Power is the strength of the weightlifter. Energy is the endurance.
If you’re chasing horsepower numbers, you’re looking at the peak of that lift. But if you’re talking about how far you can actually go on a tank of gas, you’re talking about energy. It’s the capacity to do work over time. Power is just the rate at which you burn it.
One kilowatt-hour (kWh) might sound abstract until you see it in action. Use 1 kW of power and that kWh lasts an hour. Crank the load to 10 kW and you’ve burned through it in six minutes. The tank doesn’t care how fast you drain it. It only cares that it’s empty.
The Two Faces of Power
Everything boils down to two states. Potential energy and kinetic energy.
Potential energy is just power sitting on the shelf. It’s waiting to be converted. Gasoline in your tank. Food in your gut. A compressed spring. A weight hanging from a tree.
The human body is an energy-conversion device. It takes fuel and turns it into movement. Your car engine does the same with gasoline. A pendulum clock uses falling weights.
The higher you lift something, the more potential it holds. A bowling ball dropped from your hand does nothing. Drop it from 100 feet and you’re replacing the roof.
Calculating the Gain
Remember running up the stairs earlier? We calculated power by dividing work by time. But we can also look at what that work was.
You gained potential energy by raising your body mass against gravity. The formula is straightforward.
PE = Force * Distance
Force is your weight. Mass times gravity (mgh). Distance is the height. Raise the mass. Raise the height. The energy climbs.
Units That Actually Matter
You need to know how these values stack up. Here is the breakdown.
SI Units
- Newton meter (Nm): 1 Nm equals 1 Joule.
- Joule (J): 1 J is roughly 0.239 calories.
- Calorie (cal): 1 cal equals 4.184 Joules.
- Watt hours (Wh): 1 Wh equals 3,600 Joules.
- Kilowatt hours (kWh): 1 kWh is 1,000 Wh or 3,600,000 Joules. That’s 3,412 BTU.
English Units
- Foot-pound (ft lb): 1 ft lb is about 1.356 Nm.
- British Thermal Unit (BTU): 1 BTU equals 1,055 Joules. Or 0.0002931 kWh.
It’s all the same thing. Different labels for the same physical reality. You can convert between them. You just have to pick which unit makes sense for the car you’re tuning or the grid you’re measuring.
