When you buy a diesel generator, the “KVA” rating on the box is measured under “Standard Reference Conditions” (typically at sea level and 25°C). However, South Africa’s unique geography, from the scorching heat of the Northern Cape to the high altitudes of Gauteng, means your generator might not actually deliver its full labelled power once installed on-site. Let’s take a look at how climate impacts the performance of Diesel Generators.
This phenomenon is called de-rating. Failing to account for it is one of the most common reasons for generator failure and “nuisance tripping” in South Africa.
The Science of Air Density
A diesel engine is essentially a massive air pump. To produce power, it needs to mix diesel with oxygen. The amount of power an engine can produce is directly proportional to the density of the air it breathes.
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Thin Air = Less Oxygen: As air becomes thinner (less dense), there is less oxygen available for the combustion process.
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The Result: To prevent the engine from smoking or overheating, the fuel system must reduce the amount of diesel injected. This results in a direct drop in the generator’s total kilowatt (kW) output.
READ THE STUDY: A Model for Estimation of De-rating in Diesel Engines used for Power Generation
Altitude De-rating (The Highveld Factor)
Altitude is the most significant factor for our customers in Johannesburg and Pretoria. Because these cities sit at roughly 1,500 to 1,700 metres above sea level, the air is significantly thinner than in Durban or Cape Town.
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The 1% Rule: According to the SANS 8528 standard, a naturally aspirated engine begins to de-rate almost immediately, while turbocharged engines (common in SDEC and Baudouin units) typically start to de-rate after 1,000 metres.
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The Calculation: For every 100 metres you climb above that 1,000m threshold, you should expect a 1% reduction in available power.
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Real-World Example: If you install a 100kVA generator in Randburg (approx. 1,600m), you have already lost roughly 6% of your capacity before you even turn it on.
Ambient Temperature De-rating
South African summers often see temperatures soaring above 35°C, especially in the interior. Like altitude, high heat makes air expand and become less dense.
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The 40°C Threshold: Most industrial alternators are designed to operate at an ambient temperature of up to 40°C. Once the air temperature exceeds this, the cooling system struggles to dissipate heat from the copper windings.
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The Penalty: For every 5°C increase above 40°C, the alternator capacity typically de-rates by an additional 3%.
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Compound Effect: On a 42°C day in the Limpopo bushveld, your generator is fighting both altitude and extreme heat, which can lead to a combined power loss of 15% or more.
Humidity and “Wet” Air
While humidity has a smaller impact on power output than altitude or heat, it remains a factor in coastal regions like KwaZulu-Natal.
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Displacing Oxygen: High humidity means there is more water vapour in the air, which displaces oxygen.
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Corrosion Risk: In coastal areas, the bigger “weather” threat isn’t de-rating—it’s salinity and moisture. Generator King ensures that units destined for the coast feature anti-condensation heaters and specialised varnish on the alternator windings to prevent oxidation and short circuits.
How to Protect Your Investment
To ensure your backup power South Africa solution doesn’t let you down on a hot Highveld afternoon, we recommend three strategic steps:
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Oversize by Design: Always size your generator with at least a 20% safety margin to account for de-rating factors. If your load test shows you need 80kVA, a 100kVA unit is the correct choice for Gauteng.
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Ensure Proper Ventilation: Never “box in” a generator. It needs a massive volume of cool air to function. Ensure air intakes are clear of debris and that hot exhaust air is directed away from the intake.
Site-Specific Power Solutions
A generator that works perfectly at the coast might struggle in the mountains. At Generator King, we don’t just sell “boxes” we specialise in engineering solutions that account for your specific South African climate.
Don’t get caught with an undersized unit. Contact Generator King today for a professional assessment or to Get a Quote. We will calculate the exact de-rating factors for your location to ensure your SDEC, Baudouin or FAW generator delivers 100% of the power you need.
By the Generator King Technical Team
