Curtailment

Two contexts for the same word

Curtailment appears in two distinct contexts, and conflating them leads to confusion.

Grid-level curtailment

At the wholesale level, AEMO occasionally curtails large-scale wind and solar farms - instructing them to reduce output - when there’s more generation on the grid than load can absorb and the system frequency is rising. This happens most often on mild spring days with high solar output and low demand. The generator is paid a below-cost or zero price and told to dial back.

As renewable penetration has grown, curtailment hours in the NEM have increased. South Australia, with very high renewable penetration relative to its demand, experiences more curtailment events than other states. AEMO’s solution involves expanding transmission (to export surplus interstate), building storage, and developing flexible load mechanisms.

Residential curtailment

For a home solar system, curtailment happens when the inverter is forced to produce less than what the panels could generate, because:

Export cap hit: A 10 kW system on a 5 kW export limit, with only 3 kW of house load, will export at 5 kW and curtail the remaining 2 kW. The panels could produce 10 kW, but only 8 kW is used.

Zero export with low load: On a zero-export connection with 2 kW of load, a 10 kW capable system produces only 2 kW and curtails 8 kW.

Battery full, grid limit reached: If both the battery is full and the export limit is saturated, the inverter throttles down.

Quantifying the loss

The Solar Export Loss Calculator can estimate how much generation is curtailed based on system size, export limit, battery capacity, and consumption profile. For typical residential systems (6.6 kW panels, 5 kW inverter, 5 kW export limit), curtailment is relatively minor because the inverter itself is the 5 kW bottleneck - the panels can produce more, but the inverter output is capped.

Curtailment becomes significant when the installed panel capacity substantially exceeds the export limit and there’s no battery to absorb the surplus. Adding battery storage is the most direct fix for residential curtailment.