Solar lighting is a tariff question before it is a technology question
Exterior lighting at a Mexican industrial site runs at night. Under CFE's Gran Demanda en Media Tensión Horaria tariff (GDMTH), which applies to medium-tension users with demand of 100 kW or more, most of the night sits in the intermedio and punta periods. At a plant running one or two shifts, yard and perimeter lighting can be the load that sets billed demand in a period when nothing else is running. That is the first thing a facilities manager should check, because it decides whether solar lighting is a sustainability line item or a cost line item.
Three things moved between 2016 and 2022, and they changed the arithmetic rather than the idea. LED efficacy rose far enough that Mexico wrote a higher floor into law. Battery and controller costs fell. And the carbon value of displaced grid electricity became a published number that an auditor will accept. What follows is what to specify, what to reject, and what to look at on the CFE bill. The companion piece on solar LED lighting and IoT controls covers the specification and siting detail in more depth.
The efficacy floor Mexico now sets
NOM-031-ENER-2019, the Mexican energy efficiency norm for LED luminaires used in roadway and public exterior area lighting, was published in the Diario Oficial de la Federación on 3 March 2021 and took effect 90 days after publication. It requires a minimum luminous efficacy of 95 lumens per watt for roadway luminaires with a rated life of 75,000 hours or less, and 105 lumens per watt where rated life exceeds 75,000 hours. Its scope reaches luminaires fed from the public grid and luminaires fed from batteries, accumulators or self-generation, so a solar luminaire is inside it, not outside it.
The norm governs public roadways and public exterior areas rather than a private plant yard. Specify against it anyway. It is the published Mexican floor, and a supplier who cannot produce a NOM-031 certificate or a photometric report for the exact catalog number being quoted is asking you to buy from a brochure. Watch one substitution in particular. Efficacy quoted for the LED package is not efficacy for the assembled luminaire, and the assembled figure is what determines how many watt-hours the battery has to carry from dusk to dawn.
What automation saves, and what it does not
Motion sensing and scheduled dimming cut the hours a luminaire spends at full output. Where a yard is occupied for a small fraction of the night and the controls hold a low standby level the rest of the time, energy drawn from the battery falls close to proportionally. Battery life does not follow that curve. Service life is governed by depth of discharge and cycle count, not by lamp burn hours, and a controller that shallow-cycles the pack every night is still cycling it every night. A supplier claim that cutting duty cycle by a factor of ten multiplies battery life by the same factor does not hold, and it should be replaced with a cycle-life curve at the actual depth of discharge the design implies.
The question to put in the tender is autonomy. How many consecutive days can the unit hold design output with no charge, at what depth of discharge, at what ambient temperature, and what is the warranted cycle count at that depth. In a wet season, autonomy is the specification that decides whether a light works in August.
Tilt, siting and panels you would rather not see
Panel tilt and azimuth change annual yield, and the gain from correcting them is site-specific. No single percentage carries from Hermosillo to Guadalajara to Mérida, and any supplier quoting one has skipped the modeling. Require a modeled annual yield at the site's own coordinates, with the assumed soiling loss stated, and require it before the sizing is fixed. Where a mast carries both the panel and the head, they usually want to point in different directions, which is a real reason to specify independently adjustable mounts rather than a styling preference.
Building-integrated photovoltaics and printed panel overlays exist for places where a conventional module is visually unacceptable. Both cost output. An overlay filters light before it reaches the cell, and the loss is material and specific to the product and the pattern. Ask for the derated output of the exact overlay being supplied, in writing, and size the system on that figure rather than on the bare module rating.
The carbon number, and where it comes from
Carbon reduction from lighting means one thing in Mexico. It is the grid electricity the site stops buying, valued at a factor the government publishes. SEMARNAT's aviso of 15 April 2021 set the Factor de Emisión del Sistema Eléctrico Nacional for 2020 at 0.494 tonnes of CO2 equivalent per MWh, and that is the factor applied to indirect emissions from purchased electricity under the Registro Nacional de Emisiones. A retrofit's reported reduction is the MWh removed from the meter multiplied by the factor for the reporting year. The factor is reissued annually, so a multi-year claim built on a single year's factor will not reconcile.
Our assessment is that exterior lighting is a weak headline decarbonization project and a good one to bank. At most plants the tonnage is small next to process load. It is also cheap, measurable and defensible in an audit, which is more than can be said for much of what fills a first sustainability report. If the site is pursuing a green building certification, the same metered reduction is the evidence base for both, so run the certification documentation off the same meter data rather than a second study.
Where CRE gets involved, and where it does not
A self-contained solar luminaire has no connection to the Red General de Distribución. There is no interconnection contract, no meter change and no study from CFE Distribución. That administrative saving is part of why remote poles, perimeter runs and detached yards are where the case is strongest, alongside the trenching and cabling the site does not have to install.
Roof-mounted solar serving the plant itself is a different regime. Generación distribuida covers plants below 0.5 MW of generating capacity, under SENER's Manual de Interconexión de Centrales de Generación con Capacidad menor a 0.5 MW (DOF, 15 December 2016) and CRE's Resolución RES/142/2017 (DOF, 7 March 2017). The threshold applies to generating capacity, not to the facility's load, and it is expressed in MW rather than in kWp of DC panel. CRE writes those rules. CFE Distribución operates the network and processes the interconnection, and it is not the regulator.
Questions for a lighting supplier
- Which CFE tariff the site is on, and whether exterior lighting contributes to the punta or intermedio demand reading on the last 12 bills.
- Assembled luminaire efficacy in lumens per watt for the quoted catalog number, with a NOM-031 certificate or a full photometric report.
- Autonomy in days at design output, the depth of discharge assumed, and the warranted cycle count at that depth.
- Modeled annual yield at the site's coordinates, with stated tilt, azimuth and soiling loss.
- Derated output for any overlay or integrated module, stated by the manufacturer rather than the installer.
- Replacement cost and lead time for the battery pack, which is the item that fails first and the one no one budgets.
Any savings range a supplier quotes is measured against your current CFE supply and moves with tariff class, region, load factor and burn hours. Treat it as illustrative until it has been rebuilt from 12 months of billing and the site's real lighting schedule. Where a site already has interval metering in place, that rebuild is a short exercise rather than a study, which is one more argument for metering the load before specifying the fix.
Sizing a lighting retrofit against your own bill
Mexico Energy Partners reviews exterior and process lighting load against a facility's CFE billing and tariff class, and separates the energy saving from the demand saving so the two are not double counted. An initial review needs 12 months of CFE bills, the tariff class, a lighting schedule or burn-hour estimate, and a count of fixtures by type.