Since Solana Beach launched its own power-buying agency to compete with San Diego Gas & Electric in June, the city’s 7,000 electricity customers have already saved about $200,000.

But the city is also already looking ahead to a rough patch where it expects the new agency to lose money until 2022. The amount is relatively small – about $350,000 over three years – but an early sign of how hard it is to predict what will happen in the energy business.