UPS sizing:
VA against watts, and minutes from the battery

Why a UPS has two power numbers

Look at the label of a UPS and you will find a rating in VA, such as 1000 VA, and often a smaller number in watts. VA means volt-amperes, the product of voltage and current drawn. Watts are the real power that does useful work. The two differ by the power factor, which describes how much of the current is doing work.

Appliances list their consumption in watts, and batteries store energy in watt-hours. The UPS rating is the odd one out, so every sizing exercise begins by converting between VA and watts using the unit's power factor.

Always use the watt rating of your own unit, from its label or datasheet. The power factor is a property of the UPS design, not a universal constant.

Sizing the UPS for a load

Add up the running power of everything that will be plugged into it. Leave out laser printers and anything with a heating element, which draw large currents and are better kept off a UPS.

The headroom matters. Running a UPS near its limit shortens the battery's life, causes overload alarms when the computer spins up, and leaves no room to add a device later.

Many people are caught out by a UPS rated in VA that is advertised as if it were watts. A 1,000 VA box that supplies only 600 W is not misleading, but it is easy to misread. If the label gives only a VA rating, ask for the power factor or the watt rating before buying.

Backup time from the battery

The result is an optimistic upper bound for an ageing battery. Lead-acid capacity falls as the battery ages and when the discharge is fast, so a battery with a year or two behind it may deliver noticeably less than the label says.

To go the other way, decide how long you need power and size the battery from it: multiply the load by the hours wanted, and divide by the usable fraction and the efficiency. A 310 W setup that must last 4 hours needs 310 × 4 ÷ (0.8 × 0.85) = 1,824 Wh, which is about 152 Ah at 12 V, so one 150 Ah battery or two 100 Ah batteries in parallel.

What changes the backup time in practice

• Load: halving the load more than halves the draw on the battery, since the battery's usable capacity rises at lower currents.
• Battery age and temperature: both cut capacity, and heat shortens life.
• Battery chemistry: lithium batteries allow deeper discharge and hold capacity better than lead-acid, but cost more.
• Inverter efficiency: it drops at low load, so a large UPS carrying a tiny load wastes a share of the energy.
• Series or parallel connection: two 12 V batteries in series double the voltage, and in parallel double the amp-hours. Energy in Wh adds in either case, but the UPS must be designed for the voltage you choose.

A UPS that is regularly cycled deeply wears its battery faster. A typical sealed lead-acid battery lasts three to five years in standby use, sooner in hot rooms. Replacing the battery at that point costs far less than replacing the unit.

Backup time for common loads on one battery

The same 1,200 Wh battery, at 80% usable and 85% efficiency, delivers 816 Wh to the load. How long that lasts is simply 816 divided by the watts drawn.

The figures show why doubling the load halves the time, and why a small, essential load such as a router and a light can run through a long cut on a battery that would only keep a desktop going for a couple of hours. In practice a heavy load also lowers the battery's effective capacity, so the real times at the top of the table will be somewhat lower.

Line-interactive, online and inverter systems

Small line-interactive UPS units pass mains power through and switch to the battery when it fails, with a switching time of a few milliseconds. They suit computers and routers. Online (double conversion) units run the load from the inverter at all times, give a clean output and no break, and are used for servers and critical equipment, at a higher price.

Home inverters with a separate battery are designed for longer backup of fans and lights, and some have a switch-over time long enough to reboot a desktop. If you will run a computer from one, check the stated changeover time or use a UPS in front of the computer.

Whatever the type, a UPS should be tested by a mains-off trial every few months. Pull the plug with the load connected, confirm it carries the load for the time you expect, and note how long it takes the battery to recharge afterwards.