The question facilities managers ask most often about commercial generator servicing is how frequently a service is needed. The answer depends on how the generator is used, how many hours it runs, and what the manufacturer recommends for the specific engine and alternator combination. This article sets out the standard intervals, what each service should cover, and the consequences of missing a service.
Research from the Electrical Contractors' Association suggests that generators without a formal maintenance programme are three times more likely to fail to start in a genuine power outage than those on a planned maintenance contract (ECA, 2022). The cost of a missed service is rarely the cost of the service itself. It is the cost of the failure it was supposed to prevent.
Manufacturer-Recommended Service Intervals
Most commercial diesel generator manufacturers publish service intervals based on operating hours and calendar time, whichever comes first. The standard intervals for the most common engine families are set out below.
Annual service (or every 250 to 500 hours, whichever comes first) is the baseline for most standby generators. Standby sets that are tested weekly but rarely carry full load may accumulate fewer than 50 hours in a year. For these sets, the calendar interval is binding. An annual service is the minimum that any standby generator should receive.
Six-monthly service (or every 500 to 750 hours) is recommended for prime power sets or standby sets in high-criticality applications such as data centres, hospitals and emergency services facilities. At this frequency, consumables are replaced before they reach the end of their service life and any developing fault is caught early.
Quarterly checks are appropriate for high-hours prime power applications and for sets that must comply with sector-specific standards such as NHS HTM 06-01 for healthcare. Quarterly checks typically cover fluid levels, battery condition, visual inspection and a load test, with a full service carried out at six-monthly or annual intervals.
What Is Included in a Generator Service?
A complete annual service for a commercial diesel generator covers the engine, electrical systems, fuel system and control panel. The scope may vary slightly depending on the manufacturer's recommendations for the specific model, but the core items are consistent across all makes.
Engine servicing includes engine oil and filter change, fuel filter replacement (primary and secondary), air filter inspection and replacement, coolant level and anti-freeze concentration check, drive belt inspection, and visual inspection for leaks, cracks and corrosion. For high-hours sets, compression testing and injector checks are added to the scope.
Electrical checks include battery condition testing using a conductance tester, battery charge voltage confirmation, alternator output voltage and frequency measurement under no-load and loaded conditions, control panel self-test, alarm and shutdown verification, and ATS operation test. Cable termination and earth connection inspection is also carried out.
A load bank test is included at every service we carry out. Running the set under artificial load confirms that voltage and frequency are stable at rated output, that the cooling system maintains temperature within specification, and that wet stacking has not developed. Results are recorded on the service sheet and compared against previous readings to identify any trend.
What Happens If You Skip a Service?
Skipping a generator service does not produce an immediate visible problem in most cases. The set will continue to start and run. This is the danger. The consequences of a missed service accumulate silently and typically become apparent at the worst possible moment.
The most common outcomes of irregular servicing include battery failure (the leading cause of generator failure to start), fuel contamination by water ingress or microbial growth, blocked fuel filters causing hard starting and reduced output, wet stacking causing loss of power and accelerated cylinder wear, and coolant pump failure causing overheating shutdown.
Each of these faults is detectable and preventable through routine servicing. Each of them, if it develops into a failure during a real power outage, will cost significantly more to resolve than the service that would have prevented it. A replacement battery costs a fraction of a service call. An engine overhaul following coolant loss costs multiples of several years of planned maintenance.
If your generator has not been serviced for more than 12 months, contact us for an inspection. Our engineers will assess the current state of the set and recommend the appropriate remedial action. Book a generator service today.
BS 7671 and Compliance Considerations
The Electricity at Work Regulations 1989 require that all electrical systems, including generator installations, are maintained in a safe condition. This does not specify a service interval but places a duty of care on the responsible person to confirm that the system is fit for purpose. A documented service record from a competent contractor is the standard way of demonstrating compliance.
BS 7671 (the IET Wiring Regulations, 18th Edition) provides the technical framework for periodic inspection and testing of electrical installations. For generator installations, this includes testing the ATS panel, distribution boards, cable insulation and earth connections. The recommended frequency for periodic inspection depends on the type of installation and the environment, but for commercial sites the standard is every five years or on change of occupancy.
Sector-specific standards add further requirements. NHS HTM 06-01 specifies monthly inspections and annual full-load tests for healthcare generator systems. Data centre standards including EN 50600 and the Uptime Institute Tier Standards specify maintenance frequencies and documentation requirements for generators supporting critical IT infrastructure. Our service programmes are designed to satisfy all of these requirements.
How to Set Up a PPM Contract
A planned preventive maintenance (PPM) contract removes the burden of remembering service dates, sourcing engineers and managing compliance records. It converts generator maintenance from a reactive task into a scheduled, documented activity with a fixed cost.
Setting up a PPM contract with Generators London starts with a free site survey. Our engineers inspect the generator, review any existing service records, and agree an appropriate service schedule based on the manufacturer's recommendations, the criticality of the application and your budget. We then provide a fixed-price annual contract covering all scheduled visits, with priority access to our emergency breakdown team included.
Contract terms of 12, 24 or 36 months are available. Longer terms offer lower monthly rates and greater budget certainty. All contracts include digital service records after every visit, a contact number for out-of-hours emergency support, and a condition review at the mid-contract point. To discuss a PPM contract for your site, call 0800 000 0000 or request a Free Generator Survey.
Frequently Asked Questions
How often should a standby generator be serviced?
Standby generator servicing should take place at least annually, regardless of hours run. For sets installed in critical applications such as healthcare, data centres or financial services, six-monthly servicing is recommended. The annual service should always include a load bank test, a battery condition check and a full engine fluid and filter change.
Can I service the generator myself to save money?
Basic checks such as fluid levels, battery terminals and visual inspection can be carried out by a trained site operative. But a full generator service requires specialist diagnostic equipment, knowledge of engine-specific torque values and clearances, and competency in electrical testing to BS 7671. Attempting a full service without the correct training and tools risks damage to the set and may invalidate the manufacturer's warranty.
Does a generator service include a load test?
It should. A service that does not include a load test cannot confirm that the generator will actually perform under load during a real outage. All of our services include a load bank test as standard, and the results are recorded on the service sheet. If your current service provider does not carry out a load test, ask why not.
What records should I keep for generator compliance?
You should retain a signed service record for every visit, a copy of the commissioning certificate from the original installation, any periodic inspection reports, and records of any faults, repairs or modifications. For regulated sectors, you may also need to retain load test certificates and manufacturer service bulletins. We provide all of this documentation as standard and store digital copies in our records management system for the life of the contract.