Load Bank Testing

Load bank testing in London confirms that a commercial generator can deliver its rated power output under realistic operating conditions. Running a set at no load or low load does not prove it will perform during a real power outage. Generators London carries out resistive and reactive load bank tests on commercial and industrial generating sets across Greater London, issuing a signed test certificate on the same day.

Our CompEx-certified engineers have been carrying out generator load testing since 2009, working on sets from 20 kVA to over 1,000 kVA for clients in healthcare, data hosting, manufacturing and facilities management. This page explains what load bank testing is, why it is important, the difference between resistive and reactive testing, and what happens during a test session.

What Is Load Bank Testing?

Load bank testing is a controlled process of applying an artificial electrical load to a generator using a dedicated load bank unit. The load bank simulates the electrical demand that would be placed on the set during a real power outage, without needing the building's actual load to be connected. This allows the test to be carried out safely, at any time, and at a precisely controlled load level.

The test verifies that the engine, alternator, cooling system and control systems can all perform correctly at rated output for a sustained period. It also identifies problems such as wet stacking, cooling system weakness, AVR instability and governor hunting that may not be apparent during routine no-load run tests.

To book a load bank test, Get a Free Generator Survey or call 0800 000 0000.

Why Is Load Bank Testing Important?

Many facilities managers run their generator under no load for 10 to 15 minutes each week as a routine check. This confirms the set will start, but it does not confirm it will carry load. A set that has been running lightly loaded for an extended period may have developed wet stacking, reduced cooling capacity or alternator deterioration that only becomes apparent when full load is applied.

Load bank testing is a requirement under several standards and guidelines including BS 7671, the NHS HTM 06-01 guidance for healthcare electrical infrastructure, and the recommendations of the BESA (Building Engineering Services Association). Many building insurers and facilities management contracts now specify annual load bank testing as a condition of cover or compliance.

For generators used as part of a planned maintenance programme, we recommend incorporating a load bank test at least annually, or at every six-monthly service for high-criticality sites.

Resistive vs Reactive Load Testing

There are two main types of load bank test, and the right choice depends on the type of load the generator is expected to supply in service.

Resistive load testing applies a purely resistive load, equivalent to the demand from heaters, lighting and similar non-motor loads. It is the most common type of load bank test and is sufficient for most standby generator applications. The power factor during a resistive test is 1.0, which means the generator's kVA and kW ratings are both being exercised equally.

Reactive load testing adds an inductive or capacitive component to the load, replicating the demand from motors, transformers and other reactive equipment. The power factor during a reactive test is typically 0.8 lagging, which is the standard power factor rating of most commercial generators. Reactive testing is recommended where the site load includes significant motor or transformer content, as it places a more demanding burden on the alternator and AVR.

What Happens During a Load Test?

Our engineers arrive on site with a portable load bank unit sized for the generator under test. The test follows a structured protocol that typically covers the following stages.

The set is started and run at no load to confirm correct voltage, frequency and oil pressure. Load is applied in steps, typically 25%, 50%, 75% and 100% of rated output, with the set held at each level for a minimum period to allow temperatures to stabilise. Engine parameters including coolant temperature, oil pressure and exhaust temperature are recorded at each load step. Alternator output voltage and frequency are recorded and compared against rated values. The automatic transfer switch is tested in both automatic and manual modes where applicable.

On completion, a signed load bank test certificate is issued recording all measured parameters, the test duration at each load step, and any observations or recommendations. The certificate is available in digital format for inclusion in your compliance records. Contact us to book a load bank test.

Frequently Asked Questions

How long does a load bank test take?

A standard load bank test for a commercial standby generator takes between two and four hours on site, depending on the kVA rating of the set and the number of load steps in the test protocol. We recommend allowing half a day for the visit, including set-up, testing and the post-test discussion with your facilities team. The certificate is issued on the same day.

How often should load bank testing be carried out?

Load bank testing frequency depends on the criticality of the site. For most commercial standby applications, annual testing is sufficient. For healthcare, data centre and other high-criticality sites, six-monthly testing is recommended. Generators that have not been load tested in the previous 12 months should be tested before any planned maintenance period or building reoccupation.

Will load bank testing disrupt my site?

Load bank testing is carried out using an external resistive or reactive load connected directly to the generator output. It does not affect your building's electrical supply or connected equipment. Tests can be scheduled outside normal working hours or at weekends where noise or access is a concern. The generator will run at rated load during the test, so normal exhaust and noise levels should be expected.

Do you issue a certificate after the test?

Yes. We issue a signed load bank test certificate on the day of the test. The certificate records the generator's rated output, the actual measured output at each load step, engine and alternator parameters, test duration, and any faults or recommendations identified during the test. Digital copies are provided for your records and can be shared with insurers or auditors on request.