Significant energy aspects (SEUs) in ISO 50001

Significant Energy Uses (SEUs) are the processes, systems or equipment that have a large impact on your energy use. This concept is a core part of ISO 50001:2018  and is defined in clause 3.32 of the standard. SEUs form the basis of your energy management system: by focusing on the biggest consumers, you achieve the largest savings. For more background, see the explanation by NEN on ISO 50001 . All terms from the standard (EnPI, EnMS, energy baseline, SEU) are explained in the ISO 50001 glossary.

What is an SEU?

A Significant Energy Use is an energy user (process, system or device) that meets one or more of these criteria:

1. Large use It represents a substantial share of your total energy use (often above 10%, but this can differ per organisation).

2. Large savings potential Even if the use is not enormous, there is a lot to gain from improvement (old, inefficient equipment, a poorly controlled process).

3. High variability Use fluctuates strongly, which points to optimisation opportunities (for example, a process that sometimes runs efficiently and sometimes does not).

4. Large future impact You are about to invest or expand, so now is the time to secure efficiency.

You do not have to meet all four. One criterion can be enough to classify something as an SEU.

Steps to determine your SEUs

Determining your SEUs follows five steps. Below, you see for each step what you do and what it delivers.

Below, we work out each step with examples.

Step 1: Energy review

You start with a full energy review: where does your energy go? You do this through:

• Invoice analysis: Historical use per energy type (electricity, gas, heat, etc.)
• Measurements: Submeter cabinets, loggers and sensors to measure use per process or department
• Walkthrough: A walk around the shop floor with technical staff

Output: Overview of all energy flows and users.

Step 2: Map the use

You make an overview (often a Sankey diagram or table) that shows: of the 100% of energy that comes in, where does it go?

Example: production company

• 35% compressors (compressed air)
• 25% climate control (HVAC)
• 20% production process (ovens, presses)
• 10% lighting
• 10% other (office, IT, canteen)

You see immediately: compressors, HVAC and the production process are the big items.

Step 3: Apply the criteria

For each large consumer, you check:

• What percentage of the total? (above 10% = probably an SEU)
• How old or efficient is it? (old equipment = savings potential)
• How stable is it? (strong fluctuations = can be optimised)
• Can we influence it? (if it is outside your control, it is not an SEU)

Example: Compressors use 35%. They are 15 years old. Their performance varies (sometimes 80% load, sometimes 20%, but they are always on). This is an SEU based on large use and savings potential (old compressors, no variable speed drive).

Step 4: Prioritise

You now have a list of potential SEUs. Make a matrix:

Priority 1: Large use + high savings potential + feasible (→ compressors) Priority 2: Medium or large use + feasible (→ HVAC, lighting) Priority 3: Large use but complex or little to gain (→ production process)

You focus on priorities 1 and 2 first.

Step 5: Document

For each SEU, you document:

• Description: What is it? (for example, “Compressor system: 3 screw compressors supplying compressed air to the factory”)
• Use: How much (kWh per year, m³ of gas per year, etc.)
• Percentage of total: For example, 35%
• Savings potential: An estimate (for example, “20% possible with a new compressor with variable speed drive”)
• Current status: Age, condition, maintenance
• Priority: High, medium or low

Output: SEU register (Excel, or in your ISO 50001 software).

Examples of SEUs per sector

Production companies (metal, food, chemicals)

Typical SEUs:

• Compressors (compressed air): Often 20 to 40% of electricity use. Old compressors run constantly, even at low demand. A variable speed drive saves 20 to 40%.
• Ovens and drying ovens: Gas or electric. Insulation, heat recovery and optimising cycle times deliver savings.
• Cooling and cold stores: Running 24/7, so insulation and efficient cooling machines are crucial.
• HVAC (climate control): Large factories with a lot of space to heat or cool. Zone control, night setback and flow control help.
• Motors and pumps: Hundreds of electric motors. Frequency drives on large motors (above 5 kW) save 10 to 30%.

Example: A metal company identifies compressors (35%), ovens (25%) and HVAC (15%) as SEUs. Together, they make up 75% of use. Its action plans focus on these.

Data centres

Typical SEUs:

• IT equipment (servers, storage, network): This is your core business and hard to reduce, but you can optimise it (replacing older servers with more efficient ones).
• Cooling (CRAC units, chillers): Often 30 to 50% of total use. Improve PUE through free cooling, hot/cold aisle containment and higher temperatures.
• UPS (uninterruptible power supply): Losses in the UPS (5 to 10%). A more efficient UPS or eco-mode saves energy.
• Lighting: Relatively small (5%), but LED with sensors is a quick win.

Example: A data centre identifies cooling (40%), IT load (45%) and UPS losses (8%) as SEUs. Goal: reduce PUE from 1.6 to 1.3 through cooling optimisation and free cooling.

Office buildings and real estate

Typical SEUs:

• HVAC: 40 to 60% of use. Smart thermostats, zone control and night setback deliver 15 to 25% savings.
• Lighting: 20 to 30% of use. LED with motion sensors and daylight control save 50 to 70%.
• IT and servers: Server rooms and workstations. Virtualisation and cloud migration reduce local server load.
• Lifts and fans: Significant in high-rise buildings. Frequency drives and optimised maintenance help.

Example: An office building (10,000 m²) identifies HVAC (50%), lighting (25%) and IT (15%) as SEUs. LED lighting with sensors plus optimised thermostat settings deliver 20% total savings.

Hotels and hospitality

Typical SEUs:

• HVAC: Rooms, lobby and conference rooms. Smart room thermostats (no heating when a room is empty) save 20 to 30%.
• Hot water: Large boilers for showers and the kitchen. Heat pumps, solar boilers and pipe insulation help.
• Kitchen equipment: Ovens, cold rooms and freezers. Insulation, efficient appliances and shutdown schedules (not running 24/7).
• Lighting: Lobby, corridors, rooms and car park. LED with sensors.
• Swimming pool (if present): A huge consumer (heating and filtration). Covers and heat recovery are essential.

Example: A hotel (100 rooms) identifies HVAC (35%), hot water (25%), kitchen (15%) and swimming pool (15%) as SEUs. Smart thermostats in rooms plus a pool cover deliver 15% savings.

Hospitals and care institutions

Typical SEUs:

• HVAC: Running 24/7, with strict requirements (temperature, air quality). Zone control helps (operating theatres need a higher standard than office spaces).
• Medical equipment: MRI and CT scanners, sterilisation. Hard to optimise (safety comes first), but efficient replacement is possible.
• Hot water: Legionella-safe, so high temperatures. Heat pumps with Legionella protection help.
• Lighting: 24/7 operation, large floor area. LED and smart management.
• Kitchen and laundry: Large consumers. Heat recovery from the laundry to the heating circuit.

Example: A hospital identifies HVAC (40%), medical equipment (20%), lighting (15%) and hot water (10%) as SEUs. Focus on HVAC zone control and LED lighting.

How many SEUs should you have?

No fixed number. It depends on your organisation.

Rules of thumb:

• Simple organisation (1 site, uniform process): 3 to 7 SEUs
• Average complexity (several departments, varied processes): 5 to 12 SEUs
• Complex organisation (several sites, different sectors): 10 to 20+ SEUs

Too few SEUs (for example, 1 or 2): You are probably missing things. 80% of your use should be covered by your SEUs.

Too many SEUs (for example, 30+): You are becoming too fragmented. Focus on the truly large and influenceable ones.

Check: Your SEUs together should cover at least 70 to 80% of your total use. If your SEUs together cover only 50%, you are being too selective.

Updating SEUs

SEUs are not static. You reassess them:

Annually: At the management review, check whether the SEUs are still correct. Are there new large consumers? Have you taken measures that mean an SEU is no longer significant?

After major changes:

• New production line installed → likely becomes an SEU
• Old compressors replaced → possibly no longer an SEU (use has dropped significantly)
• Building extension → HVAC may become a larger SEU

Example: A company had compressors as an SEU (35% of use). After investing in a new efficient compressor with variable speed drive, use drops to 20%. It remains an SEU (still large), but the priority shifts to other SEUs (for example, HVAC, now 30% of use).

SEUs and action plans

For each SEU, you make an action plan. This contains:

• Objective: For example, “Reduce compressor use by 20% within 18 months”
• Measures: Concrete actions (new compressor, leak detection, lower pressure)
• Responsible person: Who takes this on?
• Budget: What does it cost?
• Savings: What does it deliver (kWh per year, € per year)?
• Schedule: When to start, when to finish?

Not for every SEU at once: You choose which SEUs you tackle in year 1 and which in year 2. This depends on budget, feasibility and ROI.

Example action plan:

SEU: Compressors (35% of use, 520 MWh per year) Objective: 20% reduction = 104 MWh saving Measures:

1. Leak detection and repair of leaks (start Q2 2025, cost €3k, saving 5%)
2. Lower pressure from 8 to 7 bar (start Q3 2025, cost €1k, saving 8%)
3. New compressor with variable speed drive (start Q4 2025, cost €40k, saving 10%) Total: €44k investment, 23% saving (120 MWh), €14.4k per year ROI: 3.1 years

Common mistakes with ISO 50001 energy aspects

1. Everything is an SEU

If you have 20 or more SEUs in a small company, you are too broad. Focus on the real priorities.

Solution: Use the 80/20 rule. Which 20% of your processes cause 80% of your use?

2. SEUs that are too abstract

“Electricity use” is not an SEU. It is an energy type. An SEU must be a concrete process or system: “Compressors,” “HVAC system building A,” “Production line 3.”

Solution: Be specific enough that you can define actions for it.

3. SEUs you cannot influence

“Energy use of tenants in our building” is not a good SEU if you have no influence over it (unless you can take measures such as sub-metering or efficiency clauses in tenancy agreements).

Solution: Focus on SEUs where you can take action.

4. Forgetting to update the SEU register

You determine SEUs in year 1 but never reassess them. Meanwhile, new consumers have been added or old ones have disappeared.

Solution: Reassess annually at the management review. Use changes, and SEUs change with them.

ISO 50001 SEU checklist

• Energy review done (all use mapped)
• Use per process or system measured or estimated
• Top 10 biggest consumers identified
• Criteria applied (large use, savings potential, variability)
• SEUs selected (covering at least 70% of total use)
• SEU register drawn up (description, use, priority)
• For each SEU: action plan made or scheduled
• Management has approved the SEUs
• SEUs communicated to relevant staff

Further reading

• Energy review: how to map your energy use
• Energy baseline: your starting point for measurement
• EnPIs: how to measure your performance
• Step-by-step plan: the 8 steps to certification
• For production companies: sector-specific tips
• For data centres: SEUs in data centres