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On sunny days, the photovoltaic panels supply all of the Paris household’s electricity needs and charge their hybrid car’s electric battery, too. Once these basics are covered, the rooftop-generated power feeds into the stationary battery until it’s full — primed for nighttime energy demand and cloudy days. Then, when the battery is topped off, the unit’s digital control system automatically redirects any excess energy into Berlin’s power grid, for which the Parises will be compensated by the local grid operator.

The Parises are one of more than 120,000 German households and small-business owners — and an estimated 1 million people worldwide — who have dug deep into their pockets to invest in solar units with battery storage since lower-cost systems appeared on the market five years ago.

Today, one out of every two orders for rooftop solar panels in Germany is sold with a battery storage system.

The home furnishing company Ikea even offers installed solar packages that include storage capacity.

this recent growth demonstrates the strong appeal of a green vision for the future: a solar array on every roof, an electric vehicle in every garage, and a battery in every basement.

For a small house with just 20 panels, one can expect to pay about $8,000 to $11,000 for the PV array and roughly the same amount for the battery and DC/AC power inverter. The largest home batteries go for around $34,000. And for an extra $500, advanced devices connect the system to household appliances and optimize energy use, as well as regulating feed-in to the grid. With such top-of-the-line technology and lots of sunlight, an owner might save as much as 80 percent on electricity bills, according to Solarwatt, a Dresden-based outfit manufacturing smart tech.

But the economics of battery storage aren’t the only, or even the main, motivation of most battery system buyers, says Matthias Schulnick of Enerix Berlin. “More and more people want to be independent of the power companies and rising prices,” he says. “And they want a green footprint, to do something for the future.”

The consulting firm McKinsey predicts that the cost of energy storage systems will fall 50 to 70 percent globally by 2025 “as a result of design advances, economies of scale, and streamlined processes.”