- Author
- Byron Raal, CAS Founder-Editor About the author
- Checked against
- ISO 11011
- Date last checked
- 29 August 2026
A compressed air energy audit in Australia follows ISO 11011:2013 methodology: measure supply-side performance (compressor power, specific power, pressure profile), demand-side consumption (flow at branches and end uses), and losses (leaks, which can reach 20-30% on a poorly maintained system, artificial demand, inappropriate uses). At ~$0.30/kWh AU industrial tariff, electricity is commonly estimated at 70-80% of compressor lifetime cost. NSW Energy Savings Scheme funds eligible upgrades. If a grant round can part-fund the work, the audit doubles as application evidence: check the current grants and incentives map first.
How do I run a compressed air energy audit?
Compressed air energy audits in Australia follow ISO 11011:2013 methodology, which structures the assessment across four areas: demand profiling (logged kWh against measured air flow), specific power benchmarking (kW per L/s FAD against published reference), pressure band analysis (reducing setpoint by 1 bar typically saves 6 to 7 per cent of energy), and heat recovery potential (50 to 80 per cent of compressor electrical input is available as recoverable thermal output).
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Why compressed air is the most expensive utility on the plant
Compressed air burns budget twice: first at the meter, then again through heat loss, leaks, pressure overshoot, inappropriate uses, and fixed-speed part-load waste. That’s why it typically costs seven to ten times the electricity that produced it, measured per unit of useful work delivered at the end-use. The compounded inefficiencies sit in five categories: heat loss at compression (80-90% of input energy), distribution leaks (20-30%), pressure-band overshoot, inappropriate end-uses (open blowing, abandoned drops, oversized regulators), and partial-load efficiency penalties on fixed-speed compressors (per US DOE, Compressed Air Sourcebook audit-baseline guidance).
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</figure>See where the waste sits by sector in our 2026 compressed air leak-cost report.
Page conventions: all pressures are bar gauge (barg) unless otherwise noted. PSI conversions, where shown, are for reference only.
At a typical Australian C&I marginal tariff of approximately $0.30 per kWh for the 2025-26 financial year (actual rates vary by state, retailer, and contract), a 75 kW shaft compressor draws roughly 82 kW total electrical input (per the shaft-vs-total basis below); running 4,000 hours per year at $0.30/kWh, that lands in the order of $98,400 in electricity alone. The audit math is blunt: every percentage point of leak rate on this system is approximately $984 per year of pure waste. A 25% leak rate, a level a poorly maintained site can reach, represents over $24,000 annually that disappears through unmaintained joints. If the provider doesn’t turn that into a ranked opportunity register, it’s just a walk-through (per US DOE, Compressed Air Systems programme). For the formula behind that electricity figure and a full AUD cost table by compressor size, see the air compressor running costs guide.
The audit can pay for itself quickly, with the actual payback depending on the savings it measures, your site’s marginal electricity cost, and the audit and repair costs. First audits of poorly maintained systems can recover 20 to 35 per cent of compressed-air electricity spend, in some cases with payback inside 12 to 24 months. Don’t accept a savings claim that won’t show the data file behind it. For waste-heat opportunities specifically, the heat-recovery ROI calculator quantifies fuel saving and payback period for a compressor retrofit.
Receiver tank sizing: isothermal Boyle’s Law
Receiver tanks buffer short-burst peak demand, and bad receiver maths turns into either nuisance pressure drops or unnecessary steel. Don’t size receivers from catalogue folklore. Sizing them correctly is the single most-misunderstood arithmetic in compressed-air engineering. The valid formula for short-burst (isothermal) demand is:
V_receiver = (Q_excess x t_burst x P_atm) / (P_max − P_min)
Where Q_excess is the excess burst flow in L/s FAD, meaning the burst demand less what the compressor delivers during the burst, not the total peak demand. The receiver only has to supply the shortfall, so sizing on total peak flow while the compressor keeps running buys steel you don’t need. t_burst is burst duration in seconds, P_atm is atmospheric pressure (101.3 kPa absolute), P_max and P_min are the maximum and minimum acceptable tank pressures in absolute kPa.
Worked example A. Excess burst demand of 50 L/s FAD above what the compressor delivers, for a 30-second burst, P_max 800 kPa abs (700 kPa gauge), P_min 700 kPa abs (600 kPa gauge):
V = (50 x 30 x 101.3) / (800 − 700) = 151,950 / 100 = 1,519.5 L.
Worked example B. Excess burst demand of 12 L/s FAD above what the compressor delivers, for a 30-second burst, same pressures:
V = (12 x 30 x 101.3) / (800 − 700) = 36,468 / 100 = 365 L.
A 365 L receiver suits this profile, NOT 3,205 L. The 10x error common in older sizing tools comes from incorrect application of the pressure-ratio term, treating it as a multiplier rather than as the denominator of an absolute-pressure differential.
The formula assumes isothermal expansion (slow burst, full thermal exchange with tank walls, typical for bursts of 5 seconds or longer). For adiabatic fast bursts under 1-2 seconds, increase receiver volume by approximately 30% to allow for thermal-state recovery between bursts.
Power basis: shaft kW versus total electrical input kW
Don’t audit shaft kW as if it were the site electricity bill. Compressor manufacturer datasheets typically quote shaft power, the kW input to the airend rotor. To convert to total electrical input, divide shaft kW by motor efficiency (typically 0.92-0.95 for IE3 or IE4 motors above 7.5 kW) and divide again by drive efficiency (0.97-0.98 for VSDs, 1.00 for direct-drive star-delta).
A 75 kW compressor’s total package input is higher than its nominal rating; the CAGI data sheet for your model gives the real figure. Motor and drive losses alone show why the meter reads higher than the shaft number:
- Premium IE4 motor plus direct drive: 75 / 0.94 / 1.00 = 79.8 kW
- Modern IE3 plus VSD: 75 / 0.93 / 0.98 = 82.3 kW
- Older motor plus older VSD: 75 / 0.92 / 0.97 = 84.0 kW
- High-end conservative: approximately 86 kW
Audits MUST use total electrical input kW for cost calculations. How far the quoted kW understates true energy spend varies by model, so check the CAGI performance data sheet for your machine’s total package input power rather than leaning on a rule of thumb: Atlas Copco’s GA37, for example, pairs a 37.3 kW nominal motor with a 43.3 kW total package input at full load, about 16 per cent higher. Every kW figure in an audit report should declare which basis it uses. “75 kW (shaft)” and “82 kW (total electrical)” are not interchangeable, and the recommendations register changes materially when the basis is corrected.
Specific power benchmarks
If you don’t measure specific power, you don’t know whether the compressor is making air or just making heat. Specific power, kW per L/s FAD or kW per 100 cfm, tells you how much electricity your system spends per unit of air delivered, which is why it is the standard benchmarking unit in US DOE compressed air guidance and on CAGI performance data sheets. Industry-typical 2025 benchmarks at full load, with the same figures converted to cost per cfm:
- Modern VSD rotary screw: approximately 18 kW per 100 cfm (6.4 kW per m³/min FAD; 0.38 kW per L/s FAD).
- Older fixed-speed at part load: 25-34 kW per 100 cfm (9-12 kW per m³/min FAD; 0.55-0.71 kW per L/s FAD).
- Centrifugal at design point: approximately 16 kW per 100 cfm (5.65 kW per m³/min FAD; 0.34 kW per L/s FAD).
- Small reciprocating: 20-23 kW per 100 cfm (7-8 kW per m³/min FAD; 0.42-0.48 kW per L/s FAD).
A site running 9 kW per m³/min where 6.5 is achievable is consuming roughly 38 per cent more energy than the benchmark at the compressor, before considering distribution losses. The first-pass audit calculation is average package kW divided by the simultaneous FAD flow, compared against the appropriate benchmark above. If you’re working from interval totals rather than instantaneous readings, divide the logged kWh by the free air volume delivered over the same interval and convert the benchmark to those units before you compare. Either way the gap is your specific power divided by the benchmark, minus one, expressed as a percentage. Pin that number to the marginal tariff and you have the headline AUD savings figure for the recommendations register.
Audit methodology under ISO 11011:2013
An audit without logged data is a sales visit with a clipboard. So the ISO 11011:2013 framework matters because it structures compressed-air assessments around evidence, not memory (per ISO, ISO 11011:2013 catalogue abstract). Data to collect over a representative cycle (1-2 weeks single-shift, 2-3 weeks multi-shift):
- 15-minute logged compressor electrical input in kW
- Header pressure sampled fast enough for the transient you’re diagnosing, about one second where you’re chasing pressure dips; one-minute storage is fine only where the logger’s faster internal samples preserve each interval’s minimum and maximum
- Total flow at compressor discharge, which is the right point for compressor supply and package performance, though plant-demand transients across downstream storage need flow measured on the demand side of the receiver; or estimated from compressor loaded hours and nameplate FAD on fixed-speed or load/unload machines running near rated loaded output
- Leak survey via ultrasonic detection during a planned non-production window. Use the leak cost calculator to convert orifice size and pressure into the annual AUD figure for the recommendations register
- End-use audit identifying inappropriate uses (open blowing, abandoned drops, oversized regulators)
Output deliverables that justify the audit fee:
- Baseline spend: annual kWh and AUD at the actual marginal tariff.
- Opportunity register: itemised by leak repair, pressure reduction, sequencing optimisation, end-use elimination, drying technology change.
- Per-opportunity economics: payback in months and 5-year NPV, with the data file behind each number disclosed.
What none of that tells you is the invoice. Our guide to audit costs and funding sets out what an audit costs in Australia and the state schemes that can help fund the upgrades it recommends.
What to ask of an audit provider
A manufacturer’s “free” audit isn’t free if it steers the answer before the data is logged. Independent provider versus manufacturer audit: a manufacturer’s audit optimises toward that manufacturer’s equipment recommendations. An independent audit optimises toward lowest total-cost outcome regardless of brand (per CAGI, Working with compressed air buyer guidance).
Questions that distinguish the two:
- What flow-measurement instrumentation will be used and what is its current calibration certificate?
- What sample-rate logger, and for how many days?
- Does the provider have any commercial relationship with the equipment they will recommend?
- Will the audit report disclose calculation assumptions and source data files?
- Is the audit conducted under ISO 11011:2013 or equivalent?
The answers reveal whether the audit is independent or sales-led. CAS connects facilities with independent audit providers who satisfy the questions above. Weighing a vendor’s free offer against a paid audit? See the truth about free compressed air assessments.
Sourcing an independent compressed-air audit
What CAS does. Tell us about your facility, operating pattern and the evidence you need from an energy assessment. We can review your brief and discuss a suitable Australian provider if one is available. We ask for your written permission before an introduction and explain any referral payment arrangement. There is no cost to enquire.
What to send us. Facility type, installed compressor make and rated kW, operating hours, pressure set point, postcode, and the reason for the audit: for example an energy bill increase, reliability concern, capital review or reporting requirement. We can review your brief and discuss a suitable Australian provider if one is available. We ask for your written permission before an introduction and explain any referral payment arrangement. There is no cost to enquire.
Form not loading? Email us directly at byron@compressedairsolutions.com.au with your enquiry.
Frequently asked questions
How big a receiver does my compressed air system actually need?
Size receivers from the excess of short-burst demand over what the compressor delivers, using V = (Q_excess x t_burst x P_atm) / (P_max − P_min), with Q in L/s FAD, t in seconds, P in absolute kPa. A 12 L/s FAD excess burst for 30 seconds with 100 kPa swing (700-800 kPa abs) needs 365 litres, not the 3,000-plus litres the wrong formula returns. The most common mistake is treating the pressure ratio as a multiplier instead of the absolute-pressure denominator. For very fast bursts under 1-2 seconds, add about 30 percent for thermal recovery.
Why does the audit need shaft kW and total electrical input kW separately?
Because they aren’t the same number. Manufacturer datasheets typically quote shaft kW at the airend. The site electricity meter sees total electrical input, which is higher by motor and drive losses. Divide shaft kW by motor efficiency (0.92-0.95 for IE3 or IE4 above 7.5 kW) and again by drive efficiency (0.97-0.98 for VSDs, 1.00 for direct-drive). A 75 kW compressor’s total package input is higher than its nominal rating, and the CAGI performance data sheet for your machine states the real figure, which varies by model (Atlas Copco’s GA37, for example, pairs a 37.3 kW nominal motor with a 43.3 kW package input, about 16 per cent higher). Auditing against shaft kW understates energy spend and leaves money on the table at the recommendations stage.
What specific power should my compressor be hitting?
Specific power is kW per 100 cfm (or kW per m³/min FAD). 2025 benchmarks at full load: modern oil-injected rotary screw with VSD about 18 kW per 100 cfm (6.4 kW per m³/min FAD; 0.38 kW per L/s FAD); centrifugal at design point about 16 kW per 100 cfm (5.65 kW per m³/min FAD; 0.34 kW per L/s FAD); older fixed-speed rotary screw at part load 25-34 kW per 100 cfm (9-12 kW per m³/min FAD); small reciprocating 20-23 kW per 100 cfm (7-8 kW per m³/min FAD). If your measured specific power, average package kW divided by simultaneous FAD flow, lands above the achievable benchmark for your machine class, the size of the gap is what matters: divide your figure by the benchmark, subtract one, and that is your excess energy at the compressor before distribution losses are even counted. A site running 9 kW per m³/min where 6.5 is achievable sits 38 per cent above.
What does an ISO 11011:2013 compliant audit actually involve?
Logged data over a representative cycle: 15-minute interval compressor electrical input in kW, header pressure sampled about every second where pressure dips are in question, with one-minute storage acceptable only where interval minima and maxima are preserved, total flow at the compressor discharge for supply and package performance, measured on the demand side of the receiver where plant-demand transients are the question, or estimated from compressor loaded hours and nameplate FAD on fixed-speed or load/unload machines near rated loaded output, an ultrasonic leak survey during a planned non-production window, and an end-use audit identifying inappropriate uses (open blowing, abandoned drops, oversized regulators). Single-shift sites need 1-2 weeks of data; multi-shift sites need 2-3 weeks. The deliverable is a baseline kWh and AUD spend at your actual marginal tariff plus an itemised opportunity register with per-opportunity payback in months and 5-year NPV.
Should I use the equipment manufacturer or an independent auditor?
A manufacturer audit optimises toward that manufacturer’s equipment recommendations. An independent audit optimises toward lowest total-cost outcome regardless of brand. Test the distinction with these questions: what flow-measurement instrumentation will be used and what is its calibration certificate; what sample-rate logger and for how many days; does the provider have any commercial relationship with the equipment they will recommend; will the audit report disclose calculation assumptions and source data files; is the audit conducted under ISO 11011:2013 or equivalent? If the answers waver, the audit is sales-led, not independent.
Get Matched with an Energy Audit Specialist
Describe your compressed air system and the evidence your finance team needs for an investment decision. We can review your brief and discuss a suitable Australian provider if one is available. We ask for your written permission before an introduction and explain any referral payment arrangement. There is no cost to enquire.
Related: The Hidden Cost of Compressed Air Leaks in Australian Industry (2026) sizes the national cost of the leaks an energy audit uncovers.