A single complex bill in ERCOT can require 250,000 determinants. Multiply that by hundreds of customers. Add resettlements. Run it through hundreds of pre- and post-billing validations.

Now ask the operations team managing it on spreadsheets how they’re doing.

That’s the reality facing retail energy suppliers today—and it’s creating a tension that’s becoming harder to ignore legacy billing systems that work fine for most customers but buckle under the weight of the complex products that C&I customers increasingly demand.

The pressure is real. And it’s accelerating.

Five years ago, full pass-through products were reserved for the largest C&I customers. Today, smaller commercial and industrial customers are asking for these structures because they’re more cost-effective than fixed pricing in a market where costs—and risks—keep climbing.

The problem? Many billing systems were designed 15 or 20 years ago, when markets first deregulated. They weren’t built for this level of complexity—or for a regulatory environment where stakeholder proceedings that once took years now wrap up in months.

ISO changes aren’t waiting

Consider what’s happened in just the past year:

ERCOT’s RTC+B implementation went live December 5, 2025—the most significant market enhancement since the nodal market launched in 2010. Real-Time Co-optimization fundamentally restructures how energy and ancillary services are dispatched, co-optimizing both every five minutes. For billing teams, this means new settlement components, new price signals, and retired legacy COP statuses. Systems that couldn’t adapt through configuration faced a long December.

ISO New England’s DASI went live March 1, 2025, replacing the Forward Reserve Market with a new Day-Ahead Ancillary Services Market. Two new categories—Flexible Response Services and Energy Imbalance Reserves—each with their own clearing prices, obligations, and close-out charges. For retailers passing through ISO costs: new line items, new calculations, new data feeds.

MISO’s LOLE calculation error, discovered in June 2025, revealed that third-party software had been applying the wrong methodology since 2017. The financial impact: approximately $280 million in adjustments, with settlement corrections hitting via miscellaneous charges. For retailers with MISO customers, this meant rapid recalculation and customer communication—on a timeline nobody planned for.

PJM’s large load integration challenges are reshaping resource adequacy conversations entirely. With 30 GW of data center load projected by 2030, FERC’s December 2025 colocation order and new transmission service categories will create billing constructs that systems will need to accommodate.

Each of these changes requires billing system modifications. For systems that require custom code rather than configuration, each one becomes a project.

The spreadsheet trap

What happens when a legacy system can’t support a new product type? Someone builds a spreadsheet.

It starts innocently. Sales closes a bespoke deal. Operations figure out a workaround. One customer becomes two. Two becomes twenty. Twenty becomes two hundred.

Teams that started with creative workarounds now have five or ten people whose full-time job is keeping manual processes running. Growth gets bottlenecked—not by market opportunity, but by back-office capacity. Sales can’t confidently offer complex products because operations are maxed out servicing existing customers.

And when ERCOT restructures its COP statuses or ISO-NE adds new ancillary services products, each change adds another calculation. Another spreadsheet. Another late night.

Why “rip and replace” isn’t always the answer

Full system conversions are expensive—often millions of dollars. Timelines stretch three to five years. The subject matter experts needed for the project are the same people needed to navigate day-to-day market uncertainty, including implementing RTC+B or integrating DASI components.

And there’s sunk cost reality. Many retailers have invested millions in current systems. Those platforms may handle 80% of the portfolio just fine. It’s the other 20%—complex, bespoke, high-touch customers—that create 80% of the operational burden.

A modular middle path

What’s emerging is a more surgical approach: identify what’s broken and fix that—without disrupting what’s working.

Modern architecture increasingly allows component-based solutions. Rather than wholesale replacement, retailers can supplement legacy platforms with specialized modules for complex billing, data aggregation, and resettlement management that integrate through APIs.

The “single source of truth” philosophy that drove CIS implementations for decades is giving way to pragmatism: best-in-class tools for specific problems, connected by modern integration.

The key is configurability. When ERCOT implements RTC+B, the response should be configuration, not custom code. When MISO issues a $280 million settlement adjustment, you need a system that can recalculate affected invoices and display adjustments as line items—not one requiring cancel-rebills for every impacted customer.

The cost of standing still

The risks of modernization are real. But so are the risks of inaction.

The changes in 2025 alone—RTC+B, DASI, the MISO LOLE error, PJM’s large load proceedings—represent just one year of market evolution. Next year will bring more. By not thinking about how to support complex products at scale, retailers’ risk having their software limit their business.

The question isn’t whether customers will demand more complexity. It’s when.

The retailers who figure out how to adapt—whether through modular solutions or targeted upgrades—will have a meaningful competitive advantage. The ones who don’t may find themselves watching opportunities pass by, limited not by market conditions, but by the systems they chose not to change.

By Kate Schmid, Director of Client Services, Customized Energy Solutions