Just used my Revive Solar Charger 4000mAh External Battery for the first time. Specs are here: http://www.amazon.com/gp/product/B0083ACGCO/ref=oh_details_o00_s00_i00 First I fully charged the solar charger via USB for > 12 hours. Then I used it to - Charge my iPhone 4 from 35% to full - Charge my iPad 2 from 49% to 71%. (Yes, it has a 2.1amp USB port.) Then the solar charger was out of power. What is the math for checking if it truly used 4000mAh that it is supposed to have for that amount for charging?