Why a gas hob is less efficient than it looks

Where the heat goes that never reaches the pan.

A gas hob is less efficient than it looks because a large share of the heat a flame produces never touches the pan at all, escaping sideways and upward into the surrounding kitchen air the moment the flame's shape does not match the shape and size of whatever is sitting above it.

Three ways heat escapes an open flame

A flame releases heat in every direction, and only the portion that finds a solid surface to transfer into gets used. A pan sitting directly above captures the hot gas rising straight into its base well enough, but a flame's cone typically spreads wider than the burner itself, and any part of it licking up past the pan's edge is heating open kitchen air.

Hot combustion gases also rise past the rim and mix into the room before they have finished giving up their heat to the metal, and the flame radiates outward in every direction that is not pointed at a solid surface. None of this requires anything to be broken. It is the ordinary behaviour of an open flame heating an object that only intercepts part of the heat spreading out around it, and depending on how well pan and ring are matched, often close to half of the gas's heat ends up in the kitchen instead of the food.

An oven avoids most of this by trapping hot gas inside a sealed cavity and forcing it to keep circulating past the food before it can escape. The hob's openness is convenient for watching and stirring, and that same openness is what lets its heat wander off.

A small pan on a big ring

Putting a small saucepan on a ring sized for a much larger pot makes the waste visible. The flame's tips lick up around the sides of the pan into open air, heating nothing but the kitchen. Swap in a pan that matches the ring and the same flame disappears under the base, with no gap left for it to escape through. The loss is still there, just less visible, whenever flame and base are not an exact match, which in ordinary cooking is most of the time.

The opposite mismatch, a pan much larger than its ring, loses little heat by escape, but the centre runs hotter than the rim because heat has to spread sideways through the metal to reach the parts of the base outside the flame's footprint.

Getting more of the flame into the pan

Matching pan size to ring size is the simplest way to improve a gas hob's real efficiency. Turning the burner up to compensate for a mismatched pan only produces a larger flame with more of it spilling past the edge, burning more gas to lose more heat. A lid helps by holding rising hot gas above the food a little longer before it escapes into the room. A dark, well-used pan base also absorbs more of the radiant heat spilling sideways out of the flame than a bright, polished one, a small effect next to the size mismatch but a real one.