How the calculation works
Monthly supply cost = therms × dollars/therm + monthly supplier fee. Stress cases change usage or unit price by 25%, one at a time. They are sensitivity tests, not probabilities or forecasts.
Use inputs you can defend
Read the units and use a consistent billing period. Defaults are examples; replace them with values relevant to your decision. If a cost is unknown, the result cannot establish that it is zero. Keep excluded charges in a separate budget and compare the same service scope before judging two alternatives. You can print this page for your own records without creating a website inquiry.
Keep units consistent
A therm measures heat energy; a gas bill may also display a volume measurement and a conversion factor. Quote units must match the billed usage before calculating cost. The commodity price, monthly supplier fee, utility distribution charge and tax are separate components. If a quote uses a wholesale reference or a different energy unit, ask for an explicit conversion and the full delivered pricing formula rather than comparing the raw numbers.
Distinguish choice from transportation
The ICC describes small-customer choice programs in Nicor Gas, Peoples Gas and North Shore Gas territories. Ameren publishes a separate business gas transportation pathway. Transportation can involve supplier nominations, balancing, storage or other obligations beyond a simple commodity rate. Eligibility and timing follow the utility and account. Do not infer gas service options from your electric utility or treat every Illinois business as enrolled in the same program.
Stress-test winter exposure
Build a monthly usage profile rather than multiplying a mild month’s bill by twelve. Compare a base month, higher consumption at the same price, and higher consumption with a higher variable price. Fixed supply can reduce commodity price uncertainty while leaving weather-driven consumption uncertain. Document whether each price includes basis, transportation-related tasks and fees; missing inclusions can be larger than the difference between two advertised commodity prices.
Keep the inputs private
These calculations run on this page using your browser. They do not save values to local storage, attach figures to a review request or send them to a server. If you choose to ask a question, the review link may select only the inquiry type. Closing or refreshing the page discards your edits. The print button opens your browser’s print dialog.
Sources & further checks
Primary references checked for this guide on October 15, 2026. Follow the source for current tariffs, eligibility and program terms; rates here are not live offers.