Audit a Shared EV Coincident Demand Charge
Compare charger sessions with the utility's winning interval so EV drivers pay only the incremental demand they actually created.
A shared EV driver should not be charged the entire household demand fee merely because the car charged that month. Identify the utility’s billable peak interval, compare the home’s demand with and without each charging session, and allocate only the incremental peak that the EV actually created.
Read the tariff’s demand rule first
Energy and demand are different. Energy is accumulated kilowatt-hours. Demand is power during a defined interval, often the highest interval in the billing period. The federal Alternative Fuels Data Center’s utility module explains that coincident EV charging can raise a site peak and that managed charging can distribute charging across a vehicle’s parked period.
Copy the utility’s exact interval length, demand rate, peak window, seasonal rule, minimum demand, and any ratchet based on earlier months. Do not assume every visible spike is billable. Some tariffs measure only specified hours or preserve part of an earlier peak.
Obtain interval data from the utility meter and timestamped session data from each charger. Normalize time zones and daylight-saving changes before comparing them. Charger kWh for a whole session cannot reveal whether it overlapped the single billable peak.
Calculate the EV’s incremental peak
Build a coincident-peak stack for the billable interval. Start with total site demand from the utility. Add each charger’s average kW during that same interval. Estimate the non-EV baseline by subtracting those charger loads from site demand, checking that units and interval lengths match.
Suppose the home reached 14 kW during the billable interval. EV A averaged 5 kW, EV B averaged 3 kW, and non-EV load was 6 kW. If the same home’s highest eligible demand without EVs would have been 8 kW during another interval, the EVs created 6 incremental kW, not 8. Allocate that 6 kW between A and B in proportion to their contribution during the billable interval, unless the written household rule uses another defensible method.
Run a counterfactual check. Remove one EV session at a time and recompute the highest eligible site interval. An EV that charged all night but never affected the billable maximum may owe energy charges but no incremental demand charge.
If charger data is missing, do not infer peak contribution from miles driven or monthly charging kWh. Mark the allocation provisional and use charger-rated power only as an agreed cap, not as fabricated measured demand.
Handle ratchets, taxes, and future prevention
A ratchet can keep billed demand above this month’s measured peak because of a prior month. Put that difference in a ratchet tail row. Assign it under the prior peak agreement and keep it visible for every month it persists. A former EV owner may remain responsible only if the household agreement clearly addressed that continuing effect.
Multiply allocated incremental kW by the applicable demand rate. Add demand-related taxes only if the bill identifies them. Energy charges, charger network fees, and fixed customer charges remain separate. This prevents one EV session from absorbing unrelated household costs.
For the next cycle, set charger power limits or stagger start times before the home’s normal peak. HomeCo’s guide to managing multiple EV loads in a shared home addresses the operating side. Keep the ledger focused on what the utility actually billed.
Each month, save the tariff, bill, utility interval export, charger exports, calculation sheet, and one screenshot of the winning interval. That small evidence packet makes a large fee reviewable.
FAQ
Is a demand charge based on total EV kWh?
No. A demand charge is based on power during the tariff’s measurement interval, while kWh measures energy over time. A long, low-power session can use substantial energy without setting the monthly peak.
What if two EVs charge during the peak?
Use each charger’s average kW in the exact billable interval. Allocate only the incremental site peak they created, then divide that increment according to their coincident contributions.
Should the EV owner pay the home’s preexisting demand charge?
Not under an incremental-causation method. The household’s demand without EV charging remains a shared base under its normal rule. The EV allocation begins only where coincident charging raises the billable peak or an agreed ratchet tail.