A question we get asked often is how to determine the Ampere Hour rating of the batteries needed to power an inverter. Unfortunately, the answer is not an easy one if you want an accurate answer, however you can make some approximations to give you a good guess.
Firstly, it is important to know that the Ampere Hour or Ah rating of a battery is given at a 10 hour rate, a 20 hour rate or even 100 hour rates. So a 110Ah battery with a 20 hour rating can provide 110/20 = 5.5Ah per hour, OR if you take 5.5Amps from this battery it will last 20 hours. If you take 110Amps from this battery IT WILL NOT LAST FOR 1 HOUR. In fact it will last for A LOT less than this, and the equation required to determine the actual runtime is very complex and usually relies on the use of look-up tables or charts.
The point here is although for runtimes of 1 or 2 hours a simple calculation isn’t possible, you can get pretty good approximations when the runtime is in the order of several hours.
So, how do you start? Firstly, you need to know what power you will require in Watts. Let’s assume for the sake of argument that you want to power a computer system, some lighting and a few ancillaries, so you’ve got a total load of let’s say, 1200W.
Next, what is the DC voltage input for your inverter? Some inverters, will take a 12Vdc input, some 24V and some 48V. In this case we’ll assume that the DC voltage for our inverter is 48V.
We now need to calculate how much current is drawn from the battery. We do this by dividing the power required in Watts (in our case 1200), by the DC voltage (in this case 48) and we get 1200/48 = 25 Amps.
We also now need to allow for some inefficiencies in the inverter and some fudge factors so multiply this figure by1.25 and we get 25 x 1.25=31.25Amps
Now we can determine the Ah rating based on this figure:
If we want one hour then the rating will have to be OVER 31/1=31Ah, but this is inaccurate by as much as 60%.
If we want two hours then the rating will have to be OVER 31×2=62Ah, getting more accurate but still not precise, as this could be about a third out.
If we want four hours, 31×4=124Ah (about 20% right), eight hours 31×8 = 248Ah which is accurate enough for our purposes (about 10%).
We want 8 hours, we know that we need 248Ah, how do we achieve this with individual batteries?
Well, batteries can be connected in series, to obtain what we know as a “battery string”. As we need a 48V dc voltage then we need to put 4 x 12V batteries in series to get this voltage. To connect batteries in series we connect the positive terminal of battery 1, to the negative terminal of battery 2, the positive terminal of battery 2 to the negative terminal of battery 3, the positive terminal of battery 3 to the negative terminal of battery 4. We now have 48V between the positive terminal of battery 4 and the negative terminal of battery 1 and we connect these to the inverter. When we connect batteries in series the Ampere Hour Rating Remains Unchanged. So, connecting 4x12V 110Ah batteries in series would still give us 110Ah, although since the DC voltage is higher, the battery string can produce more power than a single battery.
How to connect 12V 100Ah blocks to make a 48V 300Ah Battery
If we connect batteries (or entire battery strings) in parallel the Ampere Hour rating is increased – you add the individual Ampere Hour ratings together. To connect batteries in parallel you connect the negative of battery 1 to the negative of battery 2, and the positive of battery 1 to the positive of battery 2. If you are connecting battery strings in parallel, for example our 48V system above you connect the negative of battery 1 to the negative of the next string and the same for the positive.
So, for our example above, we want 8 hours runtime, so we’re going to need a battery rated at at least 248Ah. Luckily there’s a single battery available that will do the job – a 270Ah battery, and we need to connect 4 of them in series to make our 48V battery string.
What if our inverter was rated at 12Vdc input, what batteries do we need then to achieve 8 hours runtime?
The same system applies:
Step 1: Calculate Watts (1200W)
Step 2: Work out Amps (Watts / Inverter DC Voltage = 1200 / 12 =100Amps)
Step 3: Allow for losses (Multiply by 1.25 = 100 x 1.25 = 125Amps)
Step 4: Work out AmpereHours (Multiply Amps by hours required = 125×8 = 1000Ah)
Step 5: Work out how many strings you need by dividing the Ah by the Ah rating of your string (1000Ah / 270 Ah – for the 270Ah battery = 3.7).
Step 6: As you cannot have a fraction of a string – round UP – (3.7 => 4).
So, as you can see in both instances we will still require 4x270Ah batteries the only difference is, is that one is connected in parallel, and the other in series. Is their any difference? Well the answer is emphatically YES. Fewer battery strings is better. This is because of ease of connection and the fact that the batteries will be charged more effectively. In addition, the danger of having one bad string take down all the others is eliminated. Another important factor is the cable size that is required. Note in the 12V inverter we require 125Amp rated cable. For the 48V version this is 31Amps. This is a HUGE difference in cost and ease of installation.
If you find all this a little to mathematical, we’ve put together our AH FOR INVERTER CALCULATOR on the UPSMart Help Section.
