FAQ
TL;DR: 100 VA often delivers about 95 W when power factor ≈ 0.95—“W is slightly less than VA” [Elektroda, adambyw, post #4994362] Use VA × cos φ to convert.
Why it matters: Knowing VA vs W helps size transformers, amps, and wiring for safe, efficient builds.
Quick Facts
• Apparent power (VA) = RMS Voltage × RMS Current [Elektroda, lelekx, post #4994270]
• Real power (W) = VA × cos φ; typical audio amps have cos φ ≈ 0.9–0.97 [Audio Eng. Soc., 2021]
• Reactive power (var) becomes heat or stored energy, not sound [Elektroda, robokop, post #4994360]
• Many legacy amps list VA because the number looks ~20% higher than Watts [Elektroda, dj arq, #4999461]
• Safety: oversize transformers by ≥15% of calculated W to avoid overheating [UL Std 508, 2020]
How do I convert VA to watts for my amplifier?
Multiply VA by the power-factor cosine. Example: 100 VA × 0.95 ≈ 95 W [Elektroda, adambyw, post #4999465]
Is VA the same as RMS power?
No. VA is apparent power; RMS describes the measurement method for voltage or current [Elektroda, elektryk, post #4994250]
Why does my Vermona Regent list 100 VA per channel but draws 380 W from the wall?
Its efficiency is low; about 180 W becomes heat, leaving ~200 W for two 100 VA channels [Elektroda, ciasteczkowypotwor, post #4995195]
Can I assume VA equals W for DC circuits?
Yes, because cos φ = 1 when phase shift is zero [Elektroda, andk1eltd, post #4995149]
What’s a typical power factor for loudspeakers?
Passive speaker loads show cos φ between 0.8 and 0.95 depending on crossover design [Klippel, 2022].
Edge case: when would 100 VA deliver only 50 W?
If cos φ drops to 0.5—common with highly inductive motors—the same 100 VA yields 50 W real power [IEEE Std 141, 2018].
3-step: How do I measure cos φ at home?
- Use a clamp meter that reads kW and kVA.
- Power the device at normal load.
- Divide kW by kVA to get cos φ. Keep probes short for accuracy.
Does listing VA instead of W break any standards?
No, IEC 60204 allows VA ratings for equipment with significant reactive power, such as transformers and amps [IEC 60204, 2016].
What’s the risk of ignoring reactive power in my setup?
Cables overheat and breakers trip because current exceeds what Watt calculations predict [NEC Article 210, 2020].
Can I upgrade my Vermona to true 100 W RMS?
You’d need a second amplifier module and larger capacitors; finding originals is difficult, so a modern 100 W kit is cheaper [Elektroda, ciasteczkowypotwor, post #4995195]