Maybe energy has types. Such as ki, genki and android energy.
"Power" is how much energy they bring out per second. But "energy" could be used to refer to how much has been collected in one place. So Goku could have twice the power (ie energy output rate) as 17 (Goku from ki, 17 from android energy), but 17's barrier could stably hold high amounts of energy so in 10 seconds he could have a barrier than can block the kamehameha that Goku charges for 2.5 seconds. Some techniques, like 17's barrier, Genki Dama or Final Flash can hold more energy before needing to be fired than other techniques, such as kamehameha or masenko. And when the attack hits the opponent all the energy is usually delivered in one moment, so two attacks of equal energy also have equal power, even if they were made by fighters of unequal power.
17 and 18 both can produce android energy forever but have a limit to how much they can produce per second. 18 can't produce as much energy per second, so has less power (but maybe whatever generates android energy can be trained to produce more power per second eg maybe she has organic cells that produce android energy and are trainable).
If they say "Goku's ki is bigger than 17's" they mean "his ki energy reserve is bigger", which is somehow proportional to his power ie his energy output rate. The bigger your max energy reserve, the more quickly you can put out energy.
Goku can have twice the ki of 17 and therefore be twice as powerful, but he'll eventually run out of ki if he doesn't rest and eat - he can't generate more energy without resting and eating.
