An energy audit is a systematic review of how a building, site or organisation uses energy. The auditor collects consumption data, inspects the site, works out where the energy goes, and reports a list of savings measures, each with its estimated energy saving, cost and payback. The European process standard is EN 16247-1:2022, the international one is ISO 50002-1:2025, and in North America ASHRAE Standard 211 defines three audit levels.
An audit is a snapshot. It says what the site used, where, and what could change, on the dates the data covers. It does not keep the savings in place. That is the job of an energy management system such as ISO 50001, and of the metering that shows whether each measure is still working.
Energy audit levels: ASHRAE level 1, 2 and 3
ANSI/ASHRAE/ACCA Standard 211, Standard for Commercial Building Energy Audits, was published in 2018 and reaffirmed in 2023. It defines three levels. Each level includes the work of the one below it. The names below are the ones in common use for ASHRAE audits; what binds is the scope the standard sets for each level.
| Level | Usual name | What the auditor does | What the client gets |
|---|---|---|---|
| Level 1 | Walk-through survey | Analyses 12 months or more of utility bills, benchmarks energy use per m², walks through the building and talks to the operators | Benchmark, obvious no-cost and low-cost measures, a list of areas for closer study |
| Level 2 | Energy survey and analysis | Breaks consumption down by end use, inspects systems, reviews schedules and set points, measures where data is missing | Each measure with its estimated saving, cost and payback, and the interactions between measures |
| Level 3 | Detailed analysis of capital-intensive modifications | Monitors for weeks, models or simulates the systems, prices the work with contractors | Investment-grade figures for large projects such as plant replacement |
A Level 1 audit can rest on bills. A Level 2 audit cannot, because bills give one number per meter per month and the measures are per system. A Level 3 audit nearly always adds temporary or permanent metering, because a capital decision needs measured load profiles, not estimates.
EN 16247 does not use levels. The client and auditor agree the scope, aims, depth and boundaries at the start, and the standard scales the work to that agreement. ISO 50002-1 works the same way, and its companion parts, ISO 50002-2:2025 and ISO 50002-3:2025, give guidance for buildings and for processes.
The EN 16247 process
EN 16247-1 sets the general requirements. Part 2 covers buildings, part 3 processes and part 4 transport, and each adds to part 1 where it applies. Part 5 sets the competence of energy auditors. The audit follows the same steps in every case:
- Preliminary contact. Agree the aims, scope, boundaries, level of detail, deliverables and the criteria for ranking measures.
- Start-up meeting. Name the people who will supply data and access, and agree the data the client must provide.
- Data collection. Bills, interval data, submeter records, equipment lists, operating hours, production or occupancy figures, and any earlier audits.
- Field work. Inspect the plant and the processes, check how they actually run against the data, and take measurements where the data is missing.
- Analysis. Build the energy balance, define a baseline and indicators, and identify and evaluate the savings measures.
- Report. The findings, the energy balance, the ranked measures with their assumptions, and the data the auditor relied on.
- Final meeting. Present the results and agree the next steps.
The energy balance is the check on the whole audit. The end uses the auditor identifies must add up to the metered total. A large unexplained remainder means the breakdown rests on assumptions, and so does every saving calculated from it.
Who needs an energy audit
Most mandatory audit schemes come from Article 8 of the 2012 EU Energy Efficiency Directive and its recast. The table gives the current rules in Great Britain and the main EU markets.
| Scheme | Who is in scope | Duty and frequency | Current deadline |
|---|---|---|---|
| EU Energy Efficiency Directive (EU) 2023/1791, Article 11 | Enterprises above 10 TJ (about 2.8 GWh) a year, averaged over three years, without a certified energy management system | Energy audit at least every four years | Was due by 11 October 2026 (first audit under the recast rules), then at least every four years |
| Same Directive, above 85 TJ (about 23.6 GWh) a year | Enterprises above the higher threshold | Certified energy management system instead of audits | In place by 11 October 2027 |
| ESOS, Great Britain and Northern Ireland | 250 or more employees, or turnover above £44 million and balance sheet above £38 million | Audit of at least 95% of total energy, or ISO 50001, every four years | Phase 4 notification by 5 December 2027 |
| Spain, Real Decreto 56/2016 | Large companies and groups, under the national size test until later Spanish law applies the energy test | Audit of at least 85% of final energy every four years, or a certified management system that includes one | See the Spanish guide |
| Italy, Decreto legislativo 102/2014, Article 8 | Large companies and registered energy-intensive companies | Audit of Italian production sites every four years, results sent to ENEA | See the Italian guide |
| Germany, Energieeffizienzgesetz (EnEfG), section 8 | Companies above 7.5 GWh a year of final energy, averaged over the last three years | Energy management system to ISO 50001 or environmental management system under EMAS | Companies already covered in November 2023: by 18 July 2025. See the EnEfG guide |
| France, Code de l'énergie, Articles L233-1 to L233-3, as amended by Law 2025-391 | Companies at or above 2.75 GWh a year of final energy, averaged | Energy audit every four years unless a management system is in place; a certified ISO 50001 system at or above 23.6 GWh | Set by Decree 2025-1382 of 29 December 2025 |
The recast Directive replaced the old size test with an energy test, which Member States applied from 11 October 2025. A mid-sized manufacturer that was exempt may now have an audit deadline, and a large office company that uses little energy may not. The EED guide explains how to count energy against the thresholds. The ESOS guide covers the phase 4 rules in Great Britain.
Annex VI of the Directive sets minimum criteria for every Article 11 audit. The audit must use up-to-date, measured, traceable operational data, including load profiles for electricity. It must review the consumption profile of the buildings, processes and transport, use life-cycle cost analysis instead of simple payback wherever possible, and be representative enough to give a reliable picture of overall energy performance.
Outside the mandatory schemes, many audits are voluntary or tied to funding. In Ireland, the Sustainable Energy Authority of Ireland (SEAI) offers a €2,000 voucher towards an audit under its Support Scheme for Energy Audits. SEAI changed the scheme's eligibility criteria in October 2026, so check its current terms before applying.
The data an energy audit needs
An auditor can only analyse the data the site can produce. Prepare it before the start-up meeting.
| Data | Why the auditor needs it | Where it comes from |
|---|---|---|
| 12 to 36 months of bills for every fuel | Total consumption, cost and the EU threshold test | Supplier invoices, fuel card records |
| Electricity interval data, 15 or 30 minutes | Load profile, baseload, peaks and out-of-hours waste | Fiscal meter data from the supplier or network operator |
| Submeter data for the significant uses | Energy by system or process, in place of estimates | Existing submeters, or temporary metering for the audit |
| Relevant variables | Production volume, occupancy, opening hours and degree days, to normalise consumption | Production records, building management system, weather data |
| Asset list | Ratings, ages and operating hours of the main plant | Maintenance records, nameplates |
| Schedules and set points | What each system should be doing, to compare with what it does | Building management system, operators |
Interval data finds savings that bills cannot. A flat line at 02:00 on a Sunday is the site's overnight baseload, and in most audits the first measures come from it. Submeters do the same for each system: they show the compressor that runs all weekend to feed leaks, or the chiller that never turns down. Without them, the auditor divides the total by nameplate ratings and assumed hours, and the energy balance closes by construction.
The submetering guide explains which circuits to meter first. Meter the significant uses before the audit if you can. A year of measured data makes the audit cheaper and the savings estimates more reliable.
Worked example: an audit of a moulding plant
A plastics moulding plant uses 2,400,000 kWh of electricity and 1,100,000 kWh of natural gas a year, 3,500,000 kWh in total. At 3.6 MJ per kWh that is 12,600 GJ, or 12.6 TJ. If its three-year average is similar, it is above the EU 10 TJ audit threshold and well below the 85 TJ threshold for an energy management system. The energy unit converter puts other fuels on the same scale.
The site has half-hourly electricity data and submeters on the compressors, the moulding machines and the chilled water system. The auditor finds five measures. The prices below are the plant's own assumptions for this example: €0.20 per kWh of electricity and €0.06 per kWh of gas.
| Measure | How it was found | Annual saving | Value per year | Cost | Simple payback |
|---|---|---|---|---|---|
| Switch off barrel heaters at weekends: 45 kW × 40 h × 50 weeks | Moulding machine submeter, weekend profile | 90,000 kWh electricity | €18,000 | €1,500 | 0.08 years |
| Repair compressed air leaks: off-shift load from 30 kW to 10 kW for 2,900 h | Compressor submeter, non-production hours | 58,000 kWh electricity | €11,600 | €4,000 | 0.34 years |
| Insulate boiler house valves and flanges | Field inspection, thermal camera | 60,000 kWh gas | €3,600 | €3,000 | 0.83 years |
| Variable speed drive on the chilled water pump | Chilled water submeter, constant 15 kW | 43,920 kWh electricity | €8,784 | €9,000 | 1.02 years |
| Replace 20 kW of lighting with LED, saving 11 kW for 4,000 h | Lighting survey | 44,000 kWh electricity | €8,800 | €26,000 | 2.95 years |
| Total | 295,920 kWh | €50,784 | €43,500 | 0.86 years |
Simple payback is cost divided by annual value: €26,000 ÷ €8,800 = 2.95 years for the lighting. Together the five measures save 295,920 kWh, 8.5% of the site's total.
The pump figure assumes the pump can run at 80% flow and that its power falls with the cube of speed: 15 kW × (1 − 0.8³) = 7.32 kW, for 6,000 hours a year. A real system with static head saves less, so a Level 3 audit would measure the pump at reduced speed before committing to it. The Directive also asks for life-cycle cost analysis wherever possible. Simple payback ignores the lifetime of each measure, so the LED lighting, which lasts many years, looks worse than it is next to a leak repair that must be repeated.
The two fastest paybacks came from submeter data, not from the walk-through. Neither shows on a monthly bill. The same submeters then become the measurement for each saving. The savings baseline guide explains how to adjust the before-and-after comparison for production, and the compressed air specific power calculator checks whether the compressed air plant has improved per m³ delivered.
Energy audit or ISO 50001?
An audit produces a report. ISO 50001 produces a management system: an energy policy, an energy review, energy performance indicators, targets, action plans, internal audits and a management review, certified by an external body and kept up every year. ISO 50002-1 audits can supply the energy review that ISO 50001 requires.
The two routes are alternatives in law. The EU Directive requires a certified energy management system above 85 TJ and accepts one in place of audits above 10 TJ. ESOS accepts ISO 50001 certification as a route to compliance. A site that audits every four years and does nothing in between usually finds the same measures at the next audit, because nobody measured whether the last ones stayed in place.
Measurement for audits with EpiSensor
EpiSensor makes measurement hardware and software, not audits. ZEM electricity monitors submeter the significant electricity uses, and report over a Zigbee mesh, so they are installed without running data cables to each panel. ZPC pulse counters read the pulse outputs of existing gas, water and heat meters. A Gateway running Edge keeps the interval history on site, and the auditor can export any selection of it to CSV from the Edge Data page.
Install the meters before the audit, so the auditor works from a measured year, and keep them after it, so each measure has a before and after. The energy management page describes the full measurement plan.
Common questions
What is an energy audit?
A systematic review of how a building, site or organisation uses energy. The auditor collects consumption data, inspects the site, finds where energy is used and wasted, and reports a list of savings measures with their estimated energy savings, costs and payback. EN 16247-1 and ISO 50002-1 set out the process.
What are the levels of an energy audit?
ASHRAE Standard 211 defines Level 1, a walk-through survey with a utility bill analysis and low-cost measures; Level 2, an energy survey and analysis that breaks consumption down by end use and costs each measure; and Level 3, a detailed analysis of capital-intensive measures, usually with extra monitoring or simulation. European audits follow EN 16247, which scales the depth to the agreed scope instead of fixing levels.
Who needs an energy audit?
In the EU, Article 11 of Directive (EU) 2023/1791 requires an audit from every enterprise whose average annual energy consumption over the previous three years exceeds 10 TJ, unless it has a certified energy management system. National law sets the details. In Great Britain, ESOS applies to undertakings with 250 or more employees, or with turnover above £44 million and a balance sheet above £38 million.
How often is an energy audit required?
At least every four years under the EU Energy Efficiency Directive. The first audit under the recast rules was due by 11 October 2026, then at least every four years. ESOS also runs in four-year phases; phase 4 notifications are due by 5 December 2027.
How long does an energy audit take and what does it cost?
It depends on the level, the number of sites and how much data already exists. A walk-through of one building with a year of bills is short. A detailed audit of an industrial process that needs new temporary metering takes weeks of measurement before the analysis starts. Ask auditors to quote against a written scope, the boundaries, the level and the deliverables.
What is the difference between an energy audit and ISO 50001?
An audit is a point-in-time assessment that produces a list of measures. ISO 50001 is a continuous management system: an energy policy, an energy review, indicators, targets, action plans and management review, certified by an external body. ISO 50002-1 audits can feed the ISO 50001 energy review, and the EU Directive exempts an enterprise with a certified energy management system from the separate audit duty.