Solar panels change sunlight into electricity. They do not need fuel, they make no smoke, and once they are installed they cost little to run. Countries with many sunny days, such as those in the Middle East and North Africa, can produce a lot of power in this way. Panels can be placed on the roofs of homes and shops, so a family can make some of its own electricity. However, solar power has limits. At night there is no sunlight, and on cloudy days the panels produce less. To solve this problem, some systems store extra energy in large batteries for use later. Panels are also fairly expensive to buy, although their price has fallen a great deal over the past decade. Many governments therefore help people to pay for them.

Question: Why do some systems use large batteries?