To estimate a pneumatic rock drill’s compressed-air electricity cost, multiply its verified free-air demand by the compressor package’s specific power, then multiply by actual trigger time and the electricity tariff. For a stronger result on a shared compressed-air system, meter package energy and delivered free-air volume over the same interval, calculate kWh per reference cubic metre, and allocate only the volume used by the drill. The answer is an operating-cost estimate, not a compressor-sizing calculation or a complete cost-per-metre model.
This guide owns the buyer job of converting air demand into electricity cost per shift, month or drilled metre. It does not own compressor capacity selection, high-air-consumption diagnosis, hose pressure-drop design or consumable cost per metre. Use the air-consumption terminology guide to verify the flow basis, the compressor-sizing guide for capacity decisions, and the excessive-air-consumption guide when measured demand is unexpectedly high.
What this calculation can and cannot prove
The calculation is useful for budgeting, comparing operating scenarios, preparing an RFQ and testing whether a claimed efficiency improvement is large enough to investigate. It can answer questions such as:
- What is the estimated electricity cost attributable to one drill during one shift?
- How much does the result change when trigger time, tariff or verified air demand changes?
- Which supplier data must be collected before two alternatives can be compared?
- Is a plant-wide meter study justified because the estimate is sensitive to part-load operation?
It cannot prove that one drill produces a lower total cost per metre. Penetration rate, hole quality, bit and rod consumption, labour, downtime, compressor maintenance, fuel or electricity generation, and production delays may outweigh the air-energy difference. It also cannot show that an existing compressor has enough capacity; simultaneous demand and required working pressure must be checked separately.
Input-quality gate: do not calculate until the bases match
ISO 1217 specifies acceptance-test methods for compressor volume flow and power requirements. ISO 11011 treats a compressed-air assessment as a complete system from energy input through supply, transmission and demand. These boundaries matter: mixing a catalogue flow at one reference condition with compressor performance at another can create a neat-looking but invalid answer.
| Input | Minimum acceptable evidence | Common failure |
|---|---|---|
| Drill air demand | Measured average reference flow during representative drilling, or a supplier value with stated pressure and reference conditions | Using displacement or inlet volume as though it were delivered free air |
| Compressor energy | Package kWh from a revenue-grade or verified power meter over the same interval as flow | Using motor nameplate kW as actual package power |
| Specific power | Package input kW divided by delivered reference flow at the stated pressure and operating point | Using full-load specific power during deep part-load operation |
| Trigger time | Time the drill is actually consuming air, not paid shift hours | Multiplying full-load demand by the entire shift |
| Electricity tariff | Applicable energy charge in currency/kWh; document whether demand charges and taxes are excluded | Calling an energy-only estimate the total utility cost |
If the flow and specific-power documents do not state compatible reference conditions, stop and request corrected data. Conversion between reference conditions requires the full pressure, temperature, humidity and compressibility basis; a simple unit conversion is not enough.
Method 1: matched energy and flow measurement
This is the preferred method for a shared compressor station because it captures the package and control behaviour during the interval being studied.
- Choose a representative interval that includes normal drilling and normal compressor sequencing.
- Record compressor-package energy in kWh over that interval. Include the package boundary being evaluated and identify excluded auxiliaries.
- Record delivered free-air volume over the same timestamps and on a documented reference basis.
- Calculate system specific energy: interval kWh ÷ interval reference m³.
- Measure or estimate the drill’s reference air volume during the same interval.
- Calculate allocated drill energy: drill reference m³ × system kWh/reference m³.
- Multiply allocated kWh by the applicable currency/kWh tariff.
Illustrative example: a matched interval records 420 kWh for 3,500 reference m³ delivered, giving 0.120 kWh/reference m³. A drill consumes an estimated 540 reference m³ during that interval. Its allocated energy is 64.8 kWh. At an illustrative tariff of 0.12 currency units/kWh, the energy-only cost is 7.78 currency units. These values demonstrate the method; they are not product ratings or industry benchmarks.
For a shared network, the result remains an allocation. It becomes stronger when the drill branch is metered and the compressor package energy is logged over identical timestamps. If only total station flow is known, disclose how the drill share was estimated and show a range rather than a single false-precision figure.
Method 2: estimate from compressor specific power
Use this method at RFQ stage or when matched meters are unavailable. The required formula is:
Cost per shift = drill reference flow × compressor package specific power × drill trigger hours × electricity tariff.
Units must cancel correctly. If drill flow is in m³/min and specific power is in kW per m³/min, their product is kW. Multiplying by trigger hours gives kWh. If the data sheet reports kW per 100 cfm, divide the drill’s cfm by 100 before multiplying.
Illustrative example: verified drill demand is 3.0 reference m³/min at the stated operating condition. The compressor package’s documented specific power at the relevant operating point is 6.5 kW per m³/min. Trigger time is 3.2 hours in an eight-hour shift.
- Allocated input power: 3.0 × 6.5 = 19.5 kW
- Shift energy: 19.5 × 3.2 = 62.4 kWh
- At 0.12 currency units/kWh: 62.4 × 0.12 = 7.49 currency units per shift
The example assumes the selected specific-power value represents the actual compressor package at the applicable pressure and load. It should not be applied unchanged to a different pressure, control mode or part-load condition.
Decision table: which method should the buyer use?
| Available data | Use | Decision quality | Required caveat |
|---|---|---|---|
| Matched package kWh, station reference volume and drill-branch volume | Matched-meter method | Best for operating decisions | State meter accuracy, interval and package boundary |
| Verified drill flow and package specific power at relevant pressure/load | Specific-power estimate | Suitable for RFQ comparison and budgeting | Part-load and sequencing effects may differ |
| Motor nameplate kW and drill catalogue flow only | Do not issue a firm cost | Screening range at most | Nameplate power is not measured package input |
| Only compressor rated cfm | Do not allocate to the drill | Insufficient | Rated capacity does not identify the drill’s consumption or trigger time |
Why shared compressor systems need extra care
A second drill does not necessarily double station energy. Compressor controls, unloaded running, storage, pressure bands, dryers and sequencers change the relationship between added flow and power. A fixed-speed compressor that unloads inefficiently can show a different incremental cost from a well-matched variable-capacity system. Therefore:
- Use matched timestamps for energy and flow.
- Document which compressors were loaded, unloaded or stopped.
- Record header pressure and drill inlet pressure during drilling.
- Separate baseline plant demand from the incremental drill demand when possible.
- Do not add individual catalogue flows and multiply by one full-load specific-power value unless the resulting load point has been verified.
CAGI notes that demand, pressure and air quality are the three key sizing variables, and that usage factor is needed for intermittent applications. That same discipline applies to costing: full-load flow and actual trigger time must remain separate inputs.
Worked comparison for an RFQ
When comparing two offers, hold the evidence boundary constant. The following block is more useful than asking for a generic “low air consumption” claim.
| RFQ field | Supplier A | Supplier B | Buyer rule |
|---|---|---|---|
| Air demand | Value + unit + pressure | Value + unit + pressure | Reject unstated reference conditions |
| Test status | Method and report identifier | Method and report identifier | Separate measured results from nominal claims |
| Representative trigger time | Buyer-controlled field | Same buyer-controlled field | Use the same duty scenario |
| Compressor specific power | Use the same station value for both tools | Match pressure and operating point | |
| Calculated energy cost | Formula result | Formula result | Keep tariff and currency identical |
| Production denominator | Verified metres or holes | Verified metres or holes | Do not infer production from airflow |
If the aim is cost per drilled metre, divide the shift air-energy cost by verified acceptable metres drilled in the same conditions. Record rock class, hole diameter, depth, steel and bit condition, operator, flushing and downtime. Without those controls, an apparently cheaper air bill may simply reflect lower production.
Common costing mistakes
- Using the whole shift as trigger time. Breaks, collaring checks, steel changes and delays are not necessarily full-flow drilling.
- Using motor nameplate power. It does not show actual package energy, control losses or auxiliary loads.
- Mixing cfm and m³/min reference bases. Convert units only after confirming both values describe equivalent free-air conditions.
- Ignoring pressure. Tool flow and compressor specific power depend on the applicable pressure and system losses.
- Treating a shared system as linear. Part-load efficiency and sequencing can change incremental cost.
- Comparing electricity cost without production. The lowest cost per shift is not automatically the lowest cost per acceptable metre.
Buyer checklist before approving the estimate
- Identify the decision: budget, supplier comparison, leak investigation or cost per metre.
- Define the compressor package boundary and list excluded auxiliaries.
- Verify drill flow, pressure and reference conditions.
- Use actual trigger time or document the assumed duty factor.
- Use matched energy and flow intervals where possible.
- State tariff components included and excluded.
- Calculate a sensitivity range for uncertain flow, trigger time and tariff.
- Keep production data separate, then combine only when both periods and conditions match.
- Archive meter identifiers, calibration status, timestamps and calculation version.
Frequently asked questions
Can catalogue cfm be used to calculate cost?
It can support a preliminary range only when working pressure, reference conditions and test basis are stated. Measured representative flow is stronger evidence for an operating decision.
Should compressor motor kW or package kW be used?
Use measured package energy or a documented package-specific-power value. Motor nameplate kW is a rating, not proof of actual system energy during the drilling interval.
How is trigger time different from duty cycle?
Trigger time is the accumulated period of air-consuming operation. Duty cycle expresses that time as a fraction of a defined observation period. Always state the observation period.
Can two rock drills be costed by doubling one-drill energy?
Only as a clearly labelled screening assumption. The compressor station may move to a different operating point, sequence another compressor or incur different unload losses. Matched station measurements are preferred.
Does lower air demand guarantee lower cost per metre?
No. Cost per metre also depends on penetration rate, acceptable hole output, tool consumption, labour and downtime. Compare air-energy cost and production under equivalent drilling conditions.
Technical references
- ISO 1217:2009, Displacement compressors — Acceptance tests
- ISO 11011:2013, Compressed air — Energy efficiency — Assessment
- Compressed Air & Gas Institute: Working with compressed air
Prepare a comparable rock-drill RFQ
Use the calculation block above to standardize air-demand evidence, trigger time and commercial assumptions before comparing offers. Review PerfoMax’s pneumatic rock drill range, then send the required hole diameter, rock condition, working pressure, available compressor data, shift pattern and destination with your RFQ. PerfoMax can help organize the product and interface information; site energy allocation should be confirmed with your compressor or energy specialist.