Fire alarms, security systems, emergency lighting, servers and specialist equipment may need to operate continuously. But pumps, fans, heating, cooling and hundreds of smaller electrical loads can also continue running unnoticed.

Out-of-hours energy use is one of the easiest forms of waste to overlook because nobody is normally present to see it happening. Pumps run, fans continue operating, heating schedules remain active and hundreds of small electrical loads quietly accumulate.

In my experience, a focused effort to reduce unnecessary overnight electricity use can commonly save around 20% of total electricity consumption, particularly in buildings that have never investigated their out-of-hours load properly.

“The target does not always need to be zero. It needs to be low, stable and justified.”

What does overnight energy use mean?

Overnight energy use is the electricity or gas consumed when a building is closed or largely unoccupied.

For a conventional Monday-to-Friday office, this usually means evenings, nights, weekends and bank holidays. For museums, shops, theatres, factories and other buildings, the relevant out-of-hours period depends on their operating pattern.

Some after-hours consumption is necessary. The objective is not always to eliminate it entirely. The objective is to establish which loads are essential and stop equipment operating when nobody benefits from it.

A simple first check

The simplest check does not require sophisticated equipment.

Read the electricity and gas meters when the building closes, then read them again when it opens the following morning. The difference tells you how much energy was consumed overnight.

Repeating the exercise across a weekend can be even more revealing.

Checking overnight gas consumption

For gas, the answer in many ordinary commercial buildings should be approximately zero outside the heating season.

Even during winter, overnight gas consumption should be limited, intentional and explainable.

Unexpected out-of-hours gas use commonly indicates:

poor Building Management System schedules;
heating operating too late or starting too early;
boilers maintaining unnecessary temperatures;
frost or fabric-protection settings that are too high;
hot-water systems running continuously;
controls that have been overridden and not reset.

Half-hourly gas meters are relatively uncommon, so direct meter readings remain a useful diagnostic method.

Checking overnight electricity consumption

Electricity is easier to investigate because most larger commercial electricity supplies have half-hourly metering. The supplier can normally provide the data, often as a spreadsheet or CSV file.

Half-hourly data is particularly useful because it shows exactly when electricity use rises and falls. It can reveal whether plant starts before the building needs it, continues running after people leave or operates throughout the weekend.

What should a healthy overnight baseload look like?

There is no universal correct figure.

A hospital, hotel, laboratory or data centre will naturally have a much higher overnight load than a conventional office. The appropriate baseload depends on what genuinely needs to remain switched on.

Necessary overnight loads in a typical office might include:

fire and burglar alarms;
emergency lighting;
network equipment;
servers;
refrigeration;
essential monitoring equipment;
limited ventilation or heating under specific conditions.

The most useful approach is to list the equipment that must remain operational and estimate its combined electrical demand.

Example

If essential equipment totals approximately 3 kW, but the measured overnight demand is consistently 15 kW, there is potentially 12 kW of unexplained demand.

If that excess continues every weekday night and throughout the weekend, the annual cost can become substantial.

Common causes of excessive overnight electricity use

Pumps

Heating and cooling circulation pumps are often left running even when boilers, chillers or occupied areas do not require them. A pump may appear relatively unimportant, but continuous operation over 8,760 hours per year can create a significant cost.

Fans and ventilation

Air-handling units, extract fans and ventilation systems are commonly scheduled for far longer than the building is occupied. Sometimes the main heating or cooling plant switches off while the fans continue running throughout the night.

Computers and office equipment

Individual computers do not normally dominate a building’s energy use, but dozens or hundreds of computers, monitors, printers and other devices can create a sizeable continuous load.

Split-system heat pumps

Local air-conditioning and heat-pump units are frequently controlled independently of the central BMS. They may be left running by occupants or operate from forgotten local schedules.

Many small loads

Often there is no single dramatic culprit. Kitchen appliances, vending machines, water coolers, chargers, display screens, heaters, printers, pumps and miscellaneous plug loads may each use little electricity, but together they create a substantial baseload.

Weekend schedules copied from weekdays

One of the most common problems in Monday-to-Friday offices is that weekday BMS schedules have been copied across to Saturday and Sunday. The building may therefore heat, cool and ventilate itself normally throughout the weekend, despite being empty.

Temporary settings that become permanent

Controls are often overridden for evening events, maintenance work or exceptional opening hours. The event finishes, but nobody resets the controls.

Case study

When the building uses almost as much energy closed as open

At 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 investigation found that the building already had efficient air-source heat pumps, but inaccessible controls, unsuitable settings and extensive use of direct-electric heaters were preventing them from operating effectively.

Read the full Stourbridge Glass Museum case study

How to investigate overnight electricity use

1. Establish the average overnight load

Use half-hourly data to find the building’s typical electricity demand during the quietest part of the night. Express this as an average power in kilowatts.

For example, consumption of 5 kWh during each half-hour period represents an average load of 10 kW.

2. Estimate what genuinely needs to remain on

List the essential overnight equipment and estimate its combined load. This gives you a target baseload.

3. Calculate the avoidable difference

Subtract the justified load from the measured load, then estimate the potential annual saving using the avoidable load, out-of-hours operating time and electricity price.

4. Hold a hard switch-off night

Choose one evening and deliberately switch off everything that can safely be turned off. Compare the resulting overnight consumption with the normal profile.

A hard switch-off night is often very revealing. It demonstrates what the building can achieve and provides a practical target for normal operation.

5. Carry out an overnight audit

Sometimes the best method is simply to visit the building late at night and see what is operating.

Walk through the building and record:

lights;
fans;
pumps;
heaters;
air-conditioning units;
computers;
display equipment;
kitchen appliances;
plant-room equipment;
unusual sounds, airflow or heat.

One client produced a 70-page report following a single overnight audit, documenting equipment that was operating unnecessarily. The value was not the length of the report. It was that the waste became visible.

Practical ways to reduce overnight consumption

Use LED lighting with occupancy sensors

LED lighting reduces energy use directly, while occupancy controls prevent lights remaining on in empty areas.

Check BMS schedules every week

Schedules should not be treated as something configured once and forgotten. A simple weekly check can identify unexpected weekend operation, extended run times, event overrides, contractor changes and seasonal settings that are no longer appropriate.

Check for factory settings

New or reset controllers may operate using default factory schedules that bear little relationship to the building’s actual opening hours.

Install time clocks

Simple time clocks can control equipment that does not need to be integrated into a full BMS. They are particularly useful for local pumps, heaters, vending machines and other predictable loads.

Use last-person-out switches

A last-person-out switch disconnects multiple non-essential electrical circuits when the building closes. These are common in retail buildings but can be useful elsewhere.

Monitor rather than rely on memory

Manual switching can produce an initial reduction, but it is rarely a complete long-term solution. People forget. Staff change. Controls are overridden. New equipment is installed.

The most effective approach combines:

clear responsibility;
automatic controls;
regular data checks;
occasional physical inspections;
follow-up after events and maintenance work.

Why overnight energy use matters

Out-of-hours energy is particularly valuable to investigate because it often provides savings without affecting the building’s operation.

Reducing unnecessary night use does not normally require staff to work in colder offices or customers to receive a poorer service. It simply stops equipment operating when nobody benefits from it.

It is also a useful indicator of how well the building is managed.

A low, stable and explainable overnight baseload generally suggests that schedules, controls and responsibilities are working properly. A high or erratic baseload often points towards wider control and management problems.

The first step is simple

Read the meters, obtain the half-hourly data and establish what the building is doing when everybody has gone home.

Once the night load is visible, the waste is usually much easier to find.

Need help investigating your building?

Start with a practical building energy audit

Oxford Energy Services reviews energy data, plant, controls, schedules and out-of-hours consumption to identify practical opportunities for reducing waste.

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