A solar power system may comprise a solar panel set, a controller, a lithium battery set, and at least a DC load. The controller has a control unit built therein to control a double-contact relay, a single-contact relay, and a transformer. The rated voltage of the solar panel set is higher than the rated voltage of the lithium battery set between 115% and 130%. When the actual voltage of the solar panel set is lower than 115% of the rated voltage of the lithium battery set, the solar panel set is configured to low-loss charge the lithium battery set under the low illumination condition. When the actual voltage of the solar panel set is higher than 115% of the rated voltage of the lithium battery set, the solar panel set under the high illumination condition is adapted to have voltage-drop through the transformer and high-efficiently charge the lithium battery set.