Many modern computers have removed screen savers altogether as a proactive way to reduce energy usage. While sleep mode does an effective job of reducing energy waste by as much as 70 percent, it still expends energy when not in use. If you plan on not using your computer for at least two hours, shut it down. This will cut power usage completely. If you plan on coming back, it's OK to keep it in sleep mode so it turns back on with the stroke of a key.