Electrical submetering and energy monitoring field guide

Submetering, and what sits below the utility meter

Submetering is the practice of measuring electricity at a point below the utility's revenue meter, so you know what a tenant, a load, or a single system used instead of only what the whole building used. The utility meter sees one number for the service. A submeter sees the slice you care about: a suite, a floor, the HVAC, the kitchen, the EV chargers.

The physical job is small and the logic is simple. You clamp a current transformer around each conductor you want to watch, feed the meter a voltage reference from the same phases, and the meter multiplies current by voltage to get power and energy. The service-entrance and metering install is covered in its own guide. This is everything past that revenue meter, inside the building, where the data belongs to the owner and not the utility.

What goes wrong is rarely the meter. It is the install and the commissioning. A current transformer put on backward, or matched to the wrong voltage phase, reads a confident wrong number, and nobody catches it until a tenant disputes a bill or an energy report makes no sense. Get the CT right and the rest is software.

What is the difference between a submeter and the utility meter?

The utility's revenue meter measures the entire service and is what the power company bills against. The submeter measures a portion of that service, a tenant or a load downstream, and belongs to the building owner, not the utility. The utility meter sets the bill from the grid. The submeter divides that bill, or tracks a load, inside the property.

Ownership and purpose are the real divide. The utility owns its meter, seals it, and tests it to its own standard and the state commission's rules, because that reading is money flowing to the utility. A submeter is yours. You install it, you maintain it, and if you use it to bill tenants you take on the accuracy obligation the state puts on that. The service-entrance guide covers the utility side, including self-contained versus CT metering at the service. Submetering reuses the same CT idea further down.

The sum of the submeters will not match the utility meter exactly, and that surprises people. You never meter every branch, losses and unmonitored loads live in the gap, and the two meters have different accuracy classes. Treat the utility meter as the truth for the total and the submeters as the allocation, not as a second opinion that has to reconcile to the watt.

Why buildings submeter

Buildings submeter for money and for management, and usually both. The drivers sort into a short list, and which one is paying for the install tells you how careful the meter selection has to be.

Tenant billing and cost recovery is the big one. When several tenants share one service, submetering lets the owner charge each for what they actually used instead of folding power into rent or splitting it by square footage, which always overcharges the light user and undercharges the heavy one. Energy management is the second: you cannot manage what you do not measure, and a single utility bill tells you nothing about which system is wasting it. Cost allocation between departments or cost centers in an owner-occupied building runs on the same data.

Then there is code and certification. Some energy codes and green-building programs such as LEED call for submetering or measurement of major end uses, and that requirement is jurisdiction- and program-specific, so confirm what the adopted code edition and the rating system actually demand. New loads drive the rest. EV charging, on-site solar, and battery storage all need their own metering to bill, to settle, or just to know what they are doing.

Tenant billing and cost recovery

Tenant billing is the number one reason submeters get installed, and it is the use with the most rules attached. The owner pays the utility for the whole service, then bills each tenant from a submeter for the energy that tenant used. Done right, it is fair and it recovers a real cost. Done wrong, it is a dispute and sometimes a violation.

The model shows up in multi-tenant retail, office, apartment, and mixed-use buildings, and in mobile-home and marina settings where it has its own history of regulation. The meter sits at the tenant's panel or feeder and counts kilowatt-hours the same way the utility meter does. The owner reads it monthly, applies a rate, and issues a bill.

The catch is legality. Submeter billing of tenants is regulated by the state public utilities commission or equivalent, and the rules vary widely. Many states cap what the owner can charge to the utility's own rate so the owner cannot mark up power, require the meter meet a stated accuracy, and dictate what the bill must disclose. Some jurisdictions restrict resale of electricity outright. Before you sell a tenant-billing system, confirm what the state and local authority allow, because that is the part that bites later, not the wiring.

Energy management and finding the waste

The other half of submetering is measuring by system instead of by tenant, so you can see where the energy goes and prove what a fix saved. You cannot manage what you do not measure, and one monthly utility bill hides everything inside it.

Meter the loads that move the bill: HVAC, lighting, plug loads, refrigeration, process equipment, the chiller, the air handlers. Now the data answers real questions. Which floor runs its lights all night. Whether the new variable-speed drive actually cut the fan energy or just moved it. What the building does at 2 a.m. when it should be near idle. That last one finds more waste than any audit, because phantom overnight load is almost always a control left in hand or a schedule nobody set.

This is the measurement and verification, or M and V, behind any efficiency project. You meter the load before the retrofit, make the change, and meter after, and the difference is the savings you can stand behind. Without a submeter you are arguing from a utility bill that changed for ten reasons at once. With one, you have the load you touched, isolated and trended.

What is in a submeter system

A submeter system is four parts: the meter that does the math, the current transformers that sense the current, the voltage reference that feeds the meter the line voltage, and the communications that move the data out. Miss any one and you do not have a working point.

The meter is the measuring and computing device. It takes current from the CTs and voltage from the taps, multiplies and integrates them, and stores energy, power, and whatever else it is built to report. The current transformers, the CTs, are the sensors that go around each conductor and produce a small signal proportional to the current flowing. The voltage reference is a set of taps, usually fused, that bring line voltage to the meter from the same phases the CTs are on. The communications, the last part, is how the reading reaches a dashboard, a billing system, or a building automation system.

Get the pairing right and the rest follows: each CT has to be matched to its own voltage phase, oriented the right way on the conductor, and sized to the load it watches. The meter is rarely the failure. The CT and its wiring are.

What is a current transformer?

A current transformer, a CT, is a sensor that goes around a conductor and puts out a small signal proportional to the current flowing through that conductor. It lets a meter read hundreds or thousands of amps without the full current ever passing through the meter. The CT is the part that decides whether a submeter is right, so it gets the most attention on the install.

Two physical types cover the work. A split-core CT opens on a hinge so it clamps around a live conductor without disconnecting anything, which is why it is the retrofit choice on an energized panel. A solid-core CT is a closed ring the conductor has to thread through, more accurate and cheaper but only practical when the conductor is loose or the circuit is down. On an occupied building you reach for split-core almost every time.

Two outputs are common and they behave very differently. A traditional CT has a current output, most often 5 A at full rated primary, and that 5 A secondary is the one with the open-circuit hazard below. Many modern submeter CTs instead put out a low voltage, commonly 333 mV, or a milliamp signal, which is safer to handle but must be matched to a meter expecting that exact output. Mixing a 333 mV CT with a meter set for a 5 A CT gives a reading off by orders of magnitude.

Split-core CT

A hinged current transformer that clamps around a live conductor for retrofit without a shutdown

Solid-core CT

A closed-ring current transformer the conductor threads through, more accurate, used when the circuit is open

CT ratio

Primary amps to secondary output, such as 100:5 or 200 A to 333 mV, set in the meter to match the CT

Burden

The load the CT secondary drives, the meter input plus the leads, which the CT is rated to supply

CT ratio, accuracy class, and polarity

A CT is specified by three things that all have to be right: its ratio, its accuracy class, and which way it faces. The ratio sizes the CT to the load. The accuracy class sets how true the reading is. The polarity decides whether the meter sees real power or its mirror image.

Size the CT so the load runs in the useful part of its range. A 100:5 CT sized to a circuit that only ever pulls 10 A reads near the bottom of its accuracy band, where error is worst, and a CT undersized to a load that exceeds its primary rating saturates and reads low. Pick a primary rating a comfortable margin above the expected load, not ten times it. Accuracy class follows the use: a billing CT wants a tight class such as 0.2 or 0.5, a monitoring CT can live with class 1.

Polarity is the one that bites. Every CT has a marked side, an arrow or a dot or a labeled face, that has to point toward the load, the same direction current flows on that conductor. Put it on backward and the meter reads the current 180 degrees out, which on a power meter shows up as negative or wildly wrong power even though the kWh count may look plausible. Reversed CTs are the single most common submeter error, and they are invisible until someone checks the phase against the voltage at commissioning.

Installing the CT, and the rule that keeps you alive

Here is the blunt one. Never open the secondary of a current-output CT while primary current is flowing. A 5 A CT with its secondary open is not a dead wire. It is a constant-current source forced into an open circuit, and it drives its core to saturation trying to push current that has nowhere to go, generating a high voltage at the open terminals that can exceed 1000 V. That will shock or kill you and it will destroy the CT.

So before you disconnect a 5 A CT secondary, or pull a meter that has live CTs landed on it, short the CT secondary first. Use the shorting block or shorting terminals built for it, or land a jumper across the secondary, so the current has a path while you work. A CT secondary shorted is safe. A CT secondary open under load is a hazard. There is no in-between and no shortcut on this one.

The low-voltage and milliamp output CTs do not have this open-circuit hazard the same way, which is part of why they have taken over submetering, but you do not get to guess which kind you have. Read the CT. If it says 5 A secondary, treat every disconnect as the dangerous case. For the split-core install itself, the win is that you clamp around the live conductor without a shutdown, so the only safety task is keeping that secondary closed and respecting the panel you are working in.

The voltage reference the meter needs

A power meter cannot work on current alone. It needs the line voltage too, because power is voltage times current times the phase angle between them, and energy is power over time. So every submeter point has a voltage reference brought in from the same phases the CTs are sitting on.

The voltage comes in on taps, usually through small fuses or a fused disconnect right at the meter, because you are tapping live conductors and the tap leads need their own protection. On a three-phase point you bring all three phases plus neutral where the meter wants it, and on single-phase you bring the legs you are measuring. The fusing is not optional. An unfused voltage tap is a fault waiting for a loose lead.

Phase matching is the part that ties back to the CT. The voltage reference for phase A has to be the same phase A that the phase A CT is clamped on. Cross them, put the A voltage with the B current, and the meter computes power from a current and a voltage that are not on the same conductor, so the power factor and the power read wrong while each individual quantity looks fine. This is the other half of the commissioning check below.

Submeter types, from basic kWh to power quality

Submeters range from a simple energy counter to a full power-quality instrument, and you buy to the use, not to the spec sheet. Paying for harmonics analysis on a tenant kWh bill is wasted money, and trying to run an energy project off a meter that only logs total kWh leaves you blind.

A basic kWh submeter counts energy and maybe demand, which is all tenant billing strictly needs. A step up adds real power, apparent power in kVA, power factor, and per-phase values, which is what energy management wants so you can see how a load behaves, not just how much it drew. The top tier is a power-quality meter that also measures harmonics, total harmonic distortion, voltage sag and swell, and transients, used where the load is sensitive or the power is suspect.

The other axis is single-phase versus three-phase, and how many points one meter covers. A single-point three-phase meter watches one feeder. A multi-circuit or branch-circuit submeter, sometimes a metering strip in the panel, watches dozens of circuits from one device with one voltage reference and many CTs, which is the economical way to meter a panel densely. Match the channel count to how granular you actually need the data, because every channel is a CT to install and commission.

Branch-circuit monitoring

Branch-circuit monitoring is submetering taken down to the individual circuit, where one multi-circuit meter watches every breaker in a panel through its own CT. Instead of one number for the panel, you get a number for each circuit, which is the granular load data energy management and data centers run on.

The hardware is built for density: a meter with one shared voltage reference and a ribbon or strip of small CTs, one per circuit, often installed as a single assembly across the panel's breakers. It cuts the per-point cost dramatically, because the expensive parts, the meter and the voltage connection, are shared across all the circuits instead of repeated.

Where it earns its place is anywhere you need to know not just total consumption but the breakdown: which circuits feed which loads, how each one trends, and when one drifts. In a data center that is per-rack and per-PDU visibility for capacity and PUE. In a commercial building it is the lighting circuit that never turns off and the receptacle circuit that spikes after hours. The install discipline is the same as any CT point, multiplied by the channel count, which is exactly why commissioning a strip is where errors hide.

How does submeter data get to the software?

Submeter data leaves the meter over one of a few standard paths, and the path has to match whatever is collecting it, or you have a meter reading itself with nobody listening. The common outputs are pulse, Modbus, and BACnet, with the network carrying it the rest of the way.

A pulse output is the simplest: the meter closes a contact once per unit of energy, and a counter or controller tallies the pulses. It is cheap, one-way, and fine for a basic kWh feed, but it carries only that one number. Modbus is the workhorse of submetering, either Modbus RTU over an RS-485 serial loop that daisy-chains many meters on one pair of wires, or Modbus TCP over Ethernet. It moves every register the meter has, not just energy. BACnet, as BACnet MS/TP over RS-485 or BACnet IP over the network, is the protocol the building automation world speaks, so a meter that talks BACnet drops straight into a BAS.

From there the data goes to an energy-management system, a building automation or building management system, or a cloud platform, the same way a building management system or a data-center DCIM platform ingests any other point. Plan the data path before the install. A meter with no route to the software is a meter you read by hand with a laptop, which nobody keeps doing past the first month.

The energy-management system and dashboard

The software side of submetering is the energy-management system, the platform that collects the meter data, trends it, and turns it into something a person acts on. The meters produce numbers. The EMS makes them mean something.

At its plainest the EMS polls each meter, stores the readings, and draws the trends: energy by tenant, by system, by time of day, against last month and last year. The useful versions add analytics on top, flagging the load that drifted, the schedule that never kicks in, the demand peak that set a charge. For billing it generates the tenant statements from the meter reads and the rate. For efficiency work it holds the measurement and verification baseline so you can show what a project saved.

The dashboard is where it lands for the human, and a good one answers the question the building owner actually has, which is usually where is my money going and what changed. The integration matters as much as the meter. An EMS that cannot reach the meters, or a meter on a protocol the EMS does not speak, is two halves that never make a whole. Decide the platform and the protocol together.

What is revenue-grade metering?

Revenue-grade metering is metering accurate enough to bill against, defined in the United States by ANSI C12.20, which sets accuracy classes for electricity meters. The classes are 0.1, 0.2, and 0.5, where the number is the percent error allowed at full load, so a Class 0.2 meter is within plus or minus 0.2 percent. A meter has to meet the standard to be called revenue-grade.

Monitoring-grade meters are looser and cheaper, commonly Class 0.5 or Class 1, meaning within 0.5 or 1 percent. That is plenty to find waste and trend a load for energy management, where you care about relative change, not the exact watt. It is not enough for direct cost recovery under most utility-commission rules, where the money depends on the absolute number being defensible.

Two cautions. First, the meter accuracy and the CT accuracy stack, so a revenue-grade meter fed by a sloppy CT is not a revenue-grade point. The CT has to carry an accuracy class to match, and the CT has to be sized so the load sits in its accurate range. Second, what counts as billing-accurate is set by the state public utilities commission, and the required class and even the acceptable error band vary by jurisdiction. Confirm the standard the meter meets and the requirement the authority imposes before you promise a number is billable.

Billing rules and regulation

Submeter billing of tenants is regulated, and the regulation is the part of the job most likely to come back on you. The rules live at the state public utilities commission or the equivalent state authority, and they differ enough that what is routine in one state is prohibited in the next.

The common threads are these. The owner usually cannot charge tenants more than the utility's own rate for the energy, so submetering recovers cost but does not become a profit center on the power itself. The meter generally has to meet a stated accuracy, often pointing at ANSI C12.20 revenue-grade, sometimes with a periodic testing or recertification requirement. The bill has to disclose certain things, the reading, the rate, the period. And in some places reselling electricity to tenants is restricted or barred outright, which forces a different arrangement.

None of that is a wiring question, which is exactly why it gets missed by the people doing the install. Treat the regulatory check as part of the design, confirm it with the state authority and the AHJ, and put a revenue-grade meter in when billing is on the table even if monitoring-grade would read. The cost difference is small against a billing dispute or a commission complaint.

Using submeter data for real load and capacity

Submetering and monitoring give you the one thing a load calculation cannot: the actual measured load over time, instead of the calculated demand. The load-calculation guide covers sizing a service or feeder from connected load and demand factors, which is the right method for new work and for what the code requires. Once the building is running, the submeter tells you what it really draws.

That measured load is what you use for capacity and expansion decisions. The NEC has provisions that let you size or evaluate certain feeders and services on the maximum demand actually recorded over a representative period, where you have the metered history to support it, rather than the calculated value, and the calculated method always remains available. Confirm the application against the adopted code edition, because the rules on using measured demand are specific.

The practical payoff is on a building that wants to add load, EV chargers, a tenant fit-out, more IT gear. The calculation might say the service is full. The submeter history might show it never exceeds 60 percent of rating, leaving real headroom the connected-load math hid behind diversity. Measured data turns a guess about spare capacity into a number you can defend, and it is the difference between a service upgrade and a circuit you already had room for.

Demand, peak, and demand charges

Submeters measure demand, not just energy, and on a commercial account the demand is often the bigger line on the bill. Energy is kilowatt-hours, the total you used. Demand is kilowatts, the highest rate you pulled at any point in the billing window, usually averaged over a 15-minute interval the utility defines.

Demand charges exist because the utility has to build capacity for your peak whether you hit it once or all month, so it bills the peak. A building that runs steady pays less than one that spikes, even at the same total energy. The submeter that records demand by interval shows you when the peak happens and what set it, which is the whole basis of demand management.

Once you can see the peak, you can chase it. Stagger large loads so they do not all start at once, shift what you can off the peak window, and watch for the one piece of equipment that defines the monthly maximum in a few minutes. On sites with on-site generation or battery storage, the submeter data is what tells the controls when to shave the peak. You manage the demand charge with the same meter that bills the tenant, just reading a different register.

Power quality on the advanced meters

The advanced submeters measure power quality alongside energy, which matters where the load is sensitive or the power coming in is suspect. The same meter that counts kWh can also report power factor, harmonics, total harmonic distortion, and voltage events, if you bought that tier.

Power factor is the first one most facilities care about, because a poor power factor draws more current for the same real work and some utilities bill a penalty for it. The meter shows it per phase and over time, so you can size correction or find the load dragging it down. Harmonics and THD come from nonlinear loads, drives, electronic ballasts, server power supplies, and they distort the current waveform in ways that heat conductors and neutrals and can trip protection. A power-quality meter quantifies the distortion instead of leaving you to guess from symptoms.

This is a deeper topic than a submeter section can hold, and harmonics in particular have their own analysis and mitigation. The point here is that the metering point you install for billing or energy can double as the power-quality window if you spec the meter for it, which is far cheaper than coming back with a separate analyzer later.

EV charging submetering

EV charging is one of the fastest-growing reasons to submeter, because the energy has to be attributed to a driver, a tenant, or a cost center, and the charger is a heavy, continuous, long-running load. Whoever owns the chargers usually needs to bill or settle the energy they deliver, and that is a submeter.

Some networked chargers meter internally and report through their own platform, which can be enough when you only need to bill the charging sessions. A separate revenue-grade submeter on the charger feed is the move when you need an owner-side number independent of the charger vendor, when the billing has to meet the same regulatory accuracy as any tenant bill, or when you are allocating cost across mixed users.

EV charging is a continuous load and the runs are long, so the feeder is a voltage-drop and load question first, which the voltage-drop and load-calculation guides cover. The submeter rides on top of that feeder. Size the CT to the charger circuit, not to the building, put it revenue-grade if the energy turns into a bill, and the same install discipline applies: right ratio, right polarity, right voltage phase.

Data-center submetering, EPMS, and PUE

Data centers submeter more densely than any other building because every watt is tracked, billed, or optimized, and the metering runs deep into the distribution. The system that does it is the electrical power monitoring system, the EPMS, which meters the electrical path at every level from the service down to the branch circuits and the PDUs.

The metering follows the power hierarchy: utility service, switchgear, UPS, PDU, and finally the branch circuit feeding the rack. Branch-circuit and rack-level metering is what allocates energy cost to a tenant or a customer in a colocation facility, tracks each rack's draw for capacity planning, and feeds the DCIM and building management platforms. It is submetering at scale, with the same CTs and the same commissioning discipline, just thousands of points instead of dozens.

PUE is the reason much of it exists. Power usage effectiveness, defined by The Green Grid as total facility energy divided by IT equipment energy, is the headline efficiency metric, and you cannot calculate it without metering the IT load separately from the total. A PUE of 1.0 is the ideal where every watt reaches the IT gear, and the real number, always higher, is what the submetering measures and what the facility works to bring down. The IT-load submeter and the total-facility meter are the two terms of that ratio.

Installing the submeter in the panel

The submeter install is a CT on each measured conductor, a fused voltage reference from the matching phases, the meter mounted where it can be wired and read, and the communications run back to the network. The work is methodical, and the order matters for safety and for getting it right the first time.

Mount the meter in or beside the enclosure with room to land the CT leads and the voltage taps without crowding. Clamp the split-core CTs around the conductors, marked side toward the load, and label each CT lead to its phase as you go, because untangling a bundle of unlabeled CT leads at commissioning is how phases get crossed. Bring the voltage taps from the same phases through their fuses. Land the comms.

Respect the panel you are in. Submetering is mostly retrofit on energized gear, so the arc-flash and shock hazards of working in a live panel are real and the personal protective equipment and the energized-work rules apply the same as any hot work. Split-core CTs are what let you do this without a shutdown, but the panel is still live around your hands. The CT secondary stays shorted until it is landed on the meter, every time.

Why does my submeter read wrong?

A submeter that reads wrong almost always has a CT problem, and the number one cause is a CT that is reversed, on the wrong phase, or paired with the wrong voltage. Commissioning a submeter is mostly verifying that every CT's current matches the right voltage in the right direction, and skipping it is why so many submeter installs read confidently false.

The check is concrete. With a known load on the circuit, look at the per-phase real power and power factor on the meter. A CT on backward shows negative power or a power factor near minus one on a load that should be positive. A CT swapped to the wrong phase, or a voltage tap crossed, shows a power factor that is wrong in a way no real load would produce, often around 0.5 leading or lagging when the load is plainly resistive. Correct power and a sane power factor on every phase is the pass.

Verify three things at every point: polarity, that each CT faces the load and reads positive power; phase, that each CT's current is paired with the voltage from the same conductor; and ratio, that the CT ratio set in the meter matches the CT installed. The ratio error is the quiet one, a meter set for 200:5 with a 100:5 CT on it reads exactly double and looks plausible. This verification is the single highest-value hour on the whole job, and on a tenant-billing system it is the difference between a defensible bill and a dispute you will lose.

What to document

A submeter point nobody documented is a point nobody can troubleshoot or defend later, and on a billing system that is a real exposure. The record ties each meter to what it measures, how it was set, and that it was verified.

Capture each point and its components so the next person can read the system without tracing wires. The table below is the working set. The commissioning result is the line that matters most on a billing install, because it is the evidence the reading was true when the bills started.

Common mistakes

• Installing a CT backward or on the wrong phase, so the meter reads negative or impossible power.
• Crossing a voltage tap to the wrong phase, so power factor and power read wrong while each quantity looks fine.
• Setting the wrong CT ratio in the meter, which scales the reading by a clean multiple that looks plausible.
• Sizing the CT far above the load, so the circuit runs in the inaccurate bottom of the CT range.
• Using a monitoring-grade meter for tenant billing where the state requires revenue-grade accuracy.
• Opening a 5 A CT secondary under load instead of shorting it first, risking over 1000 V at the terminals.
• Energizing the point and never commissioning it, so a confident wrong reading goes uncaught.
• Installing the meter with no planned data path, so it is read by laptop until everyone stops.

Field checklist

Standards and references

Meter accuracy in the United States is set by ANSI C12.20, which defines the revenue-grade accuracy classes 0.1, 0.2, and 0.5 for electricity meters. When a billing system has to be defensible, that standard is what the meter is held to, and the CT carries a matching accuracy class so the point as a whole holds the class, not just the meter alone.

The installation itself falls under the NEC, NFPA 70, the same as any equipment landed in a panel, and EV-charger feeds carry the continuous-load and Article 625 rules covered in the load-calculation and voltage-drop guides. The current transformers and their wiring follow the CT instrument-transformer practice, and the never-open-secondary rule on current-output CTs is a safety fundamental, not a preference. Working in the live panels these retrofits usually require puts the job under the electrical-safety rules for energized work, including NFPA 70E and the OSHA requirements, so the arc-flash and shock protection applies.

Tenant submeter billing is governed by the state public utilities commission or equivalent, and the accuracy required, the rate cap, the disclosure, and whether resale is even allowed all vary by jurisdiction. Energy-code and green-building submetering requirements vary by adopted edition and program. The federal efficiency references, the Department of Energy and ENERGY STAR guidance, frame measurement and verification practice. Confirm every accuracy and billing requirement against ANSI C12.20, the adopted code edition, and the state authority before you commit a number to a bill.

Units, terms, and conversions

Submetering carries a handful of units and acronyms that show up across meter sheets, CT labels, and energy reports, and the same quantity reads differently depending on the source.

Energy is kilowatt-hours, kWh, and at scale megawatt-hours, MWh. Real power is kilowatts, kW, apparent power is kilovolt-amps, kVA, and the ratio of the two is power factor, a number between 0 and 1. Demand is kW averaged over the utility's interval, commonly 15 minutes. CT ratio is written primary to secondary, such as 100:5 for a current-output CT or as a primary rating against a 333 mV or milliamp output for a low-voltage CT. Accuracy class is a percent, where Class 0.2 means plus or minus 0.2 percent at full load.

Submeter

A meter measuring a tenant or load below the utility's revenue meter, owned by the building, not the utility

Revenue-grade

Meter accurate enough to bill against, meeting ANSI C12.20, typically Class 0.2 or 0.5

CT (current transformer)

A sensor around a conductor that outputs a signal proportional to the current, sized by ratio

kWh / kW

Kilowatt-hours measure energy used; kilowatts measure the rate, and the peak rate is demand

Power factor

Real power divided by apparent power, 0 to 1, low when the load is reactive or distorted

PUE

Power usage effectiveness, total facility energy divided by IT equipment energy in a data center

EPMS

Electrical power monitoring system, the dense metering and software stack in a data center

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