Humans can work steadily to shift about 75 joule a second, or 75 watt. That is about 10 percent of what a horse could manage, so humans work at 0.1 horse-power. Having a standard horse-power was important early on in the development of steam engines, as the engines were used largely for pumping water out of mines, replacing horse driven pumps. Any prospective buyer of these new engines needed to know how many horses the engine could replace. Consequently the horse-power of the engine became a critical selling point. In saying how much water each could lift in a fixed length of time, a common currency once again came into play. Now you would make the comparison in watts.
Later, steam locomotives came to drag trains around, and again the common currency became important, this time to inform the train operator of the ability of the locomotive engine to get a train up to speed. This would require a certain number of joules, and the operator would want this to be done within a certain time. In other words, they needed to choose a locomotive whose maximum power was greater than or equal to the power needed.
power needed = energy to get the train up to speedtime in which you want to get the job done
The key point about energy presented here is that analysing a problem in terms of energy does not tell you what will happen, but it does tell you what cannot happen. Choose a locomotive with insufficient power and you cannot get the train up to speed in time. Choose a locomotive with plenty of power and you will be able to, and you may still choose not to push the engine to its maximum power. Simply having enough power or energy available does not make an process happen. In exactly the same way, having lots of cash does not mean that you will necessarily go out to spend it on certain goods. It simply means that you have the chance to spend. If you have no cash you certainly cannot spend it, and so some courses of action are not open to you.