DRAFT
How to size up a UPS. These are a few concepts to understand.
Watts measures the real power, which is the power actually used or consumed by a device or system to perform work. Watts describes how fast the device is using energy.
But there is also reactive power, which arises from reactive components in an electrical circuit, such as inductors and capacitors.
Volt-amps measure apparent power, which is the combination of real power and reactive power.
The first information to calculate is the minimum Volts-Ampere (VA) value for the UPS to support the equipment you have.
You can use inexpensive power meters to measure the VA for your equipment. Plug them into the power strip, and plug the power strip cord into the power meter.
If you do not have one, then calculate this value. Make a list of the equipment you need to connect to the UPS. Now add the power in Watts for all these items. Divide this number by 0,8 to get the VA.
You can also get the VA by dividing the Watts by the power factor.
Take a margin of 20% (VAx1,2).
For example, should all your equipment consume 500W, you divide it by 0.8 and get 625VA. With the margin, you need a 750VA UPS.
Now, the second question is how to calculate the operating time.
This calculation depends on battery size in watt-hours.
Watt-hour is a measure of quantity. It is the total energy you have or have used over time. So you can retrieve the operating time by dividing the Watt-hours by the Watts your equipment consumes.
Unfortunately, batteries are usually labeled in Amp-hours, not Watt-hours. To calculate the Watt-hours, you must multiply the battery's Amp-hour rating by its nominal voltage to get its rating in Watt-hours (the more batteries you have and the bigger Ah, the better).
For example, for a small UPS, 12V x 7Ah = 84Wh
However, there are many losses, as small batteries are very heavily used, so we need to multiply by about 0.7
84 x 0.7 = 59Wh usable
If you do not find the nominal voltage on the datasheet, you may find a list of replacement batteries for the UPS of your choice, which will probably indicate battery voltage and ampere-hours.
Divide the watt-hours by the watts of your equipment to obtain the running time in hours.
59Wh ÷ 500W = 0.118 hours. If we multiply by 60, approx. 7 minutes.
Unfortunately, one usually has to buy a much more powerful UPS than needed to get a battery large enough to last 20 minutes or more. You can attach a large external battery to an inverter to obtain greater autonomy. But this doesn't work well in Africa because the battery charger is very slow and can take several days to fully charge it after an outage.
SIl power systems recommend using lithium batteries (LFP) for greater autonomy and durability.
Look at the SIL Power system website to get more insights. https://power.sil.org/learn/whats-watt/
https://power.sil.org/choose-and-use-a-ups/
When sizing up your backup power systems, make it the most reliable possible with the funds you have. Make sure it has enough autonomy. Should you need assistance for funding a UPS, please be aware that ICT services can help you, depending on the project priority and available resources. Feel free to fill in a project request.
Thanks to Paul Zwierzynski for all his valuable inputs in this topic.