When organisations invest in a diesel generator, the instinct is often to err on the side of caution and specify a larger unit than the calculated load demands. On the surface, this appears to be a prudent approach - more capacity means more headroom, and more headroom feels like more security. In practice, however, oversizing a generator introduces a specific and often misunderstood problem: wet stacking.
Understanding wet stacking, what causes it, how to identify it, and how to prevent it through accurate generator specification is essential for any organisation that depends on reliable backup or prime power generation.
Wet stacking occurs when a diesel engine is operated consistently at low load levels - typically below 30% of its rated output. Under these conditions, the engine does not reach its optimal operating temperature. Combustion is incomplete, and unburnt fuel, carbon deposits and engine oils accumulate within the exhaust system, turbocharger and other internal components.
The term refers to the wet, dark residue that builds up within the exhaust stack as a visible symptom of this ongoing problem. Left unaddressed, wet stacking accelerates engine wear, reduces performance and can significantly shorten the operational lifespan of what is otherwise a major capital asset.
A generator that is too large for the electrical loads it serves will spend most of its operating life running at a fraction of its rated capacity. This is particularly relevant for standby generators that are tested regularly but rarely operate under full demand conditions.
Consider a facility with a genuine peak electrical demand of 150 kVA. If a 400 kVA generator is installed - perhaps to accommodate a theoretical future expansion or simply because it was available at the right price - the unit will routinely operate at or below 40% of its rated load during testing and real events. At those load levels, wet stacking becomes a persistent risk rather than an occasional concern.
The consequences extend beyond the exhaust system. Incomplete combustion affects injectors, piston rings and cylinder liners. Maintenance intervals may need to be shortened. Fuel consumption per unit of output rises. The generator that was intended to provide reassurance instead becomes a source of ongoing maintenance cost and reliability concern.
The most visible sign is dark, oily residue around exhaust outlets and connections. Excessive black smoke during operation, a noticeable drop in engine performance, increased oil consumption and elevated fuel use relative to output are all indicative symptoms.
In some cases, facilities managers only become aware of the problem when the generator is required to perform during an actual power interruption and delivers compromised output. At that point, the consequences can extend well beyond maintenance costs - operational continuity, data integrity and safety systems may all be at risk.
Preventing wet stacking begins at the specification stage. Correct generator sizing - based on a thorough load analysis rather than conservative guesswork - is the most effective means of ensuring the selected unit operates within an appropriate load band throughout its service life.
Effective load analysis goes well beyond reading nameplate ratings from a list of connected equipment. A properly conducted electrical load assessment considers:
This level of detail cannot be achieved without a professional site survey. Visiting the site, assessing the electrical infrastructure, understanding the operational profile and engaging with the client's technical team provides the information base needed to arrive at a genuinely accurate generator specification.
Where wet stacking has already developed in an existing installation, there are remediation options available. Running the generator under a controlled high-load condition - either by applying a load bank or by coinciding a test period with high site demand - can help burn off accumulated deposits and restore engine performance.
However, this is a corrective measure rather than a solution. If the underlying cause - a generator that is fundamentally oversized for its application - is not addressed, the problem will recur. In some cases, re-evaluation of the installation may be appropriate, particularly if the site's electrical demand has changed significantly since the original specification was completed.
One underappreciated advantage of working with an independent, multi-brand generator supplier is the ability to select the most appropriate kVA rating for a given application - rather than being constrained to the nearest available model within a single manufacturer's range.
Suppliers tied exclusively to one manufacturer must fit each project to the available product catalogue. This can result in clients being moved up to the next model in a range, even when a smaller unit from a different manufacturer would be a more precise match for the site's actual demand. Across manufacturers such as Perkins, Cummins, Volvo, Doosan and Baudouin, the available kVA increments differ, and access to multiple ranges significantly improves the likelihood of achieving an accurate match.
The ability to specify precisely - rather than approximately - is not simply a technical preference. It directly affects load percentages during operation, long-term engine health, fuel consumption and maintenance requirements.
Wet stacking is a preventable problem. It does not arise from poor-quality equipment or inadequate maintenance alone - it arises from a mismatch between the generator's rated capacity and the electrical load it is actually required to serve. That mismatch is established at the specification stage.
Organisations that approach generator selection as an engineering decision - supported by accurate load analysis, a professional site survey and impartial technical advice - are far less likely to encounter the hidden costs that wet stacking creates. Those that select equipment based on price alone, or without a rigorous assessment of actual demand, risk investing in a system that quietly degrades from the day it is commissioned.
Reliable backup and prime power generation begins with getting the specification right. The engineering process that precedes equipment selection is not an optional extra - it is the foundation on which long-term generator performance depends.
For more information on generator sizing, load analysis and independent generator supply across the UK, visit EA Power Systems.
