N EVs · arrival diversity · coincident site peak
This simulator models the aggregate site load from a fleet of EVs charging over a 24-hour period, accounting for staggered arrivals, charge duration, and managed charging (simultaneity) constraints. Engineers and utility planners use it to quantify the coincident peak demand, transformer impact, and transformer loading that determines whether new infrastructure is needed.
The simulator distributes EV arrivals across the day using a Gaussian (bell-curve) distribution centered on the user-specified peak arrival hour with a configurable arrival window. Each arriving EV charges at the specified Level 2 power for a duration determined by energy needed divided by charger power (e.g., 40 kWh ÷ 11.5 kW ≈ 3.5 hours).
The raw (unmanaged) coincident load at each hour is the sum of all overlapping charging sessions — this represents worst-case transformer loading if no demand management is applied. Managed charging applies a simultaneity cap (EMS limit) expressed as a percentage of connected load; demand above the cap is queued and deferred to later hours using a FIFO algorithm, preserving total daily energy while flattening the peak.
Peak current is derived from the coincident peak power at the service voltage and phase: I_peak = P_peak × 1000 / (V × √3) for three-phase, or P_peak × 1000 / V for single-phase. This current must be within the transformer nameplate rating or a transformer upgrade is required. The realized diversity factor (coincident peak ÷ connected load) quantifies how much natural staggering reduces the required infrastructure.
NEC Article 625 governs EVSE installation and load calculation, requiring a 1.25× continuous load multiplier per 625.42 and permitting demand factors with a Listed EMS per 625.42(A). The coincident peak from this simulator serves as the input to the NEC 625 demand load calculation.
IEEE 1547-2018 addresses distributed energy resource interconnection and includes provisions for managed EV charging when storage or vehicle-to-grid (V2G) capability is involved. OCPP (Open Charge Point Protocol) 1.6 and 2.0.1 are the communication protocols used by EMS systems to actively manage EVSE output and implement the simultaneity cap modeled here.
ISO 15118 defines the communication interface between EV and charger for smart charging (Plug & Charge, V2G), enabling the vehicle to communicate its state of charge and energy needs to the EMS. Transformer sizing follows IEEE C57.91 loading guidelines, with nameplate kVA rating compared against the coincident peak kVA from the simulator.
Arrival diversity is the most powerful tool for reducing infrastructure cost. A fleet of 20 EVs arriving within a 6-hour window (typical workplace arrival spread of 7:00–13:00) produces far lower coincident peaks than 20 EVs all plugging in at 8:00 AM sharp. The Gaussian arrival model captures this: wider arrival windows and longer charge durations naturally reduce simultaneity.
Managed charging (simultaneity factor below 100%) requires a Listed EVSE Energy Management System per NEC 625.42(A). Common EMS strategies include first-come-first-served power sharing, scheduled charging windows, and real-time TOU rate response. At 50% simultaneity on a 20-EV fleet, the coincident peak drops from 230 kW (full simultaneity) to 115 kW — potentially avoiding a costly transformer upgrade.
Fast charging (DCFC) has a very different profile: low dwell time and high peak power per vehicle means even a few DCFC ports create sharp, high-magnitude peaks with minimal natural diversity. DCFC sites require dedicated demand management, battery buffering, or demand charge management strategies to control utility bills.
Set the number of EVs and charger power (Level 2: 3.3–19.2 kW). Enter the charge energy needed per EV (typical BEV: 30–60 kWh, PHEV: 10–20 kWh). This determines charging duration and the duration over which each EV holds a load on the transformer.
Adjust the arrival window (narrow = all arriving at once, wide = spread across the day) and peak arrival hour to match your fleet behavior — workplace fleets typically peak at 07:00–09:00, retail at 12:00–14:00. Reduce the simultaneity factor if a Listed EMS is installed.
Set the service voltage and phase for your installation. Review the coincident peak, daily energy, and realized diversity on the summary banner and load profile chart. The orange dashed line on the chart shows the coincident peak — this is the design demand for transformer and service sizing.
Coincident peak is the maximum aggregate power draw from all EVs charging simultaneously on a given circuit. It is the design load for transformer and service sizing. Without diversity, coincident peak equals connected load (N × kW_per_charger). With natural arrival staggering and managed charging, the realized diversity factor reduces this significantly — often to 40–70% of connected load for workplace fleets.
An EMS using OCPP dynamically limits total site EVSE load below a set threshold by throttling individual charger output. This allows the transformer and service to be sized for the managed demand rather than full connected load, reducing infrastructure cost. A 200 kVA transformer might support 20 Level 2 chargers at 50% simultaneity that would otherwise require a 400 kVA unit at full load.
Analysis of real workplace charging data shows most employees arrive within a 2–3 hour window centered around the shift start, with a Gaussian-like distribution. Using a 4-hour arrival window (± 2 hours from peak) is a reasonable planning assumption for typical office environments. Fleet operations with defined shift changes may have narrower 1–2 hour windows requiring more aggressive demand management.
At 11.5 kW per charger with 100% simultaneity: 20 × 11.5 = 230 kW ÷ 0.85 PF ≈ 270 kVA — a 300 kVA transformer. With a 50% EMS simultaneity cap: 115 kW ÷ 0.85 ≈ 135 kVA — a 150 kVA unit. The EMS reduces transformer size by half, saving significant capital cost. Always confirm with the utility that the service capacity is available at the site.
Yes. Managed EV charging is increasingly used for demand response — delaying or reducing charge rate during grid peak events in exchange for rate incentives. Vehicle-to-Grid (V2G) per ISO 15118 allows bidirectional energy flow, enabling EVs to function as grid storage assets. IEEE 1547-2018 includes provisions for V2G interconnection requirements when EVs participate in grid services.
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