How to Vet a Generator Installer: 5 Questions That Predict the Failures

Buying · Updated 2026-09-21

Short answer: ask the installer to show you the airflow and exhaust calculation, the transfer switch and grounding plan, the fuel supply pressure at full load, and a written load calculation. Those four documents predict most of what goes wrong later. An installer who has them ready has done this before; one who waves them off will be back on your site, billing you, within two years.

Installation faults are a fifth of everything that goes wrong

We index 45,537 troubleshooting entries from manufacturer service manuals across 191 generator brands. 15,180 of them state a cause rather than just a symptom. Reading those for causes decided at installation time rather than during operation, 3,076 — 20.3% — describe a fault that the installation created.

These are not exotic failures. They are the ordinary ones — overheating, nuisance shutdowns, a set that runs fine unloaded and drops out under load — and the manufacturer's own manual attributes them to how the machine was put in.

That ratio is the reason vetting matters. You are not mainly buying a machine, because the machines are broadly reliable and well documented. You are buying an installation.

1. Ask for the ventilation and exhaust calculation

This is the single largest category in our data, and the easiest to get wrong, because a generator in a room is a heat engine inside a box. It needs intake air for combustion and a much larger volume of air to carry away radiated heat.

What to ask: what is the required airflow in cubic metres per minute or CFM at the rated load, what free area do the intake and discharge louvres provide, and what is the exhaust back pressure of the run you have designed.

Why it matters: of the 3,556 generator sets in our index that publish a maximum ambient temperature, the median is 50°C. That is the air temperature at the machine, not outside. A room that recirculates its own discharge air reaches that number on a mild day, and the set derates or shuts down. An installer who cannot tell you the airflow figure has not read the spec sheet for the model they are selling you.

A good answer cites the manufacturer's stated airflow for your exact model and shows the louvre free area exceeding it, with the discharge positioned so it cannot short-circuit back to the intake.

2. Ask how the transfer switch, neutral and grounding are arranged

803 entries. This one is dangerous rather than merely expensive.

What to ask: is the transfer switch a three-pole or four-pole, and is the generator neutral bonded at the generator or at the service. Those two answers have to agree with each other.

Why it matters: get it wrong and you either create a parallel neutral path that puts current on equipment grounding conductors, or you defeat ground-fault sensing. Neither is visible on a walk through. Both show up as unexplained nuisance tripping, and both are a hazard.

A good answer is specific and unhesitating, and references which article of the electrical code applies to your installation. Optional standby, legally required standby and emergency systems are governed differently, and an installer who treats them as interchangeable is telling you something.

3. Ask what the fuel supply delivers at full load, not at rest

726 entries, and the most common false diagnosis in the whole category. The set runs, accepts a small load, then stumbles or shuts down when the real load arrives.

What to ask, for gaseous fuel: what is the gas pressure at the generator inlet while the set is running at full load, and what pipe size and length did you use to get there. Static pressure with the machine off tells you nothing.

What to ask, for diesel: where is the day tank, what is the lift height, and how is fuel return heat handled.

Why it matters: gaseous sets are a meaningful part of the installed base and the failure is nearly always the supply rather than the generator. A pipe run sized off the meter rating rather than the generator's demand at full load will pass every test except the one that counts.

4. Ask for a written load calculation

What to ask: which loads are connected, what are the starting characteristics of the largest motor, and what voltage dip does the alternator produce when that motor starts.

Why it matters: sizing is where the cost is decided, and both errors are expensive. Undersized and the set nuisance-trips or fails to start your largest motor. Oversized and it wet-stacks from running lightly loaded, which is a maintenance problem you will pay for every year.

For scale, of the 13,312 sets in our index with a published standby rating, a quarter are at or below 24 kW and the median is 80 kW. The spread is enormous, and the right answer comes from your actual load list, not from a rule of thumb about house size.

A good answer is a document with a line per load. If the sizing arrives as a single number in a quote with no working shown, ask for the working.

5. Ask what the warranty actually requires of you

This is the question people skip, and it is the one that costs the most when it goes wrong.

What to ask: is the manufacturer's warranty conditional on installation by an authorised dealer, what commissioning paperwork has to be filed and by when, and what maintenance records must you keep to preserve coverage.

Why it matters: most manufacturer warranties on standby equipment are conditional. Common conditions include commissioning by an authorised party, a startup report filed within a set window, and documented maintenance at the published intervals. Miss the paperwork and the machine is still under warranty in principle while your specific claim is deniable in practice. Our generator maintenance schedule sets out what those intervals usually look like, so you can check the plan you are offered against the manufacturer's own numbers before you sign.

A good answer includes who files the startup report and a copy of it for your records. Ask for that copy at handover, not two years later when you need it.

Certifications worth asking about

• Manufacturer authorisation for your specific brand. Being a dealer for one brand does not make someone authorised for another, and warranty conditions usually name the brand.
• Electrical licensing appropriate to the work, held by the person doing it rather than the company in the abstract.
• EGSA technician certification. The Electrical Generating Systems Association runs a certification specifically for on-site generator technicians. It is not universal, so its absence is not disqualifying, but its presence is a real signal.
• Documented experience with your fuel and configuration. Natural gas, diesel, and paralleled sets are different disciplines.

Finding candidates to vet

Vetting only helps once you have someone to vet. Directories such as Generator Installer List collect installers by region, which is a reasonable starting point for building a shortlist. Manufacturer dealer locators are the other obvious source, and they have the advantage of confirming the authorisation that your warranty may depend on.

Treat any directory as a source of candidates, not of verification. The questions above are what separates them.

The pattern worth remembering

Every one of these questions asks for a number or a document that the installer should already have. None requires you to know anything technical. You are not testing whether they can answer correctly, you are testing whether they did the work before quoting.

Our troubleshooting index says that one installation fault in five ends up in a service manual as somebody's diagnosed problem. The questions above cover the four categories those faults fall into.