Generator sizing is one of those decisions that looks simple until you get it wrong. An undersized unit trips under load and shuts your site down. An oversized unit burns excess fuel, runs inefficiently, and costs far more than the job demands. Getting it right from the start saves money, prevents downtime, and keeps your crew productive. This guide walks through the key steps to calculate your site's power requirements and match them to the right generator.
Why generator sizing matters more than most people think
Many site managers default to "bigger is better" when specifying temporary power. The logic is understandable: nobody wants to be caught short. But running a generator at less than 30 percent of its rated load causes a problem called wet stacking, where unburned fuel deposits accumulate in the exhaust system and degrade engine performance over time. On the other side, a generator running at or above its rated capacity will overheat, trip on overload protection, and risk damaging connected equipment. The sweet spot for most diesel generators is 70 to 80 percent of rated load under continuous operation.
Step one: list every load on site
Start by cataloguing every piece of equipment that will draw power simultaneously during peak operations. Group your loads into three categories:
- Continuous loads: equipment running for the majority of the working shift, such as site lighting, computers, CCTV systems, and water pumps.
- Intermittent loads: equipment used periodically, such as welders, compressors, and power tools.
- Motor loads: equipment with electric motors, including pumps, air conditioners, and conveyors. These draw a significant inrush current at start-up, often three to seven times the running current, which must be factored into your calculation.
For each item, note the rated wattage or kilowatt (kW) draw. You can usually find this on the equipment nameplate or in the manufacturer's documentation. If you're running three-phase equipment, confirm whether the nameplate rating is given per phase or as a total.
Step two: calculate your total connected load
Add up the kW ratings of all continuous loads, then add a portion of your intermittent loads based on how frequently they run. A common approach is to include 50 percent of intermittent load in your base calculation.
Once you have a total kW figure, convert to kilovolt-amperes (kVA) using the power factor of your site's equipment. For a mixed site load, a power factor of 0.8 is a standard assumption, meaning you divide your kW total by 0.8 to arrive at the required kVA rating. Most generator specifications are quoted in kVA, so this step is essential for an accurate comparison.
Step three: account for motor start-up inrush
Motor start-up inrush is the most common reason a correctly sized generator still trips under load. When a large motor starts, the generator must supply a surge of current that can last several seconds. If multiple motors start simultaneously, the cumulative inrush can exceed the generator's transient capacity.
To handle this, either size the generator to accommodate the worst-case start-up scenario, or stagger motor starts using a sequential start protocol. Variable speed drives (VSDs) are another practical solution: they ramp motors up gradually, dramatically reducing inrush and allowing a smaller generator to handle larger motor loads.
Step four: apply a safety margin and consider future growth
Once you have your required kVA figure, apply a safety margin of at least 20 to 25 percent. This accounts for calculation uncertainties, equipment degradation over time, and any loads you may have overlooked. It also positions the generator in the optimal 70 to 80 percent loading range during typical operation.
If the site is expected to grow over the project duration, for example a construction project that adds more plant as it moves into later phases, it's worth sizing for the anticipated peak load rather than the current one. Swapping generators mid-project is expensive and disruptive.
Step five: consider whether a hybrid solution makes sense
For many sites, a standalone generator is not necessarily the most cost-effective answer. Pairing a correctly sized generator with a battery energy storage system (BESS) can smooth out peak demand spikes, allow the generator to run at optimal load continuously, and reduce fuel consumption significantly. This is particularly relevant on sites with highly variable loads, such as those using intermittent welding equipment or large compressors that cycle on and off throughout the day.
If you're weighing up these options, our detailed comparison of generators vs battery storage covers the trade-offs in practical terms, including load profiles, fuel costs, and when a hybrid approach pays off fastest.
Practical tips for remote and off-grid sites
Remote sites introduce additional complexity. Fuel delivery logistics mean that running an oversized generator is not just inefficient; it can genuinely create a supply problem if consumption runs ahead of resupply schedules. On remote projects, getting the sizing right is even more critical.
Altitude and ambient temperature also affect generator output. As a rule of thumb, a diesel generator loses roughly one percent of rated output for every 100 metres of altitude above sea level, and additional derating applies in high ambient temperatures. If your site is in the Pilbara or at altitude in the Snowy Mountains region, apply manufacturer-specific derating figures before finalising your specification.
For sites that require around-the-clock power without interruption, it's also worth considering whether a redundant generator set is warranted. A two-unit N+1 configuration, where one generator can carry the full load if the other goes offline for servicing, provides resilience that a single unit cannot. Keeping equipment in peak condition through a regular servicing schedule is equally important for avoiding unplanned outages on remote sites where a service technician may be hours away.
Getting expert advice before you commit
The steps above will get you to a well-reasoned ballpark figure. But every site is different, and nuances in your load profile, network configuration, and operational patterns can shift the answer significantly. Before hiring or purchasing, it's worth having a power specialist review your load schedule and confirm the specification.
EEA Lightning and Power works with construction, mining, and infrastructure teams across Australia to size, supply, and support temporary power solutions. Whether your site needs a single standby unit or a full hybrid generator-and-storage system, the right starting point is always an accurate understanding of your load.