There is sometimes confusion about the difference between “electrical power” and “electrical energy”. The following article should help clear up the differences.
Power is measured in watts (W), kilo-watts (kW), mega-watts (MW), and giga-watts (GW). The power of a load can change quickly, especially if the load is turning on and off.
- A lightbulb uses 100 watts of power.
- A compressor uses 14 kW when it is running.
A graph of power shows swings up and down (sometimes to zero) as things turn on and off.
Energy is measured in kilowatt-hours (kWh), megawatt-hours, and sometimes (less commonly) in watt-hours or watt-seconds. Energy is the primary measurement used by the utility company to determine your bill each month (although demand and power factor are also sometimes used). Unlike power, energy does not change quickly, but instead accumulates gradually.
- If you run a 100W lightbulb for 8 hours every day, it will consume 0.8 kWh per day. After 30 days, it will have consumed 0.8 * 30 = 24 kWh. After 365 days it will have consumed 292 kWh.
- If you run a 14kW compressor 24 hours a day, it will consume 14 * 24 = 336 kWh per day.
Unless you are generating electricity with PV or wind power, a graph of energy over time will always go up. It will go up faster when you are using lots of power, and it will level off if you stop using any power. But it will never go back down unless you generate electricity.
If you consider electric energy as though it is water:
- Power = Gallons per minute. The power (water flow) goes to zero if you stop using water.
- Energy = Total gallons consumed. This is what you are billed for. Even if you stop using more water, your total stays the same, it just doesn’t get any higher.
Speedometer Odometer Analogy
You can imagine that power (W) is like your speedometer (current speed), while energy (kWh) is like your odometer. The speedometer can quickly go from zero to sixty and back to zero, but the odometer only slowly counts up: faster if your speed is faster. The speedometer tells you how fast you are going right now (how much power you are using right now), while the odometer tells you how far you’ve gone (how much energy you’ve used in total).