How Interruptible Natural Gas Service Works and Whether It Makes Sense for Your Business

Every winter, Illinois utilities face periods of extreme cold that push natural gas system demand to its limits. The pipeline network is sized to serve firm (uninterruptible) customers reliably — but maintaining that capacity year-round is expensive, and the utilization of that peak capacity is only needed for a handful of days each year. Interruptible service customers provide a critical safety valve: they agree to curtail usage on those peak days, freeing up system capacity for firm service customers, in exchange for substantial rate discounts throughout the year.

For the right business, this is an excellent arrangement. Understanding whether you're the right business — and what exactly you're agreeing to — is what this guide addresses.

What Is Interruptible Natural Gas Service? A Plain-Language Breakdown for Business Owners

Firm vs. Interruptible Service: The Core Distinction

Natural gas service in Illinois is broadly classified into two tiers:

• Firm service: You receive uninterrupted gas supply regardless of system conditions. The utility is contractually obligated to deliver gas to you except in extraordinary circumstances (true force majeure events). Firm service customers pay full tariff rates.
• Interruptible service: You receive gas supply when system conditions permit, but the utility can curtail or completely interrupt your supply when peak demand requires it. In exchange, you pay significantly reduced rates.

The distinction is essentially price vs. reliability certainty. Firm service pays for guaranteed availability. Interruptible service accepts the risk of occasional disruption in exchange for lower cost.

How Interruption Works in Practice

When the utility determines that system conditions require curtailment, they issue a notice to interruptible service customers. Nicor Gas and Peoples Gas typically provide 24-hour advance notice for planned curtailments. Emergency curtailments may have shorter notice windows — sometimes just 2–4 hours. During a curtailment period (typically 24–48 hours), interruptible customers must reduce their gas consumption to or below their contracted curtailment level.

Curtailment events are relatively rare in typical Illinois winters — most years see 5–20 curtailment days. In severe winters or during extreme cold events, curtailment frequency and duration can increase significantly. Winter Storm Uri in February 2021 was an extreme case that tested the limits of interruptible programs across the Midwest.

Different Interruptible Service Tiers

Illinois utilities typically offer multiple interruptible service classifications based on how quickly and how often a customer must curtail:

• Low-priority interruptible: First to be curtailed, deepest discounts
• High-priority interruptible: Curtailed only after lower-priority customers, smaller but still meaningful discount
• Partial interruptible: A portion of your load is interruptible while a base amount is protected as firm

The appropriate tier depends on your operational tolerance for interruption and the value you place on service reliability relative to cost savings.

The Real Cost Savings of Interruptible Gas Service: Is Your Business Leaving Money on the Table?

Quantifying the Rate Discount

Interruptible service rate discounts in Illinois typically range from 10–30% on commodity costs compared to equivalent firm service rates. The exact discount depends on your service tier, contracted volume, and the specific rate schedule applicable to your account. For large commercial and industrial accounts consuming 500,000+ therms annually, this translates to:

• At 10% discount on $300,000 annual gas spend: $30,000/year savings
• At 20% discount on $300,000 annual gas spend: $60,000/year savings
• At 30% discount on $300,000 annual gas spend: $90,000/year savings

These are year-round savings that apply whether curtailment events occur or not. The savings are earned simply by accepting the risk of potential interruption.

The True Cost of Curtailment Events

The flip side of the savings is the cost incurred during actual curtailment events. Relevant costs may include:

• Backup fuel costs if you switch to oil or propane during curtailments
• Reduced production or revenue if curtailment forces operational downtime
• Administrative cost of managing the curtailment response

A thorough economic analysis needs to weigh year-round rate savings against expected annual curtailment costs. For most businesses with effective dual-fuel capability, the analysis strongly favors interruptible service enrollment.

Which Illinois Businesses Qualify for Interruptible Natural Gas Service (And Which Should Avoid It)

Best Candidates for Interruptible Service

• Dual-fuel facilities: Any business with backup fuel-oil or propane capability is an excellent candidate. You can switch fuels during curtailment events and capture the rate discount all year.
• Manufacturing plants with flexible schedules: Operations that can pre-schedule production to avoid curtailment periods, or can shut down temporarily without major losses, benefit from interruptible rates
• Warehouse and storage facilities: Temperature tolerance during short curtailments is often acceptable without operational impact
• Large office and commercial buildings: Pre-conditioning and thermal mass management can often handle 24–48 hour curtailments without significant tenant discomfort
• Greenhouse operations: With advance notice, temperature management strategies can accommodate short interruptions

Businesses That Should Avoid Interruptible Service

• Hospitals, data centers, and facilities where temperature or process continuity is critical to safety
• Food processing operations with temperature-sensitive products
• Chemical and pharmaceutical manufacturing where process interruption creates safety or quality risks
• Businesses without backup fuel capability and no operational flexibility during cold weather

How to Switch to Interruptible Natural Gas Service in Illinois: Steps, Risks, and Expert Tips

Step 1: Assess Your Curtailment Tolerance

Identify your gas-consuming equipment and processes. For each, determine: Can it be safely curtailed for 24–48 hours? What's the operational impact? What's the cost of a curtailment event (backup fuel, reduced production, etc.)? Build a clear picture of your maximum acceptable curtailment days per year and maximum curtailment duration.

Step 2: Evaluate Backup Fuel Options

If you don't already have dual-fuel capability, explore whether installing propane or fuel oil backup for your highest-priority gas loads makes sense. The investment in backup fuel infrastructure — tanks, burner modifications, service contracts — can be justified by the ongoing interruptible rate discount if you're a high-gas-intensity business. Get quotes for the infrastructure cost and calculate payback against the annual savings.

Step 3: Contact Your Utility for Interruptible Rate Options

Both Nicor Gas and Peoples Gas offer interruptible service rate schedules for qualifying commercial accounts. Contact their commercial accounts team and request information on applicable interruptible service programs, minimum volume thresholds, rate discount levels, and curtailment terms. Your commercial energy broker can facilitate this process and ensure you receive all available options.

Step 4: Model the Full Economics

Build an annual economic model comparing: firm service rate × annual consumption (your current cost) versus interruptible rate × annual consumption minus expected curtailment event costs. Use a conservative curtailment assumption (25% more days than historical average) to ensure the economics hold in above-average years. Run a stress test scenario for an extreme year (similar to 2021) to understand worst-case economics.

Step 5: Develop Curtailment Response Protocols

Once enrolled, establish clear internal response protocols for curtailment notices. Assign accountability, document procedures, train personnel, and test the response at least once before the first heating season. Successful interruptible service participation requires operational readiness that goes beyond signing the contract.

Evaluate Whether Interruptible Service Makes Sense for Your Business

Interruptible natural gas service is a legitimate, often overlooked opportunity for Illinois commercial and industrial customers who have the operational flexibility to curtail gas usage on short notice. The year-round rate discounts are meaningful — potentially tens of thousands of dollars annually — and the curtailment risk, while real, is manageable with proper planning.

The team at commercialgasrates.com can help you assess whether interruptible service is a fit for your business, model the full economics, and connect you with the applicable Illinois utility programs. Contact us for a free analysis.

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