They are certainly both 20% more efficient and 20% more powerful. Let's say that a certain size panel gets 100W of insolation. If you move from 20W to 24W then you are generating 20% more power from the same amount of insolation, so you are 20% more efficient.
People seem to treat percentages differently from other numbers, when all it means is "divide by 100." Nobody complains when you say going from 9% to 18% is "doubling in efficiency" but if you say it is "100% more efficient" or "200% as efficient" then people complain.
People seem to treat percentages differently from other numbers, when all it means is "divide by 100." Nobody complains when you say going from 9% to 18% is "doubling in efficiency" but if you say it is "100% more efficient" or "200% as efficient" then people complain.