Most of us use the words ‘energy’ and ‘power’ interchangeably, as if they mean the same thing . . . but in the precise vocabulary of physics, the words have different meanings: energy is the potential to do work, and power is the rate the energy is being used  doing work. Here's a diagram of energy flowing into work:

In the diagram, energy flows freely through the space (as light) from our Sun to Earth, where it strikes a PhotoVoltaic (PV) module and is converted to electricity – but only if the electricity can flow through a load, where it does work: a Light Emitting Diode does the work of conversion back to light, or a motor lifts or powers something doing work. (That is an important ‘but only’, but that whole PV thing is a story for another page.)

Here's an important distinction: because power is real time or kinetic it cannot be stored, but energy is potential and can be stored – electricity in a battery, or physical energy in a compressed spring.

Since the two major sources of renewable (not fossil-fuel based) energy are intermittent – solar (PV) power only comes by day, and wind power is only available when it’s windy – storing electricity is important to providing the uninterrupted electricitry we take for granted.

California is a world leader in stored electricity that replaces dirty fossil fueled peak demand generation. In 2026, California electric utilities had 13,800 MW of on-grid battery storage, and customers held another 2,200 MW – together, enough to power 15 million homes for one hour (or 1.5 million homes overnight.) That's about a tenth of California's residences, so we have a way to go to get clean!

Here's another neat way to store electricity:

Pumped storage blends well with utilities that have excess power under some circumstances – more than enough PV harvesting on a sunny day – and can ‘time shift’ this surplus by using a generator ‘backwards’ to pump water uphill from a lower reservoir to an higher one. When power is needed back, water is released from the upper reservoir to turn the generators and send electricity to the grid.