The fluorescent tubes in the dormitory are generally 18W or 21W. Mobile phone charger general 5W, fast charge 10W. Calculate by 21 w tubes, open daily from 18 to 23 points, five hours, that is 21 x2x5 = 210 wh; Charger according to the fast 10 w, charged four times a day (in view of the waiting for the National Day, in not to go around the girl's dormitory severe players) for 1 hour at a time, that is 10 x4x1 = 40 wh; A day is 210 40 = 250...... Wh: oh no, says is a 4 hours a day, so is the 21st x2x4 = 168 = 208 Wh \/ 40 d 20 KWH = 20 KWH = 20000 Wh \/ 208 Wh\/d = 96 days! -- -- -- -- -- -- -- -- -- -- --, just phone recharge, especially pay attention to the charger (huawei), under the slow charge 5 v2a, quick charge 5 v4. 5 a, which is quick charge is P = UI = 5 x4. 5 = 22.5 W (actual charger calculates power consumption to calculate the input power, but only the input range of voltage and current, It's hard to quantify, but it doesn't make a big difference. Quick charge power, of course, different manufacturers still have very big difference) so in the above calculation formula: 22.5 X4X1 = 90 wh 168 90 = 258 wh\/d 20000\/258 = 77 days