How to Choose the Right Alternator Power Rating (Sorilux Technical Guide 2025)

Choosing the right alternator output doesn't just prevent electrical problems: prevents breakdowns, mechanical overloads, overheating, and loss of performance.
At Sorilux, with over 40 years of experience working with generator sets and spare parts, we often see two very common mistakes:

1. Confusing kVA with kW.
2. Failing to properly size the drive motor that will power the alternator.

In this guide, we’ll explain in simple terms which alternator you need, how to calculate its output power, and which engine it should be paired with to ensure it works properly.

🔌1. First things first: understanding the relationship between kVA and kW

Alternators are classified by kVA, but the “actual” power you use is in kW.
The relate between the two is:

kW = kVA × cos φ

This means:

• In three-phase alternators, the industry standard is cos φ = 0,8.
• In single-phase alternators, it is generally considered cos φ ≈ 1, since loads typically have less phase shift.

Quick examples

• 30 kVA three-phase alternator → 30 × 0.8 = 24 kW useful.
• 30 kVA single-phase alternator → 30 × 1 = 30 kW useful.

This difference is crucial when calculating the power you actually need.

⚙️ 2. Alternator performance: nothing ever runs at 100%

A typical alternator has an efficiency of around 90%.
That means that To deliver 24 kW of electrical power, it needs to receive more mechanical power at the shaft.

🔥 3. Key rule: the engine (or driving force) must be 20% more powerful

It doesn't matter whether you're going to drive the alternator with:

• a electric motor,
• a diesel engine,
• a hydraulic motor,
• or any mechanical drive system…

The driving force must always have at least 20% more power than the calculated mechanical power.
Reason?

• Friction
• Ventilation
• Heat loss
• Load variations
• Inductive spikes
• Operational safety

Summary of the final formula:

Motor power (kW) = kVA × cosφ × 1.33

1.33 = This result is obtained by dividing the 20% safety margin (1.2) by the alternator's estimated efficiency of 90% (0.9).

📐 4. Real-life example: Sorilux

Let's say you need an alternator 30 kVA three-phase:

Motor power (kW) = kVA × cosφ × 1.33
Motor power (kW) = 30 × 0.8 × 1.33
Engine power (kW) = 32 kW

👉 Conclusion:

You cannot connect a 24 kW or 26 kW motor to a 30 kVA three-phase alternator.
it need at least 32 kW, and if you're going to be working with heavy loads, it's better 35–36 kW.

📊 5. Handy chart: recommended power levels

Alternadores TRIFÁSICOS (cos φ = 0,8)

SINGLE-PHASE Alternators (cos φ ≈ 1)

Just change the “Effective kW” column (kVA = kW).
The calculation for the engine is the same: output + 20%.

🧲 6. Bonus: AVR control is a must

Since you're choosing a power rating, keep this in mind:

• Alternators with compounds no longer make sense today.
• The entire industry uses almost exclusively alternators with AVR, for stability, precision, and reliability.

If you're looking for something modern and hassle-free, Choose AVR.

✅ Conclusion

To choose the right alternator output, you need to be clear on three things:

1. How to convert kVA to kW depending on whether it is three-phase or single-phase.
2. Use the actual output of the alternator.
3. Size the motor with a minimum margin of 20%, always.