The duck curve

The short version

The duck curve is a graph of net load — total electricity demand minus renewable generation — plotted over the course of a day. In high-solar markets like CAISO, the midday surge in solar output significantly depresses net load during the middle of the day, creating a deep "belly." As solar output drops in the late afternoon, net load must ramp steeply upward to meet evening demand — forming a shape that resembles a duck in profile.

The duck curve is not a problem in itself — cheap midday solar is a good thing. The problem is the ramp rate: grid operators must procure enough fast-ramping generation to fill the gap between falling solar output and rising evening demand in a matter of hours. In CAISO, the 3-hour evening ramp has exceeded 15,000 MW — one of the steepest ramping challenges of any grid in the world.

Why negative prices happen

During peak solar hours, when net load drops into the duck's belly, the grid can become oversupplied. Solar generators with zero marginal cost keep producing; inflexible thermal generators can't ramp down fast enough; and hydroelectric generators may be running for reasons unrelated to electricity demand. The result is too much supply chasing too little demand — and wholesale prices go negative. CAISO has seen negative prices exceeding $-100/MWh during spring afternoons when solar output is high and air conditioning demand is still low.

Solutions to the duck curve problem

Battery storage is the most direct solution — charge during the belly (absorb excess solar), discharge during the ramp (replace the lost solar). A 4-hour battery deployed at scale provides both the absorption and the dispatchable capacity needed for the evening peak. CAISO has procured thousands of MW of battery storage specifically to address the duck curve challenge.

Demand flexibility can shift load from evening into the belly — pre-cooling buildings, running EV charging, operating industrial processes during midday hours when solar is abundant and prices are low. This flattens both the belly and the ramp simultaneously.

Curtailment and exports are the current backstop. CAISO regularly curtails solar generation during peak midday hours when the belly is deepest — a waste of clean energy that represents both an economic loss and a policy failure. Expanding transmission connections to export surplus solar to neighboring states is a longer-term structural solution.