Watts to VA Converter

How to convert watts to VA

In AC systems, watts (W) measure real power (what actually does work), while volt‑amperes (VA) measure apparent power (voltage × current, ignoring phase angle). The link between them is the power factor (PF).

Single‑phase vs three‑phase (optional detail)

For many sizing tasks (UPS, inverter, transformer), you only need W, VA, and PF. If you also know voltage and current, you can relate them like this:

Worked examples

Example 1 – 500 W computer load to VA

Suppose your equipment draws 500 W at a power factor of 0.8.

1. Use the formula: \( \text{VA} = \dfrac{\text{W}}{\text{PF}} \)
2. Plug in the numbers: \( \text{VA} = \dfrac{500}{0.8} = 625 \,\text{VA} \)

So you should size your UPS or inverter for at least 625 VA (plus a safety margin).

Example 2 – 1500 VA UPS to watts

A UPS is rated at 1500 VA with a power factor of 0.6.

1. Use the formula: \( \text{W} = \text{VA} \times \text{PF} \)
2. Compute: \( \text{W} = 1500 \times 0.6 = 900 \,\text{W} \)

This UPS can safely supply about 900 W of real power.

Example 3 – Using voltage and current (single‑phase)

You measure 230 V, 3.0 A, and estimate PF ≈ 0.85.

• \( \text{VA} = V \times I = 230 \times 3.0 = 690 \,\text{VA} \)
• \( \text{W} = V \times I \times \text{PF} = 230 \times 3.0 \times 0.85 \approx 587 \,\text{W} \)

Typical power factor values

Real‑world loads rarely have PF = 1. Here are rough guidelines:

• Resistive loads (heaters, incandescent bulbs): PF ≈ 1.0
• Computers, office electronics, small SMPS: PF ≈ 0.6–0.9 (often corrected by PFC)
• Motors, compressors, pumps: PF ≈ 0.7–0.9 (lagging)
• LED drivers, CFLs: PF ≈ 0.5–0.9 depending on quality

When in doubt and sizing equipment, assume a lower PF (e.g., 0.8) to stay on the safe side.

Practical tips for UPS, inverter, and transformer sizing

• Always compare VA to VA: if your load is 800 W at PF 0.8, that’s 1000 VA. Choose a UPS with at least 1000 VA, preferably 1200–1500 VA.
• Leave headroom: add 20–30% above your calculated VA to handle startup surges and future expansion.
• Check both W and VA ratings: some devices advertise a high VA but a lower W rating—your real usable power is limited by the W rating.

FAQ

How do you convert watts to VA?

Divide the watts by the power factor: \( \text{VA} = \dfrac{\text{W}}{\text{PF}} \). Our calculator does this automatically when you enter W and PF.

Why are watts and VA different?

In AC circuits with reactive components (inductors, capacitors), current and voltage are not perfectly in phase. Watts measure the in‑phase component (real power), while VA measure the product of RMS voltage and current regardless of phase (apparent power). Because of this phase difference, VA is usually higher than W.

What power factor should I use if I don’t know it?

For mixed office or home electronics, PF between 0.6 and 0.8 is common. For motors and compressors, 0.7–0.9 is typical. For purely resistive loads, use 1.0. When sizing protective devices or backup power, it’s safer to assume a lower PF (e.g., 0.8).

Does system type (single‑phase vs three‑phase) change the W–VA conversion?

The relationship between W, VA, and PF is always \( \text{PF} = \dfrac{\text{W}}{\text{VA}} \). System type only changes how you compute W and VA from voltage and current (with or without the √3 factor).

Can this calculator be used for solar and inverter sizing?

Yes. Enter your load in watts and an estimated power factor to get VA, then compare that to your inverter or UPS VA rating. For solar inverters feeding household loads, PF is often close to 1, but individual appliances may have lower PF.