VA vs Watts: How to Read a UPS Rating Correctly

The short version

Every UPS lists two capacity numbers, for example 1500 VA / 900 W. They describe two different limits of the same inverter:

• Watts is real power: the rate at which energy is actually used and turned into heat, light and computation. It is what your electricity meter bills and what drains the battery.
• VA (volt-amperes) is apparent power: the RMS voltage multiplied by the RMS current. It describes how much current the inverter's transistors, transformer and wiring have to carry.

When voltage and current rise and fall together perfectly, W and VA are equal. When they don't, the device draws more current than its real power would suggest, and VA comes out higher than W. The ratio between them is the power factor.

Why the two numbers differ

Two things push a load's power factor below 1.0:

Phase shift (displacement)

Motors and transformers store energy in magnetic fields, so their current lags the voltage. Part of the current sloshes back and forth without doing useful work. This is the classic power factor you learn in physics class, and it matters for fans, pumps and compressors.

Distorted current (harmonics)

Older electronics with simple rectifier power supplies only draw current in short spikes at the peak of each AC cycle. The current waveform looks nothing like the voltage sine wave, so RMS current is high relative to the real power delivered. A desktop power supply without power factor correction can have a power factor around 0.6 to 0.7 for this reason.

Modern computer power supplies, monitors, TVs and most network gear sold in the last decade use active power factor correction, which shapes the current into a near-sine wave. Their power factor is typically 0.95 to 0.99, so for them watts and VA are almost the same number.

What the UPS's own power factor means

The two numbers on the UPS label are maximums. Dividing them gives the UPS's rated output power factor:

This is why two "1500 VA" units can support very different amounts of equipment. The watt figure tells you how much real load the inverter and battery system are designed to deliver.

Worked example: a gaming PC and two monitors

Suppose a wattmeter (see how to measure power draw) shows these peak readings during a demanding game:

Against a 1000 VA / 600 W unit, the VA load is 63% but the watt load is 102%: overloaded. A 1500 VA / 900 W unit runs at 68% of its watt rating, which is acceptable for ride-through and a clean shutdown. A 1500 VA / 1000 W pure sine model runs at 61%, with comfortable headroom for transient spikes from the graphics card.

The UPS sizing calculator does this arithmetic for you and adds headroom automatically.

Why VA still matters (sometimes)

There are a few cases where the VA limit can bind before the watt limit:

• Old non-PFC power supplies and cheap wall adapters with PF around 0.5 to 0.65, in large numbers.
• Motor and pump loads such as a furnace blower or aquarium pump, whose displacement PF can be 0.6 to 0.8. These also draw a startup surge several times their running current.
• Laser printers and heaters are resistive (PF near 1.0) but enormous, which is why they never belong on battery outlets anyway.

If your load is mostly older or motor-driven gear, check both: the total watts must stay under the watt rating and the total VA under the VA rating.

How the ratings relate to runtime

Runtime depends on real power. Batteries store energy in watt-hours, and the inverter converts that to watts at the output with some loss (typically 85 to 95% efficient on battery). A 600 W load drains the battery at the same rate whether it presents as 610 VA or 900 VA, apart from slightly higher inverter losses at higher current.

That formula overestimates at high loads because lead-acid batteries deliver less of their capacity when discharged fast. See UPS runtime explained for why, or use the runtime calculator, which models this effect.

Reading a spec sheet: where to find the real number

1. Find "Output power capacity" or "Max configurable power". It is usually given as both W and VA. Use W.
2. Check whether the rating is the same on battery. A few units derate on battery or at low input voltage; the footnotes will say.
3. Check the battery-backed outlet count. Surge-only outlets don't count toward battery capacity, but on many units they also don't count toward the overload limit. On others, every outlet is behind the same limit.
4. Note the waveform. Active-PFC power supplies can misbehave on simulated sine wave output at higher loads; see pure vs simulated sine wave.