Overnight Baseload: Finding the Electricity a Building Uses When It Should Be Empty
TL;DR: Overnight baseload is the electricity a building draws at night and at weekends, when it should be close to empty. The UK Business Climate Hub advises checking half-hourly consumption for a high overnight baseload, and the Carbon Trust sets out a simple method using meter readings at closing and opening. A meter shows how much; current clamps on individual circuits show where it goes, so each load can be marked switch off, check first or leave on.
Last updated: 14 September 2026
On this page
Key takeaways
- Overnight baseload is the electricity a building uses when it should be closed. In half-hourly data it shows as a floor the night-time profile never drops below.
- The UK Business Climate Hub advises checking half-hourly consumption for a high overnight baseload to see if equipment can be switched off overnight.
- The Carbon Trust gives a no-cost method: read the meter at closing, again at opening, subtract, and multiply by your unit rate.
- A meter shows how much. Current clamps on individual circuits show where it goes.
- The Milesight CT101 is self-powered and reads to ±1% above 5 A, but stops reporting below about 1.5 A unless powered over USB-C.
What is overnight baseload?
Overnight baseload is the electricity a building draws when nobody is using it: at night, at weekends and over public holidays. Some of it is necessary, such as refrigeration, comms rooms, fire alarm panels and security systems. The rest is equipment that could be off but is not.
On a half-hourly consumption profile, the overnight baseload is easy to see. Daytime use rises and falls with occupancy, but the night-time line flattens out at a level it never drops below. The height of that floor, multiplied by every hour the building is closed, is what it costs.
The UK Business Climate Hub, whose policy content is endorsed by the Department for Energy Security and Net Zero, puts it simply in its guide to reducing business energy costs: check half-hourly consumption for a high overnight baseload to see if equipment can be switched off overnight.
Why does overnight baseload matter so much?
Baseload matters because it runs for more hours than anything else. A load that stays on overnight and through the weekend runs whenever the building is closed, which for a site open on weekday office hours is most of the week. A modest overnight baseload multiplied by those hours can rival the daytime savings people usually chase.
Cutting overnight baseload is often the cheapest saving to make, too. A timer put back on schedule, a control override reset or a closing routine that switches kit off costs little, and needs no new plant.
How do you measure out-of-hours use without new equipment?
Measure out-of-hours use with your existing meter and two readings. The Carbon Trust’s guide to effective energy management says regularly reading your energy meters to determine out-of-hours usage can offer additional savings, and sets out the method.
- Read the meter at closing time, ideally before a weekend or public holiday.
- Read it again at opening on the next working day.
- Subtract the two readings, applying any conversion or scaling factors, to get the kWh used out of hours.
- Multiply by your unit cost in £ per kWh, and extrapolate to estimate the annual cost.
If your supply has half-hourly data from a smart or advanced meter, the same answer comes as a profile rather than two numbers, which also shows when in the night the load changes.
Why does the meter not tell you where it goes?
The main meter adds every circuit together, so it can tell you the overnight baseload is high but not which circuit is responsible. A building with lighting, air conditioning, small power, a kitchen and a comms room all on one supply gives one total, and the only way to split it by meter is to switch things off one at a time at night.
Circuit-level measurement removes the guesswork. A current clamp on each suspect circuit logs its own profile, so the overnight baseload splits into named loads, each with its own night-time floor.
How do current clamps find the baseload?
Current clamps find the baseload by measuring the current on each circuit continuously, so a circuit that should be idle at 03:00 and is not stands out immediately. The Milesight CT101 clamps round a single conductor through a 16 mm opening, handles circuits up to 100 A, and powers itself from the current it measures.
Its datasheet gives an accuracy of ±1% above 5 A and ±3% at 5 A and below. There is one catch that matters for baseload hunting: when powered by the conductor, it needs about 1.5 A at a 1 minute interval, or 1.4 A at a 10 minute interval, to keep reporting. A circuit that drops below that overnight goes quiet, so for circuits where small night loads matter, power the clamp over USB-C instead.
Three practical rules apply. Clamp a single conductor, never the whole cable. On a three-phase circuit, use one clamp per phase. And treat energy in kWh as calculated management data, not a billing reading, because it is worked out from current with an assumed voltage and power factor.
What should you do with the numbers?
Turn the numbers into decisions, one circuit at a time. The Carbon Trust suggests a traffic light system for switches and controllers, which works just as well for circuits on a clamp.
- Green: switch off at closing time. For example, general lighting and kitchen equipment.
- Amber: check first with whoever owns it before switching off. For example, air conditioning that serves a comms room.
- Red: leave on. Critical equipment, for example refrigeration, fire and security systems and servers.
Then set an alert for the overnight baseload itself. Once the red loads are known, the night-time floor has a normal level, and a night well above it means something was left on. Compare weekday nights with weekends too, because a weekend floor higher than a weeknight one can point to a schedule problem.
The Carbon Trust also suggests a three week challenge: record opening and closing readings in week one, write and implement opening and closing procedures in week two, and record again in week three to calculate the savings.
What are the limits?
Clamps show where power goes; they do not fix anything, and they have limits worth knowing.
- Not a billing meter. Energy from a clamp is calculated. Check totals against your supplier’s meter reading after the first month and correct any scaling.
- Small loads can go silent. Below about 1.5 A a self-powered CT101 stops reporting, so use USB-C power where night loads are small.
- Fitting is electrician’s work. Clamps go round conductors inside the distribution board, which must be made safe first.
- Not every baseload is waste. Refrigeration, fire alarms and comms have to run. The goal is a known, justified overnight baseload, not zero.
What does an energy monitoring kit include?
An energy monitoring kit for baseload work needs a gateway, clamps for the suspect circuits and, if gas matters too, a pulse counter. Our energy monitoring kit combines a UG63 gateway, six CT101 clamps and an EM300-DI for a gas meter’s pulse output, with live prices and one click to the basket.
For choosing clamp sizes on larger feeders, see our guide to LoRaWAN current transformers. For gas and electricity meters with pulse outputs, read pulse counters, and for the regulatory side, ESOS Phase 4.
Frequently asked questions
How do I find my out-of-hours electricity use?
Read the meter at closing time, ideally before a weekend, and again at opening. Subtract the readings for the kWh used out of hours, then multiply by your unit rate, as the Carbon Trust describes.
What is a normal overnight baseload?
There is no single figure. Refrigeration, comms rooms and security systems set a floor that differs for every building. Establish yours by listing the loads that must run, then treat anything above that floor as a question.
Can smart meter data show baseload?
Yes, where half-hourly data is available. The UK Business Climate Hub advises checking half-hourly consumption for a high overnight baseload to see if equipment can be switched off.
Do CT clamps need batteries?
Not the CT101. It powers itself from the current in the conductor it clamps, and can also run from a 5 V USB-C supply for lightly loaded circuits.
Are clamp readings accurate enough for billing?
No. The CT101 measures current to ±1% above 5 A, but energy is calculated with an assumed voltage and power factor. Use it to find waste, and use the supplier’s meter for billing.
Put a price on the night
Overnight baseload is the part of the bill that runs while nobody is there. Two meter readings put a price on it, and a clamp on each suspect circuit shows which ones to switch off, check or leave on.
See the energy monitoring kit with live prices, or send us a photo of the distribution board and an engineer will suggest which circuits to clamp within one working day.
Next step
Get a priced kit list for your site
Answer three quick questions and tell us where to send it. An engineer replies within one working day with the parts, the prices and the lead time.
Rather talk it through? Call 023 9223 3611
Received. An engineer has it.
You will hear back within one working day with the parts, the prices and the lead time. A confirmation is on its way to your inbox.
If it is urgent, call 023 9223 3611.
