The Fourth Utility's Hidden Bill: Auditing Compressed Air Before the 2026/27 Budget
Photo: Rahul Enterprises-ELGI air Compressor dealer, CC0, via Wikimedia Commons
Compressed air is frequently described as the fourth utility, after electricity, gas and water. On many UK industrial and logistics sites it is also the least measured. The compressor house sits at the back of the plant, the pipework disappears into the roof void, and the electricity it consumes arrives on the same bill as lighting, refrigeration and production machinery. Because no one sees a separate line, compressed air is treated as free capacity rather than a cost centre.
That assumption is increasingly difficult to defend. As UK organisations build 2026/27 budgets against volatile wholesale prices and constrained network capacity, every unmeasured load deserves scrutiny. Compressed air is a strong candidate: it is electricity-intensive, often poorly controlled, and unusually prone to invisible waste. The purpose of an audit is not to buy a new compressor. It is to establish what the existing system costs, where the avoidable spend sits, and which interventions earn their place in the capital queue.
Begin with a Meter, Not a Mandate
The first step is a defensible baseline. Without sub-metering, any discussion of compressed air savings is anecdotal. Install a permanent or temporary meter on the compressor input, and where the site runs multiple machines, meter each compressor separately. Include the dryer and ancillaries in the boundary; they are part of the delivered cost of air.
Log at 15-minute intervals for at least four to six weeks. The period should cover production peaks, changeovers, weekend shutdowns and any seasonal variation that affects demand. Reconcile the logged data with the site's half-hourly supply data so that the compressed-air load can be separated from the rest of the estate. The output is a kW profile and an annualised kWh figure that finance can test. Temporary logging is often enough to size the opportunity; permanent metering is what keeps the issue visible after the audit report is filed.
Leak Surveys: The Load That Never Clocks Off
Compressed air leaks are a continuous load. They do not stop at tea break, and they do not appear on a production report. An ultrasonic survey conducted outside operating hours is the standard method: walk the pipework, tag each leak, and estimate its flow. The estimate need not be perfect to be useful, but it should be consistent and recorded.
Avoid relying on generic percentage rules. Calculate the annual cost of each leak using the leak flow, operating hours, specific power and the site's tariff. A leak that costs a few hundred pounds a year may still be worth fixing because the repair is cheap; a cluster of small leaks in a high-pressure line can add up to a material sum. Rank the register by annual cost, access difficulty and safety, then re-survey after repairs to prove the reduction. A leak survey without a follow-up is merely a list.
Duty Cycle and Control Review
Metering and leak data should be read alongside the compressor control strategy. Common issues include machines running unloaded for long periods, multiple compressors trimming against each other, and pressure setpoints set higher than production actually requires. Each bar of unnecessary pressure carries an energy penalty, and the effect compounds across the year.
Review the sequencing of fixed-speed and variable-speed machines, storage capacity, pressure bands and dryer operation. Check condensate drains for continuous bleeding and confirm that the system is not running during non-production hours without a reason. The aim is to match supply to demand rather than to keep the plant pressurised for hypothetical readiness. Control changes are often low-cost, but they need to be tested and documented; a setpoint adjustment made without data is simply a different guess.
Cost per Cubic Metre as a Common Language
A useful benchmark is the cost per cubic metre of delivered air, expressed either in kWh/m3 or £/m3. The metric allows sites to compare performance, track improvement and translate engineering findings into budget language. It also exposes the difference between the compressor's rated specific power and the system's actual specific power once leaks, dryers, pressure drops and control losses are included.
Where flow measurement is unavailable, a temporary flow meter can be installed for the audit period. The resulting figure should include energy, maintenance and dryer costs, and be stated with its boundary conditions. Benchmarks are only useful if they are comparable; a cost per cubic metre that excludes the dryer or assumes unrealistic operating hours will mislead the capital plan.
Ranking Repairs Against Replacement
Once the baseline, leak register and duty-cycle review are complete, the site can rank interventions using simple payback and sensitivity. For 2026/27 budgeting, a central electricity price of around 24p/kWh is a reasonable planning assumption for many UK business sites, with sensitivity tested at 20p and 30p. The exact figure should reflect the site's supply contract and flexibility position.
A leak repair costing £250 and saving 8,000 kWh a year is worth £1,920 at 24p/kWh, with a payback measured in weeks. A control upgrade costing £8,000 and saving 25,000 kWh a year is worth £6,000, a payback of around 16 months. A replacement compressor costing £60,000 and saving 40,000 kWh a year is worth £9,600, a payback of more than six years. Replacement may still be justified by reliability, maintenance costs or the need for greater capacity, but it should not be the first answer to a leak and control problem.
The ranking should also reflect risk. Leak repairs are low-cost and reversible; control changes require operating discipline; replacement carries capital commitment and commissioning risk. A marginal abatement cost curve, even a simple one, helps the board see the order of operations rather than a single large request.
Making the Audit Operational
An audit that ends with a report will not protect the 2026/27 margin. Assign ownership of the compressed-air cost centre, report kWh/m3 monthly, maintain a leak tag register, and review the control strategy at each production change. Integrate the findings into the site's capital plan and into the next energy renewal conversation. Compressed air is not invisible once it is metered; it becomes a managed utility with a cost, a benchmark and a plan.
