Cala Systems

Cala Systems makes a heat pump water heater that behaves like a thermal battery. A flow sensor, variable-speed compressor, and predictive software let the tank learn a household's hot water patterns and heat ahead of demand, choosing its moments: when the grid is clean, when the sun is on the roof, when power is cheap. Water heating is the second largest energy use in American homes and around 4% of US greenhouse gas emissions, more than aviation, and the appliance responsible has barely changed in a century.

A conventional heat pump water heater is already efficient and still dumb, reacting to a cold tank and falling back on resistance heat when it guesses wrong. Cala moves the same job to the hours when it costs the grid least. There are tens of millions of these tanks, and every one is a battery nobody was using.