VA and Watts: Two Numbers That Are Not the Same
Your UPS says 1000VA on the front panel. Does that mean it can power your 800W server? Not necessarily. The answer depends on understanding two distinct electrical measurements that are often confused.
Watts measure "real power" — the actual energy a device consumes to do useful work such as generating heat, light, or mechanical motion. One watt equals one joule of energy per second.
VA (volt-amperes) measures "apparent power," the total electrical power supplied to a circuit. It is calculated by multiplying RMS voltage by RMS current. VA is always equal to or greater than watts; it is never less.
This distinction only matters in AC circuits. In DC systems, such as battery-powered devices and mobility scooter chargers, watts and VA are always equal because there is no reactive component.
This guide applies equally to IT managers sizing a server room UPS, NHS procurement officers specifying backup power, and individual consumers replacing a home battery.
The Power Triangle: Watts, VA, and Reactive Power Explained
The relationship between watts and VA is easiest to understand through the power triangle. It has three sides: real power (measured in watts), apparent power (measured in VA), and reactive power (measured in VAR, or volt-amperes reactive).
Reactive power performs no useful work. However, inductive and capacitive loads, such as motors, transformers, and UPS inverters, need it to establish and sustain magnetic fields. It is the overhead of running AC equipment: energy that cycles back and forth between the supply and the load without contributing to useful output.
The link between watts and VA is the Power Factor (PF):
Watts = VA × Power Factor
Power factor is a number between 0 and 1. A power factor of 1.0 means watts and VA are identical. This applies to purely resistive loads like electric heaters, toasters, and incandescent light bulbs, where all the supplied power is converted directly into useful work.
Here is a concrete example using UK supply values. The UK single-phase AC supply runs at 230V and 50Hz. A device drawing 1.09A on a 230V supply produces 250VA of apparent power (1.09 × 230 = 250.7VA). If that device has a power factor of 0.8, the real power consumed is only 200W (250 × 0.8 = 200). The remaining energy is reactive power, cycling through the circuit without contributing to useful output.
Whenever you calculate VA for UK equipment, use 230V as your voltage figure. Multiply the amps drawn by 230, and you have the apparent power in VA.
Real-World Power Factors: From Modern Servers to Legacy Equipment
Not all equipment behaves the same way. Power Factor Corrected (PFC) power supplies, which became standard in computing equipment manufactured after approximately 1996, achieve a power factor of 0.99 to 1.0. For modern servers, routers, and switches with PFC supplies, watts and VA are effectively equal, which simplifies UPS sizing considerably for up-to-date IT infrastructure.
Older capacitor-input power supplies tell a different story. Legacy equipment commonly has a power factor between 0.65 and 0.85, meaning the VA rating is significantly higher than the watt rating. A real-world measurement illustrates this well: a typical desktop PC with monitor drew 230W of real power but 365VA of apparent power, giving a power factor of roughly 0.63. That gap matters when sizing backup power.
Here are some typical power factors by load type:
- Resistive loads (heaters, incandescent bulbs): PF = 1.0
- Modern PFC computing equipment: PF ≈ 0.9 to 0.99
- Motors: PF ≈ 0.8
- Legacy equipment and uncompensated heavy loads: PF = 0.65 to 0.7
The real challenge arises in mixed-load environments. A facility running modern IT gear alongside older motor-driven equipment or legacy PCs will have a combined power factor that sits somewhere in the middle. Typical appliances have a power factor of 0.6 to 0.8, meaning their VA consumption is 25 to 60% greater than their wattage rating. This is one of the most frequently overlooked factors in UPS sizing.
Why UPS Systems Are Rated in VA, and How to Size One Correctly
UPS systems are rated in VA because the fundamental limiting factor of a UPS output is its output current. Current relates directly to VA rather than watts. This is why every UPS carries a VA rating prominently on its specification sheet.
However, a UPS also has a maximum watt rating. Neither the VA limit nor the watt limit may be exceeded by the connected load. Both figures matter, and ignoring either can lead to overload shutdowns at the worst possible moment.
Consider this example: a 1000VA UPS connected to equipment with a power factor of 0.6 delivers only 600W of real power. Connect equipment with a power factor of 0.9 to the same UPS, and it delivers 900W. The same VA rating supports very different watt loads depending on what is plugged in.
The 1.2× Rule
Best practice is to choose a UPS with a VA rating at least 20 to 30% higher than the total load. This headroom accounts for future growth, measurement tolerances, and the startup spikes that occur during normal operation.
Inrush Current
Motor-driven devices such as laser printers can draw up to three times their rated power momentarily at startup. If your UPS is sized to the exact load with no margin, that inrush current can trip the overload protection. Always factor startup surges into your calculations.
Scale and Market Context
UPS systems range from 300VA units for home use up to 5,000kVA installations for data centres. Online UPS systems represented 54% of global installations in 2025 as enterprises prioritised power stability and voltage regulation. The UK UPS market alone is projected to reach approximately £2.5 billion by 2033, driven by data centre expansion, edge computing, and growing reliance on electronic systems across healthcare, education, and government.
Restoring Capacity Without Replacing the Unit
For customers whose APC UPS is ageing or no longer holding charge, a full battery swap can restore the original VA and watt capacity at a fraction of the cost of a new unit. hardwarexpress stocks a comprehensive range of UPS Battery Replacement Kits for APC, covering models including the APC Back-UPS ES 700, APC Smart-UPS 750, Back-UPS ES 400, RBC6, and RBC48.
VA vs Watts Beyond UPS: Inverters, Chargers, and Emergency Systems
The VA vs watts distinction extends well beyond UPS systems. It applies equally to power inverters, emergency lighting systems, and alarm panel backup batteries.
In DC systems, including battery-powered devices and most mobility scooter chargers, watts and VA are always equal. There is no reactive component in a DC circuit, so buyers in this space do not need to consider power factor at all.
For AC-output inverters, used in vehicles or off-grid setups, the distinction absolutely applies. The inverter's VA rating must exceed the apparent power of all connected loads, not just their watt rating. Undersizing an inverter based on watts alone can cause it to shut down under load.
Emergency lighting and alarm panel backup systems are AC loads and must be sized using VA, not just watts, to ensure the battery can supply sufficient current during a mains failure.
One further point worth clarifying: VA and watts measure the rate of power delivery, while watt-hours (Wh) measure energy stored over time. Watt-hours determine how long a battery will last; VA and watts determine how much load it can handle at any given moment. These are separate figures that serve different purposes.
Quick Reference: VA vs Watts at a Glance
- Watts (W): Real power. The energy actually consumed to do useful work.
- VA (volt-amperes): Apparent power. The total power supplied to the circuit.
- When they are equal: DC circuits, and AC circuits with purely resistive loads (PF = 1.0).
- When they differ: AC circuits with inductive or capacitive loads (PF < 1.0).
- Which to use for sizing: Always size backup power (UPS, inverters) using VA, then verify the watt rating is also sufficient.
The formula: Watts = VA × Power Factor. If the power factor is unknown, assume 0.8 as a conservative estimate for mixed loads.
For any UK AC equipment, VA is calculated by multiplying the amps drawn by 230V. That single step gives facilities managers and trade buyers the figure they need.
Three load categories to remember:
- Resistive loads (heaters, incandescent bulbs): PF = 1.0, watts = VA.
- Modern PFC computing equipment (post-1996 servers, switches, routers): PF ≈ 0.99, watts ≈ VA.
- Motors and legacy equipment: PF = 0.65 to 0.85, VA is significantly higher than watts.
If you are unsure whether your UPS, inverter, or backup power system is correctly sized for your load, hardwarexpress has been helping UK trade and public customers find the right power solutions since 2004. With high stock levels, same-day shipping on most products, and expert support available by phone or in-store at our Coventry location, getting the right answer is straightforward.