Real-time, day-ahead, and ancillary services — what each price means and why your bill looks the way it does.
Most business owners look at their electricity bill and see a single number — cents per kilowatt-hour. What that number doesn’t show is the layered, second-by-second auction running underneath it. Understanding that auction is the difference between being a passive ratepayer and being an informed buyer.
ERCOT runs three interlocking markets at once: a real-time energy market, a day-ahead market, and an ancillary services market. Together they decide what every megawatt of power costs at every node on the Texas grid, every five minutes of every day.
The Real-Time Market
Every five minutes, ERCOT runs a process called Security-Constrained Economic Dispatch (SCED). It looks at every generator’s offer to produce power, every transmission constraint on the system, and every megawatt of demand — then solves a massive optimization problem to find the lowest-cost mix of generation that keeps the grid balanced and stable.
The output is a Locational Marginal Price (LMP) at every settlement point on the grid. That price reflects the cost of delivering one more megawatt of power to that exact location. When transmission lines are uncongested and resources are plentiful, prices stay in the $20–$40 range. When constraints bind or supply tightens, prices can move to several thousand dollars within a single five-minute interval.
The Day-Ahead Market
The day before the operating day, ERCOT runs a financial market where participants can lock in prices for power they’ll consume or produce the next day. This gives generators certainty about how much they’ll run and gives load-serving entities a hedge against real-time price spikes.
If you’ve ever wondered why your fixed-rate contract is priced where it is, the day-ahead market is a big part of the answer. Retail providers use day-ahead and forward markets to hedge the load they’ve committed to serve at fixed prices.
Ancillary Services
Beyond just energy, ERCOT also procures reserves — generation and load resources held back to handle unexpected events. The main products are Regulation (responding to second-by-second imbalances), Responsive Reserve Service (RRS), Non-Spinning Reserve (Non-Spin), and the newer ERCOT Contingency Reserve Service (ECRS), which was designed to address the increased uncertainty that comes with a growing share of renewable resources.
Ancillary services costs have grown significantly as ERCOT has procured more reserves to manage variability. These costs eventually flow through to customers as part of the total cost of delivered power.
Where the Costs Show Up on Your Bill
- Energy charge: the wholesale cost of the electricity itself, plus your provider’s margin and risk premium.
- TDU delivery charges: what your wires company (Oncor, CenterPoint, AEP, TNMP) charges to physically deliver power to your meter. These are regulated and non-negotiable.
- Ancillary and ERCOT fees: the cost of reserves, system operations, and ERCOT’s own administrative fees, often passed through.
- Demand charges (commercial): based on your peak kW draw during a billing period. For many commercial customers, these can rival or exceed the energy charge itself.
If you don’t know your load factor, your peak demand pattern, and which ancillary costs are passed through versus baked into your rate, you’re not really negotiating your contract — you’re guessing.
Why This Matters Right Now
As Texas adds tens of thousands of megawatts of new load — much of it data centers running close to 100% of the time — the structure of the wholesale market will be tested. Real-time prices are likely to become more volatile during transition hours when solar generation ramps down and demand stays high. Ancillary service costs will continue to grow. Businesses that understand which of those costs flow through to them, and which can be hedged via contract structure, will pay materially less than businesses that don’t.
Take the Next Step
Not sure what’s actually driving your monthly electricity bill? Amerigy Energy provides a free bill audit that breaks down every line item — energy, delivery, ancillaries, demand, and pass-throughs — so you can see exactly where you’re spending and where you have leverage.
