In a grid-tied system, the power generated by your solar panels is first used to meet any electricity requirements of your home. This means whatever you are running when the sun is up, you'll use the electricity generated by your own solar panels. However, if your home's power requirements are not met by solar generation itself, grid power is also used. Essentially, you get less for solar power and get charged more for the grid power. Basically, try to use your electrical appliances (whichever can be helped) when your solar system is producing the most power.