The audit process for ISO 50001

The certification audit is the moment when an external auditor from an accredited body checks whether your energy management system meets ISO 50001:2018 . You can get the official standard from NEN . This page explains what to expect, what auditors look at specifically, and how to prepare.

Most companies receive a few minor nonconformities at their first audit. That is normal. You resolve them within 90 days and receive your certificate. An audit without any findings is the exception rather than the rule.

The internal audit for ISO 50001

Before the external auditor arrives, you carry out an internal audit yourself. ISO 50001 requires this: you check your own energy management system and look for shortcomings before someone else finds them. Think of it as a dress rehearsal. If you take the internal audit seriously, you will have far fewer surprises at the certification audit. A detailed explanation of clause 9.2, energy-specific audit points and a full checklist are on the page internal audit for ISO 50001.

Who carries out the internal audit?

An internal auditor may not audit their own work. So an energy manager does not audit the part they set up themselves. Smaller companies solve this by training someone from another department, or by hiring an external auditor for one day. Already have ISO 14001? Then your existing internal auditors can take this on after a short refresher in energy management.

What do you check during the internal audit?

The internal audit follows the same logic as the external audit, but you do it yourself. You go through the whole system and record your findings.

Energy review and SEUs: is the data still correct? Have any new large consumers been added?
EnPIs and baseline: are the energy performance indicators being measured and reported?
Action plans: is there progress? Are the saving measures being carried out?
Awareness: do employees know what the energy objectives are?
Previous findings: have the points from the last audit been resolved?

What do you produce?

You write an audit report with findings and assign actions to them. You resolve the nonconformities before the external audit. The external auditor wants to see that your internal audit has delivered something real. A report that says “everything is fine” with zero findings raises more suspicion than confidence.

A common mistake: scheduling the internal audit too late, just before the certification audit. Do it at least 6-8 weeks in advance. Then you have time to genuinely resolve the findings instead of quickly papering over them.

The two phases of certification

The certification audit consists of two phases. Both must be successful before you receive the certificate.

Phase 1: documentation review

The auditor checks whether you are ready on paper. They review your energy policy, your energy review, your SEU register, your objectives and your action plans. The aim: establish whether your EnMS is worked out in enough detail to justify phase 2.

Phase 1 can take place on site or remotely. It lasts half a day to a full day for smaller organisations, and 1-2 days for larger ones.

What the auditor checks:

Is there an energy policy, signed by top management?
Is the scope clearly defined?
Has the energy review been carried out and documented?
Have SEUs been identified with clear criteria?
Are objectives formulated as SMART?
Are roles and responsibilities recorded?

After phase 1 you receive a report. If there are shortcomings, you have time to resolve them before phase 2 takes place. Sometimes phase 2 is postponed if the documentation is not yet in order.

Phase 2: implementation audit

The auditor comes to your site. They check whether your EnMS not only exists on paper but also works in practice. This is the more intensive part.

What happens:

Interviews with the energy manager, top management, facility managers and operators. The auditor does not only ask “what does the policy say?” but also “how do you do that in practice?”
A walk-through of your significant energy aspects: your compressors, HVAC installation and production line. The auditor wants to see that you know where your large consumers are and what you do about them.
A data check. The auditor reviews your EnPI dashboards, measurement data and trend analyses. Do the figures add up? Is measurement actually taking place? Are the meters calibrated?
Records. Internal audit reports, management review minutes, training records, action plans with progress.

Duration of phase 2:

Small company (fewer than 50 employees): 1-2 days
Medium-sized (50-250): 2-4 days
Large (250+, multiple sites): 4-10+ days

The auditor is not looking for perfection. They are looking for a working system that demonstrably leads to improved energy performance. A company that does not fully meet its targets but can explain why and what it is doing about it scores better than a company with good figures but no substantive story.

Possible outcomes

After the audit you receive a report with findings. There are four categories:

Conformities: points where you comply. Confirmation that things are going well.

Observations: areas for improvement that are not a nonconformity. The auditor sees room for improvement, but you meet the requirement. Action is not mandatory, but it is sensible.

Minor nonconformities: shortcomings that must be resolved, but do not fundamentally undermine the system. You have 90 days to provide evidence of correction. The certificate is issued once they are closed.

Major nonconformities: serious shortcomings. The certificate cannot be issued until you resolve the nonconformity and the auditor verifies this, often through a follow-up audit. At a first certification this rarely happens if you have prepared well.

Common findings in the ISO 50001 audit

ISO 50001 audits produce specific findings that you see less often with other ISO standards. Below are the most common.

Incomplete energy review

The auditor finds that your energy review is not detailed enough. “We use 500 MWh” is not sufficient. You must be able to show where those 500 MWh go, by process or system.

Prevention: Invest in sub-metering. Make sure you can assign at least 80% of your consumption to specific processes or installations.

SEU criteria not transparent

You have identified SEUs, but the auditor cannot follow why these particular processes are significant. The criteria (high consumption, saving potential, variability) must be documented and traceable.

Prevention: Create a matrix with scores per criterion. Then it is clear to everyone why compressors are an SEU and lighting is not.

Energy baseline not normalised

Your baseline is an absolute figure (for example 500 MWh in 2024), but you have not corrected for variables such as production volume or weather conditions. If production grows, your consumption appears to rise, while you may in fact have become more efficient per unit.

Prevention: Define the relevant variables and normalise your baseline. Use regression analysis if you have the expertise, or simple ratios (kWh per product) if that is enough.

EnPIs not reported regularly

You have defined EnPIs but do not report them in a structured way. The auditor wants to see a trend: monthly or quarterly data over at least 3-6 months.

Prevention: Set a fixed reporting moment (for example the first week of the month). Automate where possible through monitoring software.

Action plans without progress

Action plans exist, but little has been done with them. No quick wins carried out, no budget set aside, no one made responsible.

Prevention: Start with the low-hanging fruit: LED lighting, thermostat optimisation, leak detection in compressed air. Show the auditor that something is actually happening.

Staff not aware of energy objectives

The auditor speaks to an operator who does not know that energy objectives exist, or a facility manager who cannot explain what an SEU is.

Prevention: Communicate actively. A poster on the shop floor, an item in the team meeting, a dashboard next to the coffee machine. It does not need to be formal, but it needs to be effective.

Competence not demonstrated

People who work with SEUs must demonstrably be competent. The auditor may ask: “Has this operator been trained to work energy-efficiently with the compressors?”

Prevention: Create a competence matrix. Which roles work with which SEUs? What training have they had? Document this.

Preparing for the ISO 50001 certification audit

Two weeks before the audit

Review the documentation:

Energy policy up to date and signed by top management?
Energy review complete (all energy flows, consumption per process)?
SEU register with criteria and prioritisation?
Energy baseline established and normalised?
EnPIs defined with at least 3 months of data?
Action plans with progress (who, what, when, status)?
Internal audit carried out and findings addressed?
Management review held, minutes available?
Roles and responsibilities documented?

One week before the audit

Prepare people:

Inform employees that the audit is coming
Give a short briefing: what is ISO 50001, what are our objectives, what might the auditor ask?
Stress: answer honestly. Not knowing is better than making something up
Make sure the energy manager, top management and operators are available

Check the site:

Walk past your SEUs. Are the meters present and working?
Are work instructions in place where they should be?
Is the dashboard or reporting up to date?
No visible waste (machines running unnecessarily, doors open with climate control on)?

During the audit

Appoint someone to accompany the auditor and take notes
Answer questions honestly and concisely. Do not go off on tangents
If you do not know something, say so and offer to provide it later
If the auditor records a finding and you do not understand it, ask for clarification
The audit begins with an opening meeting and ends with a closing meeting in which the findings are presented

After certification: surveillance and recertification

Your certificate is valid for 3 years. During those 3 years, you follow this schedule:

Year	Audit type	Scope	Cost (indication)
1	Certification audit	Full (phase 1 + 2)	€8k-€25k
2	Surveillance audit	Selection of elements	€5k-€15k
3	Surveillance audit	Selection of elements	€5k-€15k
4	Recertification audit	Full	€8k-€20k

Surveillance audits

Every year the auditor returns for a shorter check. The whole system is not audited, but a selection of it. Over 3 years everything must have been covered.

What the auditor looks at:

Are you meeting your energy objectives? Are your EnPIs improving?
Are action plans being carried out?
Is there demonstrable improvement compared with last year?
Have the findings from the previous audit been resolved?

If the surveillance audit reveals serious shortcomings, your certificate can be suspended. This rarely happens if you maintain the system seriously.

Recertification

After 3 years a recertification audit follows. This is similar to the initial audit: the whole system is reassessed. If successful, you receive a new certificate valid for 3 years.

Tip: Plan your recertification well in advance. If you wait until the last month and there are findings, you risk a gap in your certification.

Choosing a certification body for ISO 50001

You choose who carries out the audit. Well-known certification bodies in the Netherlands:

Important: Choose a body that is accredited by the Dutch Accreditation Council (Raad voor Accreditatie, RvA) . Without RvA accreditation your certificate may not be recognised. You can read how to check an existing certificate on the page about verifying a certificate. What the auditor does during the audit is covered on the auditor page.

Ask for quotes from 2-3 bodies. Rates for the same audit can differ by €5k-€15k. Also look at experience in your sector: an auditor who knows manufacturing companies asks better questions than someone who just works through the checklist.

Integrated audits: If you also have ISO 14001, you can have a combined audit carried out. One auditor checks both standards in one visit. This saves 20-30% on audit costs.

More information

Requirements of ISO 50001: What the standard requires, chapter by chapter
Step-by-step plan: The full route to certification
Energy aspects (SEUs): How to determine significant energy aspects
Costs and timeline: What does the audit and the whole process cost?
What is ISO 50001: Back to overview