Generator Buying Guide

Generator power should not be chosen by copying the set at a similar facility, but only after the load list, the power of the largest motor and the installation location have been determined. Selections made without this information can result in sets that are insufficient during an outage or that consume excess fuel because they are oversized.

The generator purchasing process should follow a set order. When decisions are made in the right order, no missing or mismatched items come up at the quotation stage. The order below is the approach that causes the fewest problems on site.

Step 1: What is the generator for?

The first question is how it will be used.

• Standby (backup): Waits while the mains is on, comes online during an outage. Runs a limited number of hours a year.
• Prime (primary source): No mains, or an unreliable one. The set runs for long periods at variable load.
• Continuous: Constant load, almost non-stop running.

This choice decides which power rating you look at. Details: standby, prime and continuous power.

Step 2: Load list

A list, not a guess. For each item: power, quantity and whether it runs at the same time as the others. List these separately:

• Large motors: Pumps, compressors, fans. The starting method (direct-on-line, star-delta, soft starter, drive) directly affects the set size.
• UPS and rectifiers: Bring harmonics and battery charging load.
• Lifts, air conditioners, chillers.

The power calculator is a good starting point for a first estimate. The final decision is made from the load list, with the starting effect of the largest motor calculated.

Step 3: Phase, voltage and frequency

Single phase or three phase? 400/230 V or another voltage? 50 Hz or 60 Hz? This question matters especially on export projects. A set ordered at the wrong frequency cannot be used on site.

Step 4: Site conditions

• Temperature and altitude: On a hot, high site the set loses power.
• Dust, humidity, sea air: Affect the choice of filters, paint and canopy.
• Cold: Needs a block heater, low-temperature oil and the right battery.

Step 5: Installation location and noise

Where will the set go? Open ground, roof, basement? In an enclosed room, ventilation, exhaust and the fuel line have to be designed. Outdoors, especially near homes, the noise level is decisive. Noise figures are always given with a distance, such as “70 dB(A) at 7 metres”. A noise figure without a distance means nothing.

Step 6: Control, ATS and monitoring

• If the set is to come online automatically, you need an ATS panel.
• If more than one set will run in parallel, you need synchronization equipment.
• For a remote facility, remote monitoring makes a real difference: generator remote monitoring.

Step 7: Fuel and autonomy

How many hours will the set run on a full tank? How long does a fuel delivery take? If the standard base frame tank isn’t enough, a larger or external tank is planned. Consumption varies with load; size the tank for the real load you expect during an outage, not the average load.

Step 8: Choosing the engine

The Terex Diesel range includes options with Perkins, Baudouin, Cummins, FPT Iveco, Kofo, Kubota, Quartz, Scania, SDEC, Volvo Penta, Yangdong and Yuchai engines. The questions to ask: Is there service and parts support for this engine in your region? How tolerant is it of fuel quality? Does it suit the running hours and load profile? Where is the budget?

Step 9: Reading the quote

Price can’t be read apart from scope. Are the rating class, canopy, panel, tank, transport, installation and warranty the same? We explain this in a separate article: what drives generator price. If you are buying from abroad, see the supplier checklist for importing from Türkiye.

Step 10: Delivery and commissioning

The job isn’t done when the set arrives on site. At commissioning, check: oil, coolant and fuel levels, phase sequence, protection settings, ATS delays and, most importantly, running on load. If possible, open the mains breaker to simulate a real outage. How many seconds did the set take to pick up the load, how did voltage and frequency behave, was the retransfer smooth when the mains came back? Write these values down in a report. On the same day, agree when the first maintenance will be done and by whom. A test skipped at handover usually gets left to the first real outage.

Common mistakes on site

• Copying someone else’s generator. Your loads are different, your site is different.
• Ignoring motor starting. The set drops out when the big pump starts.
• Thinking about the location later. The set arrives and won’t fit through the door, or the room ventilation isn’t enough.
• Looking only at the set price. Fuel, maintenance and parts are a serious cost over the set’s life.
• Skipping the commissioning test. The set is accepted without ever being tried on load.

Buying checklist

• Is the type of use clear: backup, primary source, continuous?
• Are the load list and the starting method of the largest motor ready?
• Are the phase, voltage and frequency correct?
• Are the site temperature, altitude and ambient conditions known?
• Are the installation location, access route and ventilation suitable?
• Are the noise limit and measuring distance defined?
• Have ATS, synchronization and monitoring needs been defined?
• Have fuel autonomy and tank capacity been calculated?
• Are the quote items and warranty terms in writing?
• Will a load test be done at commissioning?

Next step

Filling in this list takes a few hours and prevents a wrong purchase. Share the details with the Terex Diesel team and we will choose the right set together. To see models, use the product finder; for your project, contact us.