If an organisation wants to understand why a building is using too much energy, it needs something different from an organisation completing a formal compliance exercise.

The difficulty is that phrases such as energy audit, energy assessment and ESOS assessment are sometimes used loosely. Before commissioning work, it is worth establishing exactly what question needs answering and what output is required.

“The right energy review depends on the decision you are trying to make.”

What is the difference in simple terms?

Building energy audit Practical building diagnosis

Used to understand how a specific building consumes energy, identify waste and develop practical improvement priorities.

Energy assessment A broad description

A general term that can describe several types of energy review. The value depends on the scope, evidence and intended outcome.

ESOS assessment Formal compliance

Part of the Energy Savings Opportunity Scheme for organisations that fall within the relevant requirements, with a different purpose and framework from a standalone building diagnosis.

What is a commercial building energy audit?

A commercial building energy audit is a practical investigation into how a building actually uses energy.

It will normally combine evidence such as:

electricity and gas consumption;
half-hourly electricity data;
operating and occupancy hours;
plant and equipment;
heating, cooling and ventilation;
lighting;
Building Management System schedules;
temperature and control settings;
site observations;
discussions with facilities or operational staff.

The purpose is not simply to calculate how much energy the building consumes. It is to understand why it is consuming it and whether that consumption is necessary.

What should a building energy audit deliver?

A useful building audit should leave the organisation with a clearer understanding of the building and a practical order in which to act.

That may include:

evidence of where energy is being used;
patterns of out-of-hours consumption;
control or scheduling problems;
unnecessary baseload;
maintenance or operational issues;
no-cost and low-cost actions;
short-term improvement measures;
capital opportunities where appropriate;
indicative savings and payback;
areas that need further monitoring or investigation.

What is an energy assessment?

Energy assessment is a much broader term.

It may describe a high-level review, a detailed technical assessment, a review of energy bills, an assessment of a proposed project or something close to a full building energy audit.

The phrase alone does not tell you how much investigation will take place.

Before commissioning an assessment, ask:

What question is the assessment intended to answer?
Will actual consumption data be analysed?
Will half-hourly data be reviewed?
Is a site visit included?
Will plant and controls be inspected?
Will operating schedules be checked?
Will recommendations be costed?
Will savings be estimated?
Will actions be prioritised?
What will the final output contain?

Two services described as an “energy assessment” can therefore contain very different levels of work.

What is an ESOS assessment?

ESOS — the Energy Savings Opportunity Scheme — is a formal energy-assessment and compliance framework for organisations that fall within its scope.

An ESOS assessment therefore starts from a different question. Its purpose is not simply:

“Why is this building using too much energy?”

It is part of a wider organisational process for reviewing energy use and identifying energy-saving opportunities.

Depending on the organisation, that wider energy picture can extend beyond one individual building.

This is why an ESOS assessment and a detailed building energy audit should not automatically be treated as interchangeable.

The biggest difference is purpose

A practical way to distinguish the different approaches is to begin with the decision that needs to be made.

“Our electricity use has increased and we don't know why.”

A building energy audit is likely to be the more useful starting point because the question requires diagnosis of the building, controls and operating patterns.

“We need an overview before considering a project or investment.”

A defined energy assessment may be appropriate, provided the scope is clear enough to support the intended decision.

“We need to meet our ESOS obligations.”

The work needs to be structured around the relevant ESOS requirements rather than treated simply as an ordinary building audit.

Compliance and building improvement are not the same objective

Compliance work can identify worthwhile energy-saving opportunities, but completing a compliance process does not necessarily answer every operational question about a particular building.

Equally, a detailed building energy audit may be very useful for improving a building while not, by itself, representing the complete organisational compliance process required for another purpose.

The important point is to avoid assuming that because one form of assessment has been completed, every other energy question has automatically been answered.

What about EPCs and other building energy ratings?

Building ratings and certificates can provide useful information, but they serve a different purpose from investigating actual operational energy use.

A calculated or rated performance does not necessarily explain:

why electricity use remains high overnight;
whether plant starts too early;
whether heating and cooling operate together;
whether BMS schedules match occupancy;
whether local equipment has been overridden;
why one site is consuming more than another.

Those questions require evidence about how the building is actually operating.

Why does actual energy data matter?

Whatever the name of the assessment, better evidence generally leads to better decisions.

Half-hourly electricity data can reveal whether the building has a persistent overnight load, when consumption rises each morning, whether weekends resemble weekdays and whether operational changes have altered the load profile.

But the data still needs to be connected with plant, controls and occupancy.

Does every type of assessment include a site visit?

Not necessarily.

This is another reason why the title of the service is less important than its scope.

If the goal is to diagnose actual building energy waste, a site visit can be particularly valuable because it allows the assessor to connect the consumption profile with:

plant operation;
control schedules;
sensor settings;
local overrides;
occupancy patterns;
portable equipment;
maintenance issues;
how staff actually use the building.

Data may show that a load exists. Inspection and discussion help explain why.

Which type of assessment do you need?

A useful starting point is to ask what outcome you actually need.

Understand one building

Choose a detailed building energy audit when you need practical diagnosis of consumption, controls, plant and operational waste.

Investigate a defined question

An energy assessment may be suitable where the scope is explicitly designed around a project, funding decision or particular technical question.

Complete formal compliance work

Use the appropriate compliance route where the organisation needs an assessment carried out within a specific statutory or reporting framework.

Move from compliance to action

Additional building-level investigation may be useful where a broader assessment has identified opportunities but more detail is needed before implementation.

Questions to ask before commissioning any energy assessment

What is the purpose of the assessment?
Is it for diagnosis, planning, investment or compliance?
What buildings or energy uses are included?
What data will be analysed?
Will half-hourly electricity data be reviewed?
Is a site visit included?
Will plant and controls be inspected?
Will actual operating hours be considered?
Will energy-saving opportunities be quantified?
Will recommendations be prioritised?
What will the final report contain?
Will a follow-up discussion be included?

Clear answers to these questions make it much easier to compare proposals that may initially appear to offer the same service.

Does the type of assessment affect cost?

Yes. Cost depends on the scope of the work rather than simply the name attached to it.

Factors can include the number and size of buildings, available energy data, complexity of plant and controls, site-visit requirements, reporting requirements and whether the work forms part of a wider compliance exercise.

Oxford Energy Services offers its standard Building Energy Audit for one commercial building at a fixed fee of £1,500.

Building diagnosis

Stourbridge Glass Museum

Half-hourly electricity analysis showed that the museum was using almost as much electricity while closed as it did while open.

The value of the investigation came from connecting that pattern with what was actually happening inside the building, including controls and direct-electric heating.

Read the Stourbridge Glass Museum energy audit case study

Energy management & compliance

Blenheim Palace

A large historic estate illustrates why energy management can extend beyond a one-off assessment. Multiple buildings, meters, operating requirements and conservation constraints require an ongoing understanding of how energy is used.

Read the Blenheim Palace energy management case study

The right assessment also depends on the building

The core principles of energy analysis remain similar, but priorities change depending on how a building is occupied and operated.

The bottom line

The words audit and assessment matter less than understanding what the work is intended to achieve.

If you need to understand how a particular commercial building is performing and where energy is being wasted, a detailed building energy audit is designed around that practical diagnosis.

If the requirement is part of a formal compliance process such as ESOS, the work needs to follow the appropriate framework and consider the wider scope required for that exercise.

In some organisations, both are useful: compliance identifies the wider requirement, while detailed building-level investigation helps turn identified opportunities into practical action.

Building Energy Audit Guides

Explore the complete building energy audit series

These focused guides explain the report, likely cost, audit process and the differences between common types of energy assessment. Each supports the main Building Energy Audits service page.

Audit reports What Does a Commercial Building Energy Audit Report Contain? Audit costs How Much Does a Commercial Building Energy Audit Cost? Audit process What Happens During a Commercial Building Energy Audit? Audit types Energy Audit, Energy Assessment or ESOS Assessment: What Is the Difference?

Explore the main Building Energy Audits service

Not sure which type of energy review you need?

Start with the question you need answered

Oxford Energy Services provides practical building energy audits alongside energy, carbon and compliance support. If you are unsure which approach is appropriate, a short discussion can help define the right scope.

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