Saudi Arabia receives a large amount of sunlight all year. For this reason, it is turning to solar energy as a clean and renewable source. Solar panels change sunlight into electricity. They do not burn fuel and they do not release harmful gases. One of the main challenges is that the sun sets at night. So power stations need large batteries to store the extra power that they make in the day. Also, dust collects on the panels and lowers their efficiency, so people must clean them often. The cost of panels falls year after year, so these projects become more useful. The Kingdom has started to build large stations in sunny areas far from the cities.

Question: In the text, the word "extra" in "extra power" means: