Load curtailment is the intentional reduction of electrical load to keep a site or grid connection within defined limits. In EV charging, load curtailment means temporarily reducing charging power (or pausing some sessions) to avoid overloading the electrical supply, manage peak demand costs, or comply with grid operator constraints. It can be triggered by building load spikes, utility signals, or site-level energy management rules.
What Is Load Curtailment?
Load curtailment reduces power consumption by enforcing a lower maximum load than would otherwise occur. For EV charging sites, curtailment typically applies by:
– Lowering the current limit delivered to chargers
– Reducing total site charging power to a predefined cap
– Stopping or delaying lower-priority charging sessions
– Applying time-based limits during peak tariff periods
Curtailment can be manual (operator sets a cap) or automatic (system reacts in real time).
Why Load Curtailment Matters in EV Charging Infrastructure
EV charging loads can be large, variable, and coincident (many vehicles arriving at once). Curtailment helps:
– Prevent trips of main breakers and avoid downtime
– Stay within contracted maximum demand or connection capacity
– Reduce demand charges and control electricity costs
– Support grid stability during congestion or emergency events
– Enable more chargers to be installed without immediate grid upgrades
For site owners, curtailment can be the difference between a scalable deployment and expensive reinforcement work.
How Load Curtailment Works
Load curtailment is usually implemented through a control layer such as an energy management system (EMS) or CPMS, using:
– Live site measurements (smart meter or CT sensors)
– Defined thresholds (e.g., “do not exceed 120 kW total site load”)
– Curtailment rules (priority, minimum current, fairness)
– Charger communication (often via OCPP) to apply setpoints
Typical workflow:
– Detect that total load is approaching or exceeding a limit
– Calculate required reduction in EV charging load
– Distribute reductions across chargers according to rules
– Restore power gradually when capacity becomes available
Common Curtailment Triggers
EV charging curtailment can be triggered by:
– Building peak loads (HVAC start-up, industrial equipment, kitchens)
– Utility-imposed limits during grid congestion
– Time-of-use tariffs and peak price windows
– On-site generation shortfalls (PV drop, generator limits)
– Site constraints such as transformer temperature or cable limits
Some systems also support automated curtailment based on demand response programs.
Curtailment Strategies for EV Charging
Different sites use different approaches:
– Equal sharing: reduce all active sessions proportionally
– Priority-based: protect critical users (fleet, staff, accessible bays) and curtail others first
– First-come-first-served: earlier sessions keep more power
– SoC / departure-based: allocate power to vehicles that need it most before departure (when supported)
– Hard cap: enforce a strict total kW limit regardless of demand
Choosing the right strategy improves user experience while meeting electrical constraints.
Benefits of Load Curtailment
– Improves electrical safety and prevents overload events
– Reduces operating costs by limiting peak demand
– Enables higher charger density without immediate grid upgrades
– Supports compliance with utility constraints and grid codes
– Improves overall site stability and uptime
Limitations and User Experience Considerations
Curtailment must be configured carefully to avoid unintended outcomes:
– Too much curtailment can cause slow charging and driver dissatisfaction
– EVs may stop charging if current drops below a minimum threshold
– Frequent ramping can create unstable sessions if setpoints change too often
– Transparent communication (app/backend status messages) can reduce confusion
Curtailment is most effective when combined with good site design and realistic capacity planning.
Related Glossary Terms
Load balancing
Power throttling
Demand response
Peak shaving
Site power limit
Smart metering
Charge Point Management System (CPMS)
OCPP
Energy management system (EMS)