Following our previous guide on calculating the right UPS capacity for you, today we move on to the next part in the series. We are going to focus on choosing the right battery size for your UPS.
Calculating the UPS power rating was fairly simple. For the average Joe (or Junaid) out there, finding out the right size of battery, especially one which can get you your desired backup time is fairly difficult to understand.
Read More: Here’s How You Can Calculate the Right UPS Capacity for Your Needs
That’s why today I will guide you about how to calculate the battery size for your UPS in a fairly simple manner.
Most of you may be already familiar with the following terms but its never a bad idea to refresh your memory. You can skip to the next section if you want to get to the nitty gritty of calculating your battery size.
Lets start here.
Battery capacities are measured in Ah, which stands for Ampere-hour or the number of amperes the battery can output in an hour. The Ah rating is usually mentioned on all lead-acid and dry batteries in a bold and large font.
The more the Ah rating, the more the battery capacity. Still with me here?
Here’s where it gets a bit complex. The Ah rating isn’t consistent and varies a lot depending on:
To quote an example, lead-acid batteries show results that vary by 10-15% under the same testing conditions.
The Ah rating is generally calculated by the manufacturer under a certain Runtime (usually 20 hours). Let’s say you have a battery with a 120Ah rating. That means that it is tested with a load current of 6A for 20 hours before it runs out of juice.
The rating is not final and a fixed runtime is used to standardize battery ratings. So if you use the same 120Ah battery with double the load (12A), it will last significantly less than 10 hours (10 to 15 percent less). Similarly, when you use the battery during loadshedding and consume its energy in a single hour, its available Ah rating decreases.
The more the load on your UPS, the faster its available runtime decreases.
For the same reason, an estimated factor is used to roughly estimate a battery’s Ah rating for long or short time periods.
The above factor is for lead-acid batteries. For AGM batteries, you can use 1.05–1.15 and for Gel batteries, 1.10–1.25 can be used.
Battery size can be deduced by using a couple of formulae.
First we need to calculate the load current and then we need to multiply it by the runtime factor (as mentioned above).
Lets take an example here:
Load Current for 1 hour = (Runtime Factor x Load) / (UPS Efficiency x UPS Input) = 1.5 x 1000/ (0.7*24) = 89.29A
Load Current for 1.5 hours = Answer x 1.5 = 89.29 x 1.5 = 133.94Ah
Now that we know our current usage for 1.5 hours, calculating the battery size is easy. We need battery size of 133.94Ah. The nearest round number is 134Ah.
In our case, we need 2 batteries so we will connect 2 67Ah batteries in series to get 1.5 hours of backup time. Since 67Ah batteries are not available, we can get 65Ah ones for slightly less backup or slightly larger ones for more backup time.
All you need to do is list the numbers like we did in our example and use the mentioned formulas to get your battery size for your desired battery backup time.
If there’s any confusion regarding this post, do sound off in the comments below. We will be bringing you the next UPS guide in the coming days so stay tuned.
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