 0.60
  • -20F = 0.40

Below -20F you need to find a specific chart for your battery. Most batteries will operate from -40F to 160F. If the battery operates above 100F it will lead to a shorter life.

In this example, let’s use 0.80.

Putting it all together:

Calculation 1

(Load WH/day)  /  Base Battery Voltage = (Load AH/day)

985 WH/day / 24V = 41.04 AH/D @ 24V.

Calculation 2

(Load AH/day)  x (Days of Autonomy) / (Max DOD) / (Cold Temp Factor) = (AH Battery Bank Size Needed)

(41.04 AH/D) x (5 Days of Autonomy) / (0.80 Max DOD) / (0.80 Cold Temp Factor) = (320.6 AH @ 24V of Battery Needed )
Let’s say the battery you selected is 120 AH @ 12V and 100HR Discharge Rate you will need 320.6 AH / 120 AH = 2.67 batteries. Round this to 3 batteries in parallel.  Because this is a 12V battery and you have a 24V battery bank, multiply the 3 batteries in parallel by 2 to create a 24V battery bank that is rated 360 AH and used 6 batteries total.
As a side note, the 100HR discharge rate has to do with how fast you discharge the battery. The quicker you discharge the battery the less AH you will receive from it. Most batteries are generically rated at the 20HR discharge rate but when you look at the datasheet you can find a more solar friendly rating. In this case we have 360 AH of battery and are only using 41.4 AH/D of  load. 360 AH / 41.4 AH/D X 24 H/D = a discharge rate of 208 HR. This far exceeds the 100HR rating of the batteries.