VA to Watts Converter — Free Online Calculator
Convert volt-amps (VA) to watts using power factor. Essential for UPS and generator sizing.
How to Use This Calculator
Enter VA and power factor. Watts = VA × PF.
The Formula Explained
VA (volt-amps) is apparent power. Watts is real power. Watts = VA × Power Factor. A UPS rated at 1000 VA with PF 0.8 delivers 800W of usable power.
VA and Watts: What Your Meter Is Actually Measuring
The distinction between volt-amps and watts comes down to one concept: power factor. Power factor is the cosine of the phase angle between voltage and current in an AC circuit. For purely resistive loads, voltage and current are in phase (0 degrees), cosine of 0 is 1.0, and VA equals watts. For purely inductive loads (an idealized motor), current lags voltage by 90 degrees, cosine is 0, and watts equal zero even with huge VA flowing. Real loads sit somewhere in between.
This matters because utility bills, equipment sizing, and safety calculations use different quantities. Residential bills are in kWh (real energy). Commercial bills add demand charges based on kVA (apparent power capacity). Wire and breaker sizing uses actual amps flowing (which corresponds to VA, not watts). If a 1,000-watt motor with 0.5 power factor is served by a 10-amp 120V circuit, the wire sees 1,000/0.5 = 2,000 VA worth of current, or 16.7 amps — enough to trip a 15A breaker even though useful power is only 1,000 watts.
Worked Example: UPS Sizing for a Workstation
A graphics workstation: 500W PC, 100W monitor, 50W peripherals = 650 watts total. All devices use modern power factor corrected (PFC) power supplies with PF approximately 0.95. Apparent power: 650 / 0.95 = 684 VA.
UPS selection: a 1000 VA / 600 W unit would not work — watts limit (600) is below the 650W load. A 1500 VA / 900 W unit works with margin. Runtime at full load: check manufacturer spec; typical is 5-10 minutes at 650 watts. For longer runtime, either buy a larger UPS or add external battery packs. Never assume runtime scales linearly — battery discharge curves are non-linear at high load.
Lesson: always check BOTH VA and watts ratings when sizing a UPS. Cheap UPS units advertise VA prominently (bigger number) but the watts rating is the real limit for modern PFC loads.
Worked Example: Motor Running Amps vs Nameplate Watts
A 3 HP motor nameplate shows: 3 HP, 230V, 9.6A FLA (full load amps), 0.82 PF, 87% efficiency. Real power (watts) calculation: 230V × 9.6A × 0.82 = 1,812 watts. Apparent power: 230 × 9.6 = 2,208 VA. Mechanical output: 3 HP × 746 = 2,238 watts (which is greater than 1,812 watts input, so something is wrong — let me recalculate).
Actually, the error is in my interpretation. Mechanical output is 2,238 watts, and electrical input is 2,238 / 0.87 = 2,572 watts. So the 9.6A FLA at 0.82 PF and 230V gives 230 × 9.6 × 0.82 = 1,812 watts, which does NOT match the expected 2,572 watts. The discrepancy means the nameplate amps or voltage assumes line-to-line 460V three-phase, not single-phase 230V. Always verify which voltage the nameplate values apply to.
Lesson: motor nameplates are complicated. The FLA value is what you size wire and breakers for directly. Do not try to derive it from HP; use the nameplate FLA number directly.
Five VA-Watts Misunderstandings
1. Assuming VA = watts on non-resistive loads. Transformers, motors, fluorescents, switching power supplies — all have power factor below 1.0. VA is always greater than or equal to watts.
2. Multiplying power factor in the wrong direction. Watts = VA × PF (multiply). VA = watts / PF (divide). A common error is dividing when you should multiply or vice versa.
3. Ignoring harmonic distortion. Non-linear loads (electronic ballasts, VFDs, LED drivers) create harmonic currents. The "distortion power factor" can be much lower than the "displacement power factor" typically quoted. Real power factor on a PC power supply might be 0.95 displacement but 0.65 true (including harmonics).
4. Summing watts across phases on three-phase loads. Three-phase power = sqrt(3) × V × I × PF for balanced loads. Summing the per-phase values directly double-counts current.
5. Using rated power factor at all loads. Motors have rated PF at full load but much lower PF at light load. A 10 HP motor running at 25% load might have PF 0.55 instead of the nameplate 0.85. Partial load operation kills power factor.
Power Factor Ranges for Common Equipment
Resistive loads (heaters, incandescent bulbs, electric ovens): PF = 1.0. Computer power supplies (PFC): 0.95-0.99. Computer power supplies (cheap, no PFC): 0.60-0.70. Induction motors at full load: 0.80-0.90. Induction motors at 25% load: 0.40-0.60. Fluorescent lamps with magnetic ballasts: 0.50-0.60. Fluorescent lamps with electronic ballasts: 0.90-0.98. LED drivers (good quality): 0.90-0.95. LED drivers (cheap): 0.50-0.70. Arc welders: 0.30-0.80 depending on type. Solar inverters: 0.95-1.00.
Power factor correction: adding capacitor banks cancels inductive reactance and improves PF. Industrial facilities typically target 0.95 PF to avoid utility penalty charges. The capacitor bank size in kVAR is calculated to bring PF from current value to target — this is a separate calculation using sine and cosine of the phase angles.
Standards and References
IEEE 1459 is the definitive standard for power quantity definitions under non-sinusoidal conditions, distinguishing between fundamental and harmonic power. IEC 60034 covers motor power factor ratings. ENERGY STAR computer requirements include PFC, mandating at least 0.9 PF for certified desktops.
NEC Article 460 covers capacitor installation for power factor correction. NEC 215.2(A)(1) specifies that feeder ampacity calculations use actual load amps (corresponding to VA), not watts. NEC 430.24 specifies motor circuit sizing using nameplate FLA, which already accounts for power factor.
VA to watts: real vs apparent power and the role of power factor
VA (volt-amperes) is apparent power, the product of RMS voltage and RMS current. Watts is real power, the energy that does useful work. For pure resistive loads they are equal. For motors, transformers, and switching power supplies they differ, sometimes significantly.
This conversion matters when sizing UPSes and transformers (rated in VA) for loads rated in watts. Undersizing a UPS by ignoring power factor is a common mistake.
The formula and what it does
Power factor is the cosine of the phase angle between voltage and current. PF = 1.0 means current and voltage are in phase (resistive). Below 1.0 means reactive load: some apparent power sloshes back and forth without doing work.
Worked example
Scenario: 1500 VA UPS loaded with computers and monitors at PF 0.7.
Real power available: 1500 x 0.7 = 1050 W. So a 1500 VA UPS supports 1050 W of real load, not 1500 W. If your equipment lists 1300 W total, the UPS is overloaded even though VA budget seems fine. This is why UPS specs include both VA and W ratings; pick the lower limit.
Common mistakes to avoid
undefinedFrequently asked questions
Why is my computer power supply rated higher in VA than W?
Switching power supplies without power-factor correction have PF around 0.6-0.7. A 600 VA supply delivers 360-420 W of real power. Modern supplies with active PFC reach PF 0.95+, narrowing the gap.
Do I size my UPS to VA or W?
Both. Pick the lower-rated limit. UPS specs are typically VA-rated, but if your load is more reactive than the UPS assumed, you can hit the watt limit first.
How is PF different from efficiency?
Efficiency is output power over input power. PF is real power over apparent power. A device can have 95 percent efficiency and 0.7 PF, or 80 percent efficiency and 1.0 PF, independently.
Can I correct a low PF?
Yes, with capacitor banks for inductive loads (motors). Common on industrial sites to avoid utility PF penalties. For residential, modern PFC supplies usually solve the problem at the device level.
What is the relationship between VA, W, and VAR?
It is the Pythagorean: VA squared = W squared + VAR squared. VAR is reactive power. So VA is the hypotenuse of the power triangle, with W and VAR as legs.
Why does my transformer say kVA, not kW?
Because transformer heat loss is driven by current (and therefore VA), not by what the load does with the energy. Transformers are sized to the apparent power they must pass.