UPS Sizing Guide: VA vs Watts and How Long the Backup Will Last

In Pakistan, a UPS is not a luxury. Between load shedding, voltage swings, and sudden tripping, computers, routers, and network equipment need protection from the power supply. But UPS units are sold in confusing ratings, and buying the wrong size is the most common mistake: too small and it overloads, too large and you pay for capacity you never use.

This guide explains VA against watts, shows how to size a UPS for an office, and sets realistic expectations for backup time.

VA is not watts

A UPS is rated in volt-amperes (VA), but your computer draws power in watts. The link between the two is the power factor. Many small and mid-size UPS units have a power factor between about 0.6 and 0.7, which means a 1000 VA UPS delivers roughly 600 to 700 watts. The rated watts are usually printed on the label or in the specification. Always size against watts, not against the VA figure on the box.

How to size a UPS in four steps

1. List every device that must stay running and find its wattage on the label or power adapter. An office desktop with a monitor commonly draws somewhere between 100 and 200 watts in normal use.
2. Add them up.
3. Add headroom of about 25 to 30 percent so the UPS is not working at its limit and you have room to add equipment.
4. Convert to VA by dividing by the power factor, or simply pick a UPS whose rated watts exceed your total.

Worked example: five desktops at 150 W each is 750 W, plus 30 W for a router and switch, for a total of 780 W. Adding 25 percent gives about 975 W. With a power factor of 0.6, that needs roughly 975 ÷ 0.6 = 1,625 VA, so a 2 kVA unit is the sensible choice.

What to expect from backup time

The internal batteries of a standard office UPS are small. They are designed to bridge short outages and give you time to save work and shut down safely, typically a matter of minutes at a full load, not hours.

If you need longer, the usual answer is a larger external battery bank with an inverter or a UPS built for extended runtime. A rough estimate of backup time is:

Backup time (hours) ≈ battery energy in Wh × usable fraction × inverter efficiency ÷ load in watts.

For example, two 12 V 100 Ah batteries hold about 2,400 Wh. If you use about half of that to protect battery life, and the inverter is about 85 percent efficient, you have roughly 1,020 Wh available. At a 780 W load, that gives about 1.3 hours. Treat this as an estimate. Battery age, temperature, and load all change the real figure.

Which type of UPS?

• Offline (standby). The cheapest and simplest. It switches to battery when the power fails. Fine for a single home computer with stable mains.
• Line-interactive. Adds automatic voltage regulation (AVR), which corrects low and high voltage without draining the battery. This is the most useful type for most Pakistani offices, where voltage swings are common.
• Online (double conversion). Continuously powers your equipment from the inverter, giving the cleanest output. Best for servers, medical equipment, and critical systems, at a higher price.

What not to plug into a UPS

Laser printers draw a large surge when the fuser heats up, and will overload a UPS sized for computers. Air conditioners, heaters, kettles, and microwaves are also unsuitable. Keep these on a separate supply and protect only the equipment that needs clean, uninterrupted power.

Do not forget the network

A computer that stays on while the router and switch go down is of little use. Put network equipment, and any CCTV recorder, on the UPS as well, and include their wattage in your sizing.

Batteries and maintenance

• UPS batteries typically last a few years, and heat shortens their life. Replace them when the backup time drops noticeably.
• Keep the unit ventilated and off carpeted floors. Do not stack anything on top.
• Test the backup now and then by disconnecting mains power briefly while the load is light.

A buying checklist

• Rated watts stated clearly, not only VA.
• AVR for line-interactive units.
• Warranty terms for the unit and the battery separately.
• Availability of replacement batteries and local service.
• An output type suitable for your equipment: modern computers usually accept a simulated sine wave, while sensitive medical and audio equipment may need a pure sine wave.

UPS or voltage stabiliser?

A stabiliser corrects voltage swings but gives no backup when the power fails. A UPS with AVR does both for small electronic loads. For air conditioners, motors, and pumps, use a stabiliser rated for that specific load, and do not run them from a computer UPS.

Common mistakes

• Sizing from the VA figure on the box instead of the rated watts.
• Plugging in a laser printer, which overloads the UPS at start-up.
• Placing the unit in a hot, closed cupboard where the battery cooks.
• Ignoring battery age until the day the backup fails.
• Choosing on price without AVR, then wondering why the battery drains during voltage swings.
• Expecting hours of backup from the small internal batteries of a standard office UPS.

BST PVT LTD supplies UPS units, batteries, stabilisers, and networking accessories to offices and institutions across Karachi and Pakistan. Send us your list of devices and we will help you choose a UPS with the right capacity.