Battery Bank Sizing: Off Grid Solar Power System Design – Step 2

Hi welcome back, I’m Amy Beaudet from the altE Store. Thank you for watching.

The second of our video series on designing an off grid system, Step 2 is to size the .

We’ll discuss the different considerations that go into sizing.

Your , We’ve, already done a loads list in our previous video, so we know how much power we will use each day.

Now, let’s see what size we need to store it.

The answer may surprise you Just a quick reminder of the components that make up an off grid system.

We will start at the battery bank.

You may recall the loads list we did in our previous video.

We came up with a total usage of 2191 5 watt hours a day.

Note, however, that 44Wh are DC, not AC and therefore not going through the inverter. So we’ll need to use that information later.

So let’s get started.

The first thing we need to decide is: what voltage we will make the battery bank Most off grid are either 12 24 or 48V.

So how do you decide which to use First is what voltage are your loads? Are you just powering a small video surveillance, camera or light that runs off 12V DC, Or is it an AC system that will be using an inverter to convert from DC to AC If it is using an inverter? What size does the inverter need to be? Generally? The higher the output wattage the higher the DC input, For example, if it is a 2000W inverter, it may be available in 24VDC, but a 6000W inverter will certainly require 48V.

Another possible consideration is the distance between the and the battery bank.

Depending on what type of charge controller you use, you may need to match the voltage of the array with the voltage of the battery bank.

I’ll get more into that in a future video, But if you do need to match voltages, keep in mind that if the panels are far away from the , you can reduce the gauge of the expensive, copper wire needed by using a higher voltage.

Since using a higher voltage array results in lower current, you can potentially save money by running the system at 48V.

Instead of 12V Let’s get into this a little deeper.

We’ll use an 8A4D battery as an example. It is 12V and 200Ah 12V x, 200Ah, 2400Wh for 1 battery.

Remember that wiring in parallel increases the amphours but keeps the voltage the same and wiring in series increases the voltage but keeps the amp hours the same.

If I needed 4800Wh capacity, I can wire 2 of these in parallel.

However, if I needed 9600Wh, I would need 4 of these .

4 x, 12V x.


Since I want to limit the number of parallel strings, I use I can’t wire them all in parallel, but I could wire them in 2, parallel strings of 2 in series.

In doing so, I made a 24V 400Ah battery bank 24V x, 400Ah 9600Wh, Or I can make a 48V system by wiring them into 1 string of 4 in series To decide which voltage you use.

You can then refer back to the other considerations of.

If you have any specific voltage DC loads or if the inverter you picked requires a certain voltage, If you did need to have a 9600Wh battery bank as in the previous example, but you need a 12V bank for your loads, you can still accomplish this. You would want to pick a lower voltage but higher amp hour battery 9600Wh.



So how can we build that? We can pick a lower voltage higher amphour battery, like the Concorde PVX 405 at 6V and 405Ah and wire them 2 parallel strings of 2 in series.

The 2 6V batteries in series makes 12V and the 2 405Ah batteries in parallel equals 810Ah.

More than enough, When wired together, you get a 12V 810Ah 9720Wh battery bank.

Now that we’ve figured out how much power we use a day, we need to know how many days we plan on running our equipment off the battery bank.

If there is no sun to recharge it or days of autonomy, This is a delicate balance, because the more days we select the bigger and more expensive the battery bank gets, But we don’t want to go too small either because the less we drain, the Batteries the longer the bank will live.

This is where that generator I mentioned can come in handy, For example, you could pick 3 days of autonomy and plan on using the genny to charge up the battery bank if needed on day 4 Depth of discharge or DoD is how far down you can drain.

The battery A lead acid, deep cycle battery that is made for renewable energy systems can be drained down pretty low, but the less you drain it the longer it will live. You’ll often hear people say you can drain a deep cycle battery down to 50.

That’s true, but if you do it will last half as long as, if you drained it to 20, Each battery will have a depth of discharge chart.

You can see here that if you drain this battery down to 50 using half it’s power, you can get about 1500 cycles or 1500 days.

If you do that every day That’s just over 4 years, But if you only drain it 20, you can get 3400 cycles over 9 years.

That sounds great, except you have to remember.

That requires a bigger bank to use a smaller percentage.

So you have to balance the upfront cost of the system with how often you have to replace the batteries.

You may also hear the term State of Charge or SoC.

That is the percentage of how full the batteries are.

It is the inverse of DoD. So a battery that is at 30 depth of discharge is at 70 state of charge.

Batteries are rated at 77 degrees, Ferenheight or 25 degrees Celcius, When the temperature gets colder than 77 degrees.

The amp hour capacity decreases, but the lifespan increases When a battery is hotter than 77.

The capacity increases, but the lifespan decreases To compensate for lower temperatures.

We will need to increase capacity.

This chart shows the change in capacity based on temperature.

You see here at 77 degrees.

The capacity is at 100 what it is rated for, For example, 100Ah, But you see here at 50 degrees.

You are at 81 of the rated capacity.

So if you still need 100Ah, you would need to multiply that by 1 19 to get a battery rated at 119Ah and at 50 degrees it will be able to store 100Ah. The colder the battery is the larger.

The rated battery needs to be to store your power.

Ok, now that we know the variables let’s do some math to figure out what size battery bank we need From our loads list.

You remember the loads list.

Don’t you, we are using 2192Wh a day, but only 2148Wh was AC.

We will divide it by the efficiency of the inverter.

We use let’s say 92 to make up for lost power used by the inverter.

Then we enter our DC loads 44Wh.

This gives us 2379Wh Next step.

Then we multiply the 2379Wh by days of autonomy and temperature compensation. I’m going to be storing it in a 50 degree room, so I use 1 19.

We’ll divide that by 5 for 50 depth of discharge Now notice that we are using 50 depth of discharge, but that’s after our 3 days of autonomy.

So I can run my loads for 3 days with no solar recharging, the batteries and after 3 days I’ll have used half my rated capacity.

That should give me plenty of stored power and a long battery life.

This gives me 16986Wh battery bank needed.

Then I divide this number by the voltage of the battery bank.

We picked I’m – going to use a 48V battery bank, so I divide by 48V to get 354Ah.

Ok, we are almost there.

We take our 354 amp hours and divide it by the maximum number of strings we want to use.

I’ll go with 2. That says I need 2 strings of at least 177Ah batteries.

Let’s pick some batteries that will fit this bill.

An MK 8AGC2 battery is rated at 6V 190Ah.

We can use that one.


We take the system voltage of 48V divided by 6V battery, which tells us we need 8 6V batteries in series.

Let’s add that all up 2 parallel strings of 8 in series 16 batteries needed That’s it for the second video for designing an off grid.

Pv system Watch the next videos in the series for how to size the solar array and charge controller and inverter using the numbers you came up with from your loads list, Also watch more of our Video Series on our web site and peruse our selection of deep Cycle batteries We’ve got a team of highly trained Technical Sales Reps available to help you plan your system.

Give us a call Thanks: .

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