About

When sizing backup power systems like Uninterruptible Power Supplies (UPS) or generators, a common pitfall is confusing Watts with Volt-Amps (VA). Watts represent the "Real Power" actually used by the equipment to perform work, while VA represents the "Apparent Power" that the wiring and source must be capable of delivering.

Because no electrical system is 100% efficient, the Apparent Power (VA) is always equal to or higher than the Real Power (Watts). This discrepancy is governed by the Power Factor. This tool converts your equipment's Wattage rating into the required VA rating, ensuring you don't buy an undersized generator or UPS that fails immediately under load.

Formulas

The relationship between Real Power and Apparent Power is defined by the Power Factor ratio.

S_(VA) = P_(W)PF

Where:

S = Apparent Power (Volt-Amps).
P = Real Power (Watts).
PF = Power Factor (0.1 to 1.0).

Reference Data

Equipment Type	Typical Power Factor	Load (Watts)	Required Capacity (VA)
Resistive Heater	1.0	1000	1000
Modern Server (PFC)	0.95	500	526
Desktop Computer	0.8	300	375
Air Conditioner	0.75	2000	2666
Refrigerator	0.7	200	285
Welding Machine	0.6	4000	6666

Frequently Asked Questions

VA accounts for energy that is "lost" or circulated in the circuit due to inefficiency (reactance). If you have a 400W device with a Power Factor of 0.8, the power source actually feels a load of 500VA.

For modern computers with Active PFC power supplies, 0.9 to 0.95 is accurate. For older computers or generic peripherals, 0.7 to 0.8 is a safer margin to ensure the UPS is large enough.

No. UPS marketing often lists VA in big bold numbers (e.g., '1500VA UPS') but the Wattage limit might be lower (e.g., 900W). You must check both limits. This tool helps you find the VA number to match your Wattage load.

You risk overloading your power source. A generator might stall, or a UPS might trigger an overload alarm and shut down immediately, defeating its purpose.