VA to Watts Calculator
How the formula works
VA is apparent power and watts is real power. They are linked by the power factor.
Formulas
Watts = VA × Power factor
VA = Watts / Power factor
Reactive power (VAr) = √(VA² − Watts²)
kVA = VA / 1,000 and kW = Watts / 1,000
Single phase current = VA / Voltage
Three phase current = VA / (√3 × Voltage)
Where:
- VA = volt-amperes (apparent power)
- W = watts (real power that does useful work)
- Power factor = real power divided by apparent power, from 0 to 1
Typical values
Resistive loads such as heaters and incandescent lamps: 1.0. Modern UPS: 0.9. Induction motors and generators: 0.8. Older UPS units: 0.6 to 0.7. These are typical values. The equipment nameplate and data sheet are final. Use the Custom option to enter your own power factor.
How to use the calculator: select the calculation, enter the value, choose the unit and the power factor, then click Calculate. Add the supply and voltage if you also want the current.
Important notes: a UPS is rated in VA, but the load it can carry is in watts. Always check both ratings.
Worked example
Example 1: VA to watts
1,500 VA at power factor 0.9.
Watts = 1,500 × 0.9 = 1,350 W (1.35 kW). Reactive power = 653.83 VAr.
Example 2: Older UPS
650 VA at power factor 0.6.
Watts = 650 × 0.6 = 390 W. Reactive power = 520 VAr.
Example 3: Watts to VA with current
750 W at power factor 0.8, single phase 230 V.
VA = 750 / 0.8 = 937.5 VA. Current = 937.5 / 230 = 4.08 A. Reactive power = 562.5 VAr.
Example 4: kVA to kW, three phase
100 kVA at power factor 0.8, three phase 415 V.
kW = 100 × 0.8 = 80 kW. Reactive power = 60 kVAr. Current = 100,000 / (1.732 × 415) = 139.12 A.
Example 5: kW to kVA, three phase
2.5 kW at power factor 0.95, three phase 400 V.
kVA = 2.5 / 0.95 = 2.632 kVA (2,631.58 VA). Reactive power = 821.71 VAr. Current = 3.80 A.
Common mistakes
- Treating VA and watts as the same. They are equal only at power factor 1.
- Choosing a UPS by its VA rating alone, without checking its watt rating.
- Using a power factor of 1 for motors, which gives too low a VA.
- Entering kVA in the VA field, or kW in the watts field.
- Using the wrong voltage for three phase. Use the line to line voltage.
- Forgetting that a UPS runs best at 70 to 80 % of its rated load.
- Ignoring the inrush current of motors and compressors.
- Using the power factor of the UPS instead of the power factor of the load.
- Rounding too early, which causes errors in large loads.
Frequently asked questions
How do I convert VA to watts?
Multiply VA by the power factor. Watts = VA × power factor.
How do I convert watts to VA?
Divide watts by the power factor. VA = watts / power factor.
What is the difference between VA and watts?
VA is apparent power. Watts is real power. Watts equals VA only when the power factor is 1.
How many watts is 1000 VA?
At power factor 0.8 it is 800 W. At 0.9 it is 900 W. At 1.0 it is 1,000 W.
How many watts is a 650 VA UPS?
At power factor 0.6 it is about 390 W. At 0.9 it is about 585 W. Check the UPS data sheet.
What power factor should I use for a UPS?
Use the value on the data sheet. Modern UPS units are often 0.9 and older ones 0.6 to 0.7.
How do I convert kVA to kW?
Multiply kVA by the power factor. For 100 kVA at 0.8 the result is 80 kW.
What is VAr?
VAr is reactive power. It is the part of VA that does not do useful work and is calculated as the square root of (VA squared minus watts squared).
Why is the power factor never above 1?
Real power can never be more than apparent power, so the ratio is always 1 or less.
How do I find the current from VA?
For single phase, divide VA by the voltage. For three phase, divide VA by (√3 × voltage).
How do I convert amps to kW?
Use the Amps to kW Calculator for single phase, three phase and DC.
Is this a final design value?
No. It is an estimate. Check the equipment nameplate and local code.