California’s main power grid operator issued an emergency watch notice for Tuesday evening as residents cranked up air conditioning during a heat wave.

Energy use could outstrip available supplies and result in possible electricity shortages, the California Independent System Operator warned in a statement posted to its website. The grid operator was encouraging market participants to offer additional grid stability support services, while utilities may ask customers to reduce demand, it said.

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California has struggled to keep the lights on during the evenings in hot summer months when the state’s abundant solar power fades yet electricity demand remains high. Residents in parts of southern California are under an excessive heat warning as sizzling temperatures grip the US southwest.