Sample A - 100W
Sample B - 200W
-- Sample A is 100% more efficient than Sample B.
If that's hard to understand, then reverse it:
-- Sample B is 100% hungrier than Sample A.
Sample C - 300W
-- Sample A is 200% more efficient than Sample C.
-- Sample C is 200% hungrier than Sample A.
-- Sample B is 50% more efficient than Sample C.
-- Sample C is 50% hungrier than Sample B.
In terms of cost?
£100, £200, £300.
"50% off" of £200 = £100, but £200 is 100% larger than £100.
"1/3 off" of £300 = £200, but £300 is 50% larger than £200.
"More efficient" makes it all relative as opposed to absolute.