On a CFE medium voltage supply the demand charge is set by a single fifteen minute interval. CFE bills the demanda máxima medida, the highest average kW recorded in any fifteen minute window of the billing period, and that one window fixes a charge the site pays for the whole month. A sub-meter does not reduce it. What a sub-meter does is show which loads were running inside that window, which is the only reliable way to decide what to move.
That is the case for sub-metering in Mexico, and it is a narrower case than the marketing suggests. The measurement itself returns nothing. The US Department of Energy's own guidance says so directly. "Metering in and of itself saves no energy, water, or dollars," in PNNL-23892, Release 3.0, published in March 2015. The return comes from the operating changes the data supports, and on a Mexican industrial bill those changes land first on the demand lines rather than on kWh.
So the decision is where the meters go and how many are enough. Meter at equipment level on the loads capable of moving the fifteen minute peak. Meter by zone only when the purpose is allocating cost between tenants or cost centers, which moves cost between budgets without lowering what the site pays.
What the CFE bill charges, and on what basis
The fifteen minute definition is written into the Mexican tariff itself. The billed quantity is "la demanda máxima medida en el periodo de facturación, durante cualquier intervalo de 15 minutos, en el cual el consumo de energía eléctrica sea mayor que en cualquier otro intervalo de 15 minutos". The 2015 field tutorial on Mexican electricity tariffs published by COMPITE with GIZ carries that wording for the medium voltage schedules that preceded GDMTO and GDMTH. The fifteen minute integration window survived the December 2017 restructuring, but under CRE Acuerdo A/039/2021, published in the DOF on 28 January 2022, the billed demand is the lower of that measured maximum and the maximum demand associated with the period's consumption.
The structure differs across the two medium voltage tariffs most plants sit on. Both carry two demand charges, capacidad and distribución. GDMTO bills a single energy rate and assigns both demand charges to the month's measured maximum. GDMTH splits energy into base, intermedio and punta periods and assigns the capacity charge to demand coincident with the punta window while distribution follows the maximum for the month, so a plant on that tariff has two distinct peaks to manage and they are rarely the same interval.
A third line responds to a different variable. Where the metered power factor for the period falls below 90%, the supplier applies a recargo of 3/5 x ((90 / FP) - 1) x 100 to the billed amount. The disposition setting that formula was published in the Diario Oficial de la Federación on 31 October 2000. Sub-meters that record reactive energy locate the load responsible. The main meter only reports that the site as a whole is behind.
Before reading any vendor proposal, calculate your own marginal cost of a kilowatt. Take the capacidad and distribución amounts in pesos from last month's invoice, divide each by the kW they were billed on, and add them. That figure is what one kW of avoided peak is worth, every month the peak repeats. A metering proposal has to beat that number, not a percentage from a slide.
What the evidence says about savings
The 20% figure that circulates in metering literature has a traceable origin, and it is worth reading in full before planning against it. PNNL-23892 collects results from several US programs. A California Energy Commission pilot reported 10% reduced energy use and 5% lower peak period demand. EnerNOC reported 11 to 20% reduction in the consumption visible from the monitoring data stream. Portland Energy Conservation data showed 12 to 20% reduced energy use. The guide summarizes the range as efficiency improvements of 10 to 20% where the indicated changes were actually made.
Three limits travel with those numbers. They are US building stock, not Mexican industrial load. They are energy reductions, which is the weaker half of a Mexican bill. And the same document states that if no action follows the analysis, no lasting reduction should be expected. Treat 10 to 20% as evidence that the mechanism works somewhere, not as a figure to put in a Mexican capital request.
Where the meters go
Rank the loads before specifying anything. List every load above about 5% of billed maximum demand, then score each by rated kW against the probability that it runs inside the punta window. Air compressors, chillers, electric furnaces, injection presses and battery charging banks are the usual candidates, because they are large and their schedules are movable. On the metering plans Mexico Energy Partners specifies, the design target is an unmetered residual below 15% of billed maximum demand, which is normally enough to attribute a peak without metering every panel in the plant.
The recording interval decides whether the data can answer the question. Log at one minute or finer and aggregate into fifteen minute windows aligned to the utility meter's own clock. Data logged on fifteen minute boundaries that do not match CFE's boundaries will show a plausible peak that is not the billed one, and the two loads that actually overlapped will stay invisible. Time synchronization is the cheapest part of the specification and the one most often skipped, a point covered in more detail in our earlier note on energy monitoring and IoT.
Automated control is worth specifying only when the action is named. A threshold on the incomer that sheds a stated load, a compressor stage or a chiller, when the rolling fifteen minute average approaches the month's running maximum, avoids one excursion. Avoiding one excursion in the punta window is the whole of the saving on the capacity charge that month. Controls written as "optimize energy use in response to real time data" do not survive contact with a production schedule.
Allocation is not saving
Zone metering for chargeback changes which budget carries the cost. At entity level nothing has been saved until load actually moves, and metering business cases in Mexico are more often wrong on this point than on any other. Present allocation as a governance benefit and price it at zero.
Billing a tenant from a sub-meter is a different matter again. That is a commercial transaction, and instruments used in commercial transactions fall under Mexico's legal metrology framework, set by the Ley de Infraestructura de la Calidad published in the DOF on 1 July 2020. A meter installed for engineering visibility is not automatically fit to invoice a third party. Decide which purpose the instrument serves before buying it, because the specification is not the same.
What actually creates a measurement duty
Compliance in this area is regularly attributed to the Ley de Transición Energética. That law sets national clean energy targets and the framework for the national efficiency program. It does not oblige a building or a plant to install a sub-meter.
The instrument that does impose measurement on a load center is the Código de Red, issued by the Comisión Reguladora de Energía as resolution RES/151/2016, published in the DOF on 8 April 2016, and reissued as RES/550/2021, published in the DOF on 31 December 2021 and in force since 1 January 2022. CRE holds compliance and sanction authority. CENACE operates the system and the market and does not issue or enforce the code. It reaches load centers at media tensión, above 1 kV and up to 35 kV, and at alta tensión, 35 kV and above, regardless of contracted demand or supply scheme. Load centers connected before 8 April 2016 had until 9 April 2019 to comply with the 2016 resolution. Studies are expected to use Class A instruments under NMX-J-610/4-3-ANCE or IEC 61000-4-30, and CRE does not authorize or certify firms to carry them out, whatever a quotation may imply about approved verifiers. Our separate note sets out Código de Red obligations for load centers in more detail.
Keep the two projects apart in the budget. A Class A power quality recorder answers the grid code. A sub-metering system answers the demand charge. Neither substitutes for the other, and a supplier who offers one instrument for both questions is answering only one of them.
How to build the case
Installed cost per metering point is not published in Mexico and moves with current transformer access, panel space, cable runs and whether the plant already has a network to carry the data. Take two quotations on the same point list rather than working from a benchmark. Then apply one test. Multiply the kW of peak the plan can name and demonstrably shift by the peso per kW from your own invoice, multiply by twelve, and compare against installed cost over the plant's payback threshold.
If the plan cannot name the loads it intends to shift, it is a visibility project rather than a savings project. Fund it as one, with a smaller number and a shorter scope. And assign a single owner for the data before the meters are ordered. Energy monitoring services that report to nobody in particular stop being read within a quarter, and the invoice goes back to whatever it was.
Have your metering plan checked against your CFE bill
Mexico Energy Partners reviews metering plans against the invoice they are meant to move. Send twelve months of CFE invoices, the plant single line diagram, and a list of loads above the threshold you use, with their operating schedules. You get back a metering point list ranked by contribution to billed maximum demand, the interval and synchronization specification the data has to meet, and an installed cost estimate to test against your own payback rule. No saving is quoted before the loads are ranked.
Sources
- Pacific Northwest National Laboratory for the U.S. Department of Energy, "Metering Best Practices: A Guide to Achieving Utility Resource Efficiency, Release 3.0", PNNL-23892, March 2015. The statement that metering by itself saves nothing, and the program results behind the 10 to 20% range.
- COMPITE and Deutsche Gesellschaft für Internationale Zusammenarbeit, "Tarifas Eléctricas: Tutorial para el trabajo en campo", 2015. The fifteen minute basis for demanda máxima medida on the medium voltage schedules that preceded GDMTO and GDMTH.
- Comisión Reguladora de Energía, Acuerdo A/039/2021 and its Anexo Único, Diario Oficial de la Federación, 28 January 2022. Distribution and capacity charges billed on demand for GDMTO and GDMTH. The capacity charge assigned to punta coincident demand on GDMTH and to maximum demand on GDMTO. Billed demand as the lower of the measured maximum and the maximum demand associated with the period's consumption.
- Secretaría de Hacienda y Crédito Público, "Acuerdo que modifica las disposiciones complementarias a las tarifas para suministro y venta de energía eléctrica", Diario Oficial de la Federación, 31 October 2000. The power factor recargo formula and the 90% threshold.
- Comisión Reguladora de Energía, resolution RES/151/2016, "Disposiciones administrativas de carácter general que contienen los Criterios de eficiencia, calidad, confiabilidad, continuidad, seguridad y sustentabilidad del Sistema Eléctrico Nacional: Código de Red", Diario Oficial de la Federación, 8 April 2016. Reissued as resolution RES/550/2021, Diario Oficial de la Federación, 31 December 2021, in force since 1 January 2022. Read with CRE's guide to the technical requirements applicable to Centros de Carga. Scope by connection voltage, Class A instrumentation and the 9 April 2019 compliance date under the 2016 resolution.
- "Ley de Infraestructura de la Calidad", Diario Oficial de la Federación, 1 July 2020. Mexico's legal metrology framework for instruments used in commercial transactions.