As snaily said above, your smartphone really doesn't need 2500kcal/day. I think what they were trying to say is that, given 100% perfect energy transfer, you'll

As snaily said above, your smartphone really doesn't need 2500kcal/day. I think what they were trying to say is that, given 100% perfect energy transfer, you'll be able to power your phone from your daily calories without missing much. As in, "this is so much, you can power a phone without realising something's missing".