Commercial energy / Buyer guidance
Commercial electricity demand charges: reproduce the billed kW calculation
Separate energy from demand, identify intervals and ratchets, and calculate a demand example without assuming every reduced peak changes the bill.
Energy and demand answer different questions
kWh measures electrical energy over time. kW measures power. An invoice can charge for energy consumed and for a defined demand determinant. That determinant may be tied to a measured interval, a time window, a minimum, a historical ratchet or another tariff rule. Read the applicable schedule before deciding that the largest number on a monitoring dashboard is the number being billed.
A reduction in annual kWh does not necessarily lower billed demand. A brief overlap of large loads may remain. Conversely, shifting a load can change a billing-window peak while leaving energy consumption similar. Keep energy and demand effects as separate lines in an operating model.
Rebuild the invoice in the tariff's order
Record the demand measurement interval, any on-peak period, billing dates, measured maximum, minimum and ratchet reference. Identify whether different demand components use different determinants. Then apply the price in the tariff's units. A contract may fix a supply energy rate while leaving utility demand components unchanged.
Use interval data from the actual meter and period. Equipment nameplate loads are not substitutes for a billed peak. A manufacturer's claimed peak reduction is not an account-level reduction if another load sets the monthly maximum after the project.
Worked example: the ratchet limits the billed change
Hypothetical tariff inputs: measured peak is 500 kW, a minimum or ratchet floor is 480 kW, and the demand price is $18/kW for the month. The initial billed determinant is the greater of 500 and 480, giving $9,000.
Suppose an operating scenario lowers the measured peak by 20% to 400 kW. The floor still makes billed demand 480 kW, costing $8,640. The modeled demand difference is $360 for that month, not 100 kW × $18 = $1,800. The demand calculator reproduces this floor logic with entered assumptions.
Annualizing $360 to $4,320 assumes the same result in all twelve months. That is a scenario, not an annual forecast: weather, peaks, floors, rates and operating constraints can vary. A historical ratchet may also reset under rules not included in the simple tool.
Decide whether a proposed action can affect the determinant
- Does the load operate in the billing window?
- Will a different load set a new maximum after the change?
- Does a minimum or historic reference prevent the billed kW from falling?
- Is the action repeatable in the relevant months without compromising operations?
- Are energy, demand-response payments and avoided costs being counted separately?
Retain before-and-after interval data and an explanation of changes in production or occupancy. Do not describe every monitored peak reduction as an invoice saving. Reconcile the actual first bill against the governing tariff and record any difference between the model and the billed result.
Sources and limits
EIA's electricity unit guide supports the measurement distinction. DOE measurement and verification provides a framework for defining what is measured. The tariff terms and values in this example are invented teaching inputs; actual utility rules control an account.