Elon Musk is the closest thing this world has to a real-life Tony Stark. Think about it. He builds cool cars. He builds cool rockets. He builds cool tunneling machines. He wants to fire people through pneumatic tubes. He built a ginormous battery factory in the desert, and now he's building the world's largest battery.

OK, technically, Elon Musk isn't building it. Tesla is. But same difference, because Tesla is his company. And Tesla plans to build a lithium-ion battery array capable of storing 129 megawatt-hours of energy.

Wait... 129 mega-whats? What is a megawatt hour, and just what could you do with all that energy?

Units for Energy

The most common unit for energy is the joule. If you pick a textbook up off the floor and place it on a table, you've expended about 10 joules of energy. Yes, that's an approximation. The precise figure would depend upon the mass of the book and the height you raised it.

OK, so what about a megawatt-hour? That's also a unit of energy. To understand it, let's first look at power. We define power as the rate at which you use energy.

Measuring the change in energy (ΔE) in Joules and the time interval (Δt) in seconds yields a power measured in watts. That means that power-time is a unit of energy and a watt-second is equivalent to a joule. And a watt-hour? Just do a simple unit conversion. Remember, the key to converting units is to multiply by the number one.