Intending to buy UPS? May be this can help!

Planning to Buy a UPS in Karachi

Summers have already started here in Karachi, and the blind man’s bluff of the electricity has already touched its peaks. In this situation, most people opt for UPS to provide backup during the power outages. So, if you too are in a quest to find a well-suited UPS (Uninterrupted Power Supply) for domestic use then here are some technicalities, some buzz words reverberating in the market which are frequently used by the UPS sellers to attract you, and which you should know before buying the UPS. So keep reading.

1. Line Interactive vs Online UPS:

The first specification which you would come across will be whether a UPS is Line Interactive or Online. This particular property is called Topology of the UPS. This property is quite important in the sense that it affects the overall cost of the system. And you should be aware of the topology, so as to take the best decision as per your requirements.

The first type i.e. Line Interactive, senses the main power line and drives the load with this main supply, and if needed also charges the battery. But as soon as the power on the main line goes off, it immediately switches to the backup and starts driving the load from the battery. This means that there is a little delay in switching from the main line to the backup. This delay won’t affect the performance of most of the home appliances, but other high performance devices which require uninterrupted and seamless supply would restart due to this minor delay.

The other type i.e. Online is also known as Double Conversion Online UPS. It basically converts the AC power from the main line into DC to charge the battery, and then from battery it again converts DC into AC to drive the load. Since the load is already driven by the battery, thus when the main line goes off then only the charging path disconnects, whereas the power continues to be delivered to the load. The online UPS are best suited for higher end applications such as performance critical servers and routers. They are also higher in prices as compared to the line interactive type. So for most domestic purposes, the line interactive type is the UPS of choice.

2. Square Wave, Modified Sine Wave or Pure Sine Wave:

Our electrical appliances require pure sine wave AC power to run properly. Thus, the closer the output of UPS to sine wave will be, the better will be its performance. In this regard, the worst of the UPS’s are of square wave type. They badly affect the performance of appliances on the longer run. The most significant indicator of the square wave UPS is an undesirable humming sound produced by electric fans.

The other one, i.e. modified sine wave UPS performs some filtering of the square wave to generate a sine wave like (but not completely sine wave) output. This is a trade-off between the cost and the performance. Modified sine wave type UPS are expensive than the square wave type but perform far better as well.

Finally, the pure sine wave UPS, as the name suggests, generates a complete sine wave on the output. Thus it provides the best performance albeit at a bit higher cost. So if you are looking to get the best performance and cost is not an issue, then opt for the pure sine wave type. Otherwise, opt for the modified sine wave UPS as a low cost alternative. But, in my humble opinion, the square wave type should be avoided as much as possible.

3. Power Rating:

The power rating of the UPS is basically the output power it can effectively deliver to the load. For example, a 1KVA UPS can be considered to theoretically drive a 1KW load (considering an ideal 1 power factor and no losses at all.) But practically, due to various losses, the power delivered to the load is lower than the expected value. The optimum value of this rating is chosen after examining the load you wish to drive. For example, a common ceiling fan takes somewhere around 70 to 100+ watts. Then a 1KVA UPS can ideally deliver power to approximately 9 or 10 fans. But again, due to the losses, the actual power delivered would be a bit low.

Considering the domestic usage for most of the common people in Karachi, where the basic requirement is to run the ceiling fans and light bulbs. Now, assuming that a normal house needs to use around 5 ceiling fans of 100 watts each and 5 energy savers of 20 watts simultaneously. This makes the power requirements equal to 600W. Taking into account various losses and providing some leeway to the UPS power rating, it is prudent to opt for a UPS with a bit higher power rating. In this particular scenario, as per my opinion, a 1KVA UPS would work just fine.

4. Backup

The final and the most important thing is the backup provided by the UPS. If the backup time is not up to the mark, then everything else is of no use. And the backup time of the UPS hugely depends upon the battery capacity (the Ampere-hour rating of the battery) and the load to be driven. Considering our previous example where we had a load of 600W. Now, let us try to estimate the battery capacity required to drive the load for approximately 2 to 3 hours backup time.

Consider we have 12V and 100Ah battery available. This makes the energy capacity of the battery equal to 1.2KVAh (12V x 100Ah). Now ideally this battery could provide the backup to a 600W load for about 2 hrs. But practically, it would be somewhere around 1 and a half hour. This means that you require a 12V and 200Ah battery to run the same load for about 3 hours.

These calculations for the battery capacity and the backup time are very crude, and many factors like power factor, inductive load and various losses have not been taken into account to keep the calculations simpler.

Ending Notes!

Now, before ending the post, I would like to sum things up. The first thing was topology. And for that parameter, a line interactive UPS is the best suited one for domestic applications. Then, for the type of output, the pure sine wave type is the best. But if you are tight on budget, then you can opt for the Modified sine wave type. Finally, for backup and power rating, it is better to get a calculation from an expert electrician for your particular case as it depends upon load, backup time, losses incurred and some other factors as well. But if you are just willing to run fans and bulbs which too are not more than 4 or 5 for 2 – 3 hours, then a 1 or 1.5KVA UPS would do just well. But again, this depends upon a lot of factors, thus an expert electrician or UPS technician would be able to guide you better. I hope that this post answers some of your questions. Thanks for reading!

Please share your thoughts, advice and suggestions in the comments section below.

Author: Saad Riaz

The author is a Hardware Design Engineer by profession, but here he expresses his thoughts and has the vision to bring about some positive change within his capacity.

10 thoughts on “Intending to buy UPS? May be this can help!”

  1. Nice written. Please keep sharing such valuable information.
    Also, if you know some points to improve the battery life for UPS, then please share. That’ll be helpful.

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