How to Calculate the kVA Required for a Generator

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Choosing the right generator involves much more than just looking at the brand or the price. One of the most important aspects is knowing how to calculate the kVA of a generator to ensure it will meet all your energy needs without oversizing the unit or compromising its performance.

At Genesal Energy, we specialise in the correct dimensioning of generator sets and in designing tailor-made solutions for each client.

This value represents the generator’s power, expressed in kilovolt-amperes (kVA). In this article, we explain step by step how to carry out the calculation, which factors to consider, and how to apply a safety margin.

Key Factors to Determine a Generator’s Power

Before going into formulas, it is essential to understand the elements that directly influence the kVA calculation for a generator. The main ones include:

  • Type of electrical consumption: it is not the same to power office equipment as industrial machinery.
  • Intended use: whether the generator will serve as the main power source or as backup.
  • Number and type of electrical devices connected: each appliance has different power requirements.
  • Starting conditions: some equipment requires start-up peaks much higher than their constant consumption.
  • Load sequence: in certain facilities, it may be advisable to prioritise loads by connecting them in stages.

By analysing these factors, you can calculate the power of a generator with greater accuracy.

Difference Between kVA and kW in a Generator

A common mistake when calculating the power of a generator is confusing kVA (kilovolt-amperes) with kW (kilowatts).

  • kVA expresses the apparent power of the generator.
  • kW indicates the actual power consumed by the connected electrical devices.

The relationship between these values is defined by the power factor (cos φ). In most installations, it is common to use a factor of 0.8, meaning that a 100 kVA generator can deliver around 80 kW of useful power.
It is also important to note that apparent powers cannot simply be added together, as each load may operate with a different power factor. Instead, the real powers in kW must be added first and then converted into kVA.
This distinction is typical of alternating current circuits. In direct current, the power factor is 1, and the real and apparent powers coincide.
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How to Calculate Power Based on Electrical Consumption

To calculate generator kVA, the starting point is the total power of all the electrical devices to be connected. This information can be found on each device’s nameplate or in its manual.
The basic procedure is:

  • 1. Add up the power ratings in kW of all the equipment.
  • 2. Apply usage or simultaneity factors, if necessary, to reflect a realistic scenario.
  • 3. Convert to kVA using the formula: kVA = kW / power factor
  • 4. Round up to the next value to ensure the generator does not operate at 100% of its capacity.

In this way, you can calculate the required generator kVA reliably and safely.

The Importance of Power Factor in kVA Calculation

The power factor is essential to convert kW into kVA. As mentioned, the usual reference value is 0.8, but it may vary depending on the type of load:

  • With electric motors, the power factor may be lower.
  • With modern electronic devices, it may approach 1.

Failing to account for this can lead to errors in sizing and selecting an undersized generator. It is always advisable to confirm this value with a specialist before choosing the equipment.
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Considerations on Start-Up Peaks and Constant Power

Many electrical devices, particularly motors, pumps, and HVAC systems, generate start-up peaks when switched on. These peaks can be two to three times higher than their rated power.

For example, a motor with a rated power of 35 kW may require more than 70 kVA at start-up.
There are two common ways to compensate for these peaks:

  • Oversizing the generator’s alternator.
  • Incorporating frequency converters or other auxiliary equipment to soften the initial demand.

How to Apply a Safety Margin When Choosing a Generator

Once the required kVA has been calculated, it is advisable to apply a safety margin. This prevents the generator from always working at its limit, extends its service life, and reduces fuel consumption.

One of the most important aspects is knowing how to calculate the kVA of a generator to ensure it will meet all your energy needs.

In general, a margin of 20–25% above the initial calculation is recommended. For example, if the result is 100 kVA, the most appropriate choice would be a 120–125 kVA generator.

Practical Example of kVA Calculation for Different Loads

Let’s suppose a facility requires a generator with the following loads:

  • Lighting and office equipment: 15 kW
  • Air conditioning: 20 kW
  • Electric motors: 30 kW
  • 1. Sum of real power: P=15+20+30=65 kW
  • 2. Apply the power factor (0.8): S=P/cosϕ=65/0,8=81,25 kVA
  • 3. Consider start-up peaks: this value may rise to around 100 kVA.
  • 4. Apply a safety margin (+25%): 100×1,25=125 kVA

In this case, the correct option would be a 125 kVA generator, ensuring it can cover both constant power and start-up peaks without compromising performance.
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Conclusion

Understanding how to calculate the kVA of a generator is essential to choose the right equipment and avoid supply issues. Remember:

  • Differentiate between kW and kVA.
  • Always consider the power factor.
  • Account for start-up peaks, not just constant power.
  • Always apply a safety margin.

Correct sizing guarantees that the generator’s power matches the real needs of the installation, optimising performance and ensuring reliability.

Understanding how to calculate the kVA of a generator is essential to choose the right equipment and avoid supply issues.

At Genesal Energy, we specialise in the correct dimensioning of generator sets and in designing tailor-made solutions for each client. If you need advice on how to calculate kVA for purchasing a generator, our technical team can help you find the best option.

Tailored Power for a Shopping Centre in Germany

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Genesal Energy understands that every client is unique and when it comes to securing power supply in critical facilities, there’s no such thing as a one-size-fits-all solution.

A key part of our work lies in listening carefully, understanding each project in depth, and designing fully customised gensets capable of meeting even the most specific requirements.

This was the case in one of our most recent projects: the design and manufacture of a generator set for a shopping centre in Germany. The client presented us with a highly specific set of technical requirements. It was an ambitious challenge, as they needed a reliable emergency power solution with clearly defined criteria regarding autonomy, aesthetics, corporate branding and weight restrictions.

The client required a high-autonomy genset, connected to a 3,000-litre belly tank fully integrated into the base frame, a design that enables the installation to operate for extended periods. Additionally, there was a strict total weight limit of 8.5 tonnes, including the fuel tank, which meant we had to completely redesign the canopy and manufacture it in aluminium to reduce weight without compromising strength or performance.

A full redesign of the canopy, which was manufactured in aluminum to reduce weight.

Accordingly, the genset was customised both functionally and aesthetically. It was painted in the client requested RAL and featured special corporate branding. From a technical perspective, the set includes an oversized alternator to ensure optimal performance under demanding conditions, a ComAp InteliGen4 200 control panel, animal ingress protection mesh, and a fully covered power section to eliminate access to live parts, among other specifications.

This project is a clear example of what we do at Genesal Energy: delivering power solutions that combine innovation, flexibility, and deep technical expertise. Whether the challenge lies in the design, available space, local regulations or visual integration, our team is always ready to respond with the best solution.

The Engineering Solution

A full redesign of the canopy, which was manufactured in aluminum to reduce weight. This ensured the final equipment – including the full 3,000-litre tank – remained under the specified weight limit.

Main Features

  • ComAp InteliGen4 200 control panel.
  • 3,000-litre belly tank integrated into base frame.
  • Spill containment tray with leak detection sensor.
  • Animal protection mesh at air inlet and outlet.
  • Special client-specific branding.
  • Painted in RAL 7035 (client’s specified colour).
  • Manual oil extraction pump.
  • Coolant recirculation heating pump.
  • Fully covered power section to prevent contact with live components.
  • Oversized alternator.

Backup power in critical healthcare environments: we designed a tailored emergency solution for a proton therapy centre

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Genesal Energy designed and manufactured a soundproofed generator set specifically adapted for a proton therapy centre located in Santiago de Compostela (Spain).

This emergency power solution delivers 550 kVA to ensure the uninterrupted operation of high-precision medical equipment in the event of a grid failure.

These types of centres, dedicated to oncology treatments using protons, require a continuous power supply, as any interruption can not only compromise the effectiveness of the treatment but also put patient safety at risk. In addition, international regulations on advanced radiotherapy impose the mandatory requirement of having backup energy systems in place.

Safety, precision, and autonomy

Genesal Energy’s engineering department developed a GEN550YI generator set housed in a 4,500mm soundproofed enclosure, providing quiet operation and reliable performance. The generator starts up automatically in the event of a power outage, supplying the necessary energy to keep facilities and medical equipment running.

A dedicated exhaust system was also designed to minimise noise emissions, in compliance with current noise regulations.

Alongside reliability, the project required a solution with high operational autonomy. Two fuel tanks were included: an 800-litre base frame tank and an external double-walled tank with a 2,000-litre capacity. Both are connected through an automatic transfer system equipped with level sensors, a transfer pump, a cut-off solenoid valve, and a fuel filter, thus ensuring a continuous and safe supply.

A robust and silent design

The solution also incorporates a redundant start-up system with batteries in parallel to ensure the generator remains operational at all times. A dedicated exhaust system was also designed to minimise noise emissions, in compliance with current noise regulations.

The unit was manufactured using state-of-the-art technology, integrating a monoblock engine–alternator system with flexible coupling, which enhances overall reliability and reduces vibrations. Thanks to this bespoke design, the proton therapy centre in Santiago now benefits from a reliable, quiet, and long-lasting backup system that protects sensitive and costly equipment, guarantees the continuity of advanced medical treatments, and complies with the highest technical and regulatory standards in the healthcare sector.

Technical features

  • Construction type: Monoblock engine–alternator in 4,500mm soundproofed enclosure.
  • Flexible coupling between engine and alternator.
  • 800-litre integrated base frame tank + 2,000-litre external double-walled tank.
  • Automatic fuel transfer system with pumps and level sensors.
  • Oversized silencer model (-30 dB).
  • Redundant battery & start system.
  • Bespoke cooling air intake and hot air exhaust.