A guided 3D journey through energy: discover on a swinging pendulum the one number that never changes, watch a bouncing ball bounce itself to rest while that number holds perfectly steady, zoom inside the stilled ball to find the motion intact but shattered into random molecular jiggle — then take the lesson to a Danish town and try to carry it through a windless day on stored wind. Energy is conserved. Quality is not.
Physics has a surprisingly honest answer, and Feynman put it bluntly: we have no knowledge of what energy is. It is not a substance or a fluid you could point at. What we do know is stranger and more useful — there is a number you can calculate for any closed system, and no matter what happens inside it, that number never changes. You can watch it happen on a pendulum: height energy (m·g·h) turns into motion energy (½·m·v²) and back again, and their sum stays perfectly flat (the pendulum in the simulation is idealised frictionless; a real one slowly hands its energy to the air and the pivot — see the bouncing ball below). Energy is measured in joule, where 1 J = 1 kg·m²/s².
Nothing is ever destroyed — so what runs out is not the amount of energy but its quality. Energy that is ordered — bulk motion all pulling the same way, or the chemical energy in a fuel, or electricity — can do work for you; energy scattered into the random jiggle of molecules is thermal energy, and it is far harder to get anything useful out of. Every conversion pushes some of your energy a step down, towards low-grade heat, and that step never runs backwards on its own. That is what the phrase 'energy crisis' really points at: a shortage of order, not a shortage of joules.
Not because its energy disappeared — the total is exactly as large after the ball has come to rest as it was when you dropped it. Each bounce squashes and stretches the ball and pushes the air aside, and that ordered downward motion gets converted into the disordered motion of countless molecules in the ball, the floor and the air. The ball and floor end up a tiny bit warmer. All the motion is still there; it is just spread out and pointing in every direction at once, so it can no longer lift the ball.
Nyttevirkning η is the useful energy you get out divided by the total energy you put in: η = ΔE_useful/ΔE_supplied. A battery driving a motor that lifts a weight might deliver 34 J of every 40 J, so η ≈ 0.85; heating water over a flame lands roughly 115 kJ of every 150 kJ in the water, η ≈ 0.77. The rest leaves as waste heat. The catch is that machines in a chain multiply their efficiencies rather than averaging them, so three steps at 0.85 each leave you about 0.61 of what you started with — which is why every extra link in an energy chain is expensive.
Wind arrives when it wants to, not when a town needs it — which makes storage the key to security of supply in a grid running on renewable energy. Pumped hydro is the cheapest storage per stored kWh at large scale — pump water uphill and recover E = m·g·h later — but it needs height, and Denmark is flat, so Danish surplus wind is instead exported to Norway, where it saves water behind existing hydropower dams and comes back as electricity when needed. The alternatives all have a catch: hot rocks (ΔE = m·c·ΔT) store heat cheaply but give back only about a third as electricity, lithium-ion has the best round trip of about 87% yet is expensive and suited to hours rather than seasons, and hydrogen packs an enormous 1.2·10⁸ J/kg but loses roughly two-thirds of it on the way there and back.