Decide on running hours and role before you look at kW. A prime-rated generator set is built to be a mine’s main source of electricity and can run for unlimited hours a year under a variable load. A standby-rated set is built to carry the site during a grid outage, for a limited number of hours. Off-grid mines need prime. Mines on a stable grid that only need to protect ventilation, pumps and safety systems can usually use standby.
The mistake that costs the most is picking a kW figure first and the rating second. The same engine and alternator carry a higher kW number under a standby rating, so a quote that looks cheap per kW may be rated for a very different job than the one you have in mind.
What the ratings mean
Generator sets are rated under ISO 8528-1. Manufacturers name the categories a little differently, but four matter for mining.
Emergency standby power is for outages only. It is commonly quoted at up to about 200 running hours a year, with no overload allowed and an average load of around 70% of the rating.
Prime power is for a set that runs as the main supply. Running hours are unlimited, the load can vary, and the average over 24 hours is normally kept to about 70% of the prime rating. A short overload of around 10% is allowed for limited periods.
Limited-time prime power sits between the two. It is often quoted at up to about 500 hours a year and suits peak shaving, or a grid-connected mine that expects regular but not continuous generator use.
Continuous power covers unlimited hours at a constant load. It is rare in mining, because crushers, mills, conveyors and pumps cycle through the shift.
These are the common ISO 8528-1 conventions. The datasheet for your exact model has the final word.
A worked example: 1000 kW standby against 908 kW prime
Take SWT’s SC1250 series, a 1000 kW class diesel set. The 50 Hz version with a Cummins QSK38-G2 engine is listed at 1000 kW / 1250 kVA standby and 908 kW / 1135 kVA prime, at 380, 400 or 415 V and 1500 r/min.
The prime rating is about 91% of the standby rating, a gap of roughly 92 kW. If you size a mine’s main supply on the 1000 kW headline, you have overstated its continuous capacity before counting anything else. Apply the 70% average-load convention and the same set should average about 636 kW over 24 hours. A crusher plant that averages 800 kW is beyond what one set of this size should carry on prime duty, so you would need two sets, plus a third if you want a spare.
Which rating fits which mine
An off-grid mine, where the generators are the only supply, needs prime power. So do exploration camps, construction phases and rental fleets, because the running hours are not known in advance.
A mine on a weak grid with frequent, long outages should look at prime or limited-time prime. This case needs honest numbers. A grid that fails for 20 hours a year is a standby job. One that fails for 20 hours a month is not, and a single multi-day outage can pass the hour limit of a standby rating by itself.
A grid-connected mine that only has to protect ventilation, dewatering, hoists and safety lighting can use standby sets. A grid-connected mine with regular peak shaving fits limited-time prime.
For standby installations, add an automatic transfer switch so the set starts and picks up the critical load without anyone at the site. SWT’s optional ATS transfers within 80 ms, depending on configuration. Run the set on load regularly to prove it works.
Sizing the generator set for the site
Add up the running kW of every load, allow for the starting current of the largest motors, and divide by the power factor, typically 0.8, to get kVA. A 1000 kW load needs about 1250 kVA of capacity. Leave room for expansion.
Camps, drilling sites and workshops up to about 200 kW usually run compact Kubota or Perkins sets that are easy to move. Crushing plants and mid-size mines at roughly 200 to 600 kW have the widest choice, across Cummins, Perkins, Weichai and Yuchai. At that size motor starting often decides the model, so give the supplier the largest motor and how it starts: direct on line, star-delta, soft starter or variable-frequency drive.
From 1000 kW up, weight and transport matter on remote sites. The SC1250 open-type versions weigh about 7,000 to 8,255 kg, and the 20-foot container versions about 11,000 to 12,650 kg, depending on configuration. They also come with factory load tests at 0, 25, 50, 75, 100 and 110% before delivery.
For several megawatts, parallel several sets rather than relying on one machine. A synchronizing system can bring up to 32 sets together, start and stop units to follow the load, and let you add capacity as the mine grows. It also avoids wet stacking, the unburned fuel and soot that build up when one oversized diesel idles lightly loaded for long periods. SWT’s Simandou Mine 24 MW power plant in Guinea shows generation at that scale.
When gas makes sense
If the mine has natural gas, associated gas, biogas or LPG nearby, a gas set can replace diesel trucking. SWT’s Cummins K50N covers 900 to 1200 kWe per unit, with a 50.3-litre V-type engine, 50 or 60 Hz output and NOx below 500 mg/Nm³. SWT supplies it with alternator matching, controls, gas treatment, commissioning and EPC services.
Gas quality decides whether a project works. If you plan to use coal mine gas, whose methane content varies, have the engine setup and gas treatment confirmed against a gas analysis before you commit.
Site conditions and derating
Rated output assumes reference conditions. For SWT’s 1000 kW class they are up to 40°C, 1,000 m altitude and 60% relative humidity. A mine at 3,000 m, or in desert heat, has to derate whichever rating it chooses, and the loss can be large. Ask for the derated output at your actual site, not the catalogue figure.
Two smaller items are easy to forget. Night shifts need lighting, and SWT also builds mobile light towers, including the 75M/H, HVP/HVL and HTM mining series. For a remote camp or pumping site, a containerized solar, storage and diesel hybrid unit can cut diesel use, and SWT lists a 30 kW version. Larger mine loads still need generator capacity at the core.
Mistakes that cost money
Running a standby-rated set as the main supply is the big one. It runs hotter, wears faster and weakens your warranty position. The opposite error is buying one large prime set for a load that is mostly small, which leaves it lightly loaded most of the time. The rest are simpler: mixing up kW and kVA, ignoring motor starting current, forgetting altitude and temperature derating, and comparing quotes that use different ratings.
What to send a supplier
For a useful quote, send the load list with the largest motor and its starting method, whether the set is main supply or backup and the expected hours a year, the maximum temperature, altitude, humidity and dust level, voltage and frequency, the fuel available, and the delivery location. A gas project also needs a gas analysis.
More detail
The ratings, with worked examples and a checklist, are covered in What Is the Difference Between Prime and Standby Power for Mines?. A model-by-model guide by mine type, load size and fuel is in What SWT Generator Set Works for Mining Site Power?.
Disclosure: I work with SWT (Shandong Superwatt Power Tech Co., Ltd.), a generator set manufacturer. Figures above come from SWT’s published product data. Always confirm ratings on the datasheet for your exact model.
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