How Electricity Meters Work: Disc to Smart Meter

Part 16 of 23 in Cobler's Electricity Fundamentals series. New here? Start with the course map.

Every ringgit TNB has ever billed passed through one small box on the wall. The meter is the cash register of the entire electricity industry, and for roughly a century that cash register was a spinning aluminium disc turning behind a pane of glass. Watch an old one under load and you can see it: the faster your building draws power, the faster the disc spins. Nobody programmed that. It falls straight out of the physics.

Understanding how electricity meters work is not trivia. It is the difference between reading your TNB bill as a single painful number and reading it as a set of measurements you can argue with, plan around, and cut.

How electricity meters work: the spinning-disc meter

The disc is a tiny electric motor, and its speed is proportional to the power passing through your building.

Look inside a classic induction meter and you find two electromagnets straddling a thin aluminium disc. One coil is wired across the supply, so the magnetic flux it produces tracks the voltage. The other coil is wired in series with the load, so its flux tracks the current your building draws. The two fluxes are offset in time, and together they induce swirling eddy currents in the disc. Those eddy currents react against the magnetic field and produce a twisting force. The torque works out to be proportional to voltage times current times the power factor (the fraction of that current actually doing useful work), which is exactly real power in watts.

That would just make the disc accelerate forever. So a permanent magnet sits over the rim as an eddy-current brake, dragging on the disc harder the faster it turns. Braking force rises with speed until it balances the driving torque, and at that point the disc settles at a speed proportional to power. Draw twice the power, the disc turns twice as fast. A train of gears then counts revolutions and rolls them onto the dials. Because speed is power and the dials integrate speed over time, what the dials show is power multiplied by hours: kilowatt-hours. The meter is a motor with a speedometer bolted to a clock, and the clock reads in kWh.

This is why energy is billed in kWh in the first place. One kWh is 3.6 megajoules of energy, but no meter was ever built to display megajoules. The register counts what the disc physically does, and what it does is accumulate power over time.

Who invented the electricity meter?

The practical AC meter came out of Westinghouse in 1888, built on a discovery made in Italy three years earlier.

The rotating-magnetic-field principle behind the disc was conceived by Galileo Ferraris in 1885. Oliver Shallenberger, an engineer at Westinghouse, turned it into the first practical AC meter: an induction ampere-hour meter patented on 14 August 1888, whose disc speed tracked the current. In 1894 he developed it into the modern watt-hour form, the design this article describes, giving the disc a speed proportional to real power (Wikipedia: Electricity meter). Around the same period Elihu Thomson at General Electric built a recording wattmeter, and the Hungarian engineer Ottó Bláthy of the Ganz Works presented an AC kilowatt-hour meter at the Frankfurt Fair in autumn 1889. The design that emerged from that decade was so sound that it outlived its inventors by a century, sitting on walls in essentially the same form into the 2000s.

How do modern electronic meters measure energy?

They throw away the disc and do the multiplication in a chip, sampling voltage and current thousands of times a second.

A digital meter puts a small sensor on the voltage and another on the current, reads both as fast numbers many thousands of times per second, and multiplies each pair of samples to get instantaneous power. Add those products up over time and you have energy, computed rather than spun out mechanically. Because the meter now holds the raw voltage and current waveforms, it can report far more than the old disc ever could: real power in kW, reactive power in kVAR (the part that does no useful work), apparent power in kVA (the two combined), the power factor between them, and consumption split by time of day. It is the same underlying quantity, voltage times current, measured with a processor instead of eddy currents. The war-of-the-currents-era engineers would recognise the arithmetic instantly; they just did it with iron and aluminium.

Why do factories get meters with current transformers?

Because you cannot pass 2,000 amps through a meter the size of a lunchbox, so the meter measures a scaled-down copy of the current instead.

A domestic supply might draw tens of amps, and the meter's own coil can carry that directly. A three-phase industrial supply can pull hundreds or thousands of amps, and no compact meter can carry that current without melting. The solution is the current transformer, or CT. A CT is a ring of iron clamped around the main busbar (the thick metal bar carrying the incoming supply); the heavy line current becomes the primary winding, and a secondary winding steps it down to a small standardised value, typically 5 A or 1 A, that faithfully mirrors the big current. The meter drinks from that thin, proportional trickle and multiplies the reading back up by the CT ratio. The meter never touches the firehose. This is why an industrial meter cabinet is full of ring-shaped CTs on the incoming cables, one per phase, feeding a small meter that could otherwise sit in a house.

What does a Malaysian commercial meter actually register?

More than just kWh. A TNB commercial or industrial meter keeps at least three registers, and each one maps to a line on your bill.

- Energy (kWh): total consumption, and on a time-of-use tariff split into peak and off-peak, where the peak window is weekdays 2:00 pm to 10:00 pm.
- Maximum demand (kW): not your total usage but your single highest 30-minute average power in the billing month. On a time-of-use tariff, only the half-hours inside the weekday 2:00 pm to 10:00 pm peak window count toward it, so an off-peak spike is not billed. The meter averages your demand over fixed consecutive 30-minute periods and remembers the worst one. Under the RP4 tariff that peak sets your Capacity and Network charges at roughly RM89 to RM97 per kW, billed for the entire month. One bad half-hour, every chiller and compressor starting together, can set a charge you pay for thirty days. We break that charge down in the RP4 maximum-demand guide.
- Reactive energy (kVARh): the meter also totals reactive energy, which is how TNB derives your power factor. If that reactive total runs too high relative to your kWh, you pay a power-factor penalty.

Read those three registers and you understand most of what your bill charges you for. The meter is not hiding anything; it is telling you exactly what it measured.

Is TNB replacing everyone with smart meters?

Yes, the rollout is well underway. As of 31 March 2025, TNB had installed about 4.57 million smart meters, heading toward a target of 9.1 million across Peninsular Malaysia under its Advanced Metering Infrastructure programme, with coverage expanding toward roughly 10.4 million premises by 2028 and beyond (TNB smart meters). A smart meter is an electronic meter with a communications link: it reports consumption in 30-minute intervals without anyone visiting the site, which is what makes time-of-use billing and remote reads practical at scale.

Why won't your own sub-meter match TNB's?

It never will exactly, and that is expected, not a fault. Different meters, different CT accuracy classes, different sampling instants, and small line losses between TNB's meter and yours all guarantee a gap of a percent or two. We explain the reasons in detail in why your sub-meter and TNB's never match.

The gap does not make sub-metering pointless. It makes the two meters answer different questions. TNB's meter tells you what you owe: one number for the whole site, once a month. Your own meters tell you why: which chiller, which production line, which shift, which half-hour drove that maximum-demand peak. TNB bills the site; only your own instrumentation attributes the cost to the equipment that caused it.

That attribution is exactly the data CobiNeural rides on. Its Energy insights sit on top of your sub-meters and the same measurements a TNB meter takes, and turn the monthly bill back into the thing it came from: real power, real demand peaks, real equipment. The meter has been doing honest measurement since Shallenberger. The job now is to read it well enough to act on.

This is Part 16 of 23 in Cobler's Electricity Fundamentals series. Previous: How Electric Motors Work (and Run Everything). Next: How the Malaysia Electricity Grid Reaches Your Plug.

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