The correct math is (DC output / power from wall) = % efficiency but the table doesn't tell us power from the wall.

To find power from the wall we get (DC output / % efficiency) = Power from wall.

To find waste heat we have (Power from wall - DC output) = watts wasted.

Now if we use 100 W DC output / 80% efficiency it equals 125W. 125W - 100W = 25 watts wasted not the 20W you might have though of. The key is that you have to divide by efficiency in this case not multiply.

So the table in the article is most likely correct. We at least know that the 25W number in the intersection of 100W and 80% is correct.

**Code:**

THE EFFECTS OF EFFICIENCY

Efficiency Power Lost as Heat

@100W output @200W output @400W output

85% 18W 35W 70W

80% 25W 50W 100W

75% 33W 67W 133W

70% 43W 86W 174W

65% 54W 108W 216W

If we use 400 W DC output / 65% efficiency it equals 615.38461538461538461538461538462W.

615W - 400W = 215W watts wasted, the table rounded up to 216W but maybe he used data for efficiency with more decimal places or maybe he just prefers to round up.