One problem is that a 18V, 2Ah battery MAY deliver 36Wh if the load has a resistance much higher than the internal resistance of the battery, but may ALSO deliv

One problem is that a 18V, 2Ah battery MAY deliver 36Wh if the load has a resistance much higher than the internal resistance of the battery, but may ALSO deliver 18Wh if the load resistance equals the internal battery resistance. For batteries that need to deliver a high current (meaning the resistance of the load is low), Wh can be imprecise. An example could be a car battery that is used primarily to start the engine. Apart from such cases, I agree that one should strive to use Wh as labels instead of Ah.