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How to calculate the generator size you need

Need power on site or at home? Calculate the right size generator for the job with our handy guide


Whether you're working on an unpowered site or want the security of an emergency backup supply, generator (or genset for ‘generator set’) hire can be essential for keeping equipment running and avoiding costly outages.

The most important decision when choosing a diesel generator for rent is getting the right size for your power requirements. An undersized generator won't deliver the amount of power you need and could damage your equipment, while one that's too large will incur unnecessary extra cost and technical issues such as glazing of the generator, engine oil consumption or lube oil leakage.

How much power do you need?

When looking at gensets for hire, you’ll see various power ratings such as watts, kilowatts, amps (short for amperes), volts and kVA (short for kilovolt amperes). Single-phase electricity is delivered to Australian homes at 230-240 volts, while commercial and industrial facilities usually use a three-phase supply. Appliances and power tools, meanwhile, often display their power ratings in amps and watts.

To calculate the size of generator you require, you’ll need to provide your equipment hire supplier with the total electrical load (in watts or kilowatts, or amps and voltage) of the equipment or appliances you'll be powering. You can use an online power calculator, or if you're hiring equipment from the same company as your generator, they can calculate it for you.

If you’re unsure about the power requirements you can always mention the plug types, outlet type (i.e. 10A, 15A, 20A 240V) or simply say you using a 3 flat pins plug or GPO. For larger plug types, let the hire company know if you have 5 round pins or 400V, etc so they can better understand your requirements.

Add up the individual power requirements for every system that will be powered by the generator at the same time to arrive at the total load. This information is not always listed on the product, and you may need to speak to your equipment supplier to check.

For large industrial or commercial sites, you should also provide the starting method or starter type –DOL (direct online) starting, star-delta starting, soft starting, VSD (variable speed drive, also known as VFD) – of each system if it’s a pump, motor, compressor or any rotary electrical equipment, as the start-up current requirement can sometimes be many times that of the running current.

What sizes of generators are available?

Generators are available in an extensive range of sizes. These include small portable units for individual power tools; backup generators and standby generators in case of a power outage or blackout; and heavy duty industrial generators enclosed in a steel container and with large fuel tanks for busy mining or construction sites.

Portable generators

If you have minimal power requirements, you may only need a small diesel generator. These are available in a range of sizes from 2 kVA to around 40 kVA and include:

Inverter generators are generally small and portable.

Inverter generators are generally small and portable.

  • Briefcase type (2 kVA) – lightweight and easy to transport, these generators are suitable for camping, handymen and workshops, capable of powering individual tools rated up to 1000w.

  • Inverter generators (4.3 kVA, 6.5kVA) – these quiet and efficient systems can power larger tools and home appliances up to a combined 5000w.

  • Rollframe type (3–9 kVA) – fitted with a heavy-duty rollframe with wheels and handles, these sturdy generators cover most domestic and tradie needs. they can also power most welding equipment up to 7000w.

  • Trailer mounted (11–100 kVA) – an efficient option for smaller construction projects, trailer-mounted generators can power equipment up to 70kW.


Larger generators

For more intensive power generation requirements, you'll need a skid-mounted or container-style generator.

Look for models with large fuel tanks so you won't have to refuel frequently, automatic voltage regulators to keep the power steady and silenced canopies to reduce noise pollution, especially if you're working in a residential area.

Traditional generators range from medium-size trailer mounted units to large containerised generators

Traditional generators range from medium-size trailer mounted units to large containerised generators.

The options are:

  • Silenced skid mounted generators (10–550 kVA) – available in a wide variety of sizes tailored to different sizes of projects, from compact units under 20 kVA to mid-size 150–275 kVA and larger 300–550 kVA units.

  • Container style (650–1400 kVA) – when only the biggest will do, these supersize generators can provide over a megawatt (one million watts) of power and can be configured for multi-use operation.

When you've calculated the size of generator you need, explore our range of generators for hire and contact your local Coates branch on 13 15 52 to discuss your ideal power solution.

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