HHV efficiency
HHV (higher heating value) efficiency is the share of the total energy in the wood — including the energy lost evaporating its moisture — that a stove delivers as useful heat. It is the conservative efficiency measure and the one the EPA database certifies; certified stoves typically run 60–85% HHV.
Manufacturers sometimes quote LHV (lower heating value) efficiency, which ignores the energy spent boiling off fuel moisture and therefore reads several points higher for the same stove. When comparing models, make sure both numbers are on the same basis — every efficiency figure on StoveAtlas is HHV, straight from the EPA certification record.
The now-expired federal 25C tax credit used 75% HHV as its qualifying threshold, which is why that number became the informal marker of a "high-efficiency" stove.