Solar energy. Photovoltaic cells on the roof of a house in Switzerland. These cells, also known as 'solar cells', convert solar energy into usable electrical energy. This electricity is used in the house and for charging up the owner's electric car. The photovoltaic cells are connected so that in sunny weather the house can export electricity to the Swiss national grid. When the weather is dull the house can import power from the grid. The intention is that overall energy exports equal or surpass energy imports.