Energy Terms Explained
We bandy about a lot of technical words when discussing electricity, and it’s helpful to know some basic terms. Electricity is essentially the flow or movement of electrons, so it can help to picture electricity in a wire like water in a garden hose. The water pressure in the hose is similar to voltage (in volts, V) and the current (in amps, A) is like the width of the hose, representing the volume of electricity moving through the wire. If the hose is not very big, it takes a long time to fill a watering can.
A wire needs both voltage and current to produce power, like a hose that needs pressure and a volume of water flowing through it in order to spray water. If either voltage or current is at zero, no electricity is produced, just like no water comes out of the hose if it has no pressure (when the tap is turned off), or no water volume (when the hose is empty). Electric power (in watts, w) is the ability to do work with electricity, and in this example electric power is like the ability to spray water with the garden hose. Power = Voltage x Current
We measure electric power in watts (w) or kilowatts (kW) (kilo equals one thousand, so 1000 watts = 1 kilowatt). Electric power (w) is required to turn an appliance on.
If you have ten 100 watt light bulbs* in your house your demand is 1 kilowatt of power to turn them all on at once. When you leave your lights on for a period of time, you use energy, which is power use over time, so the unit is in kilowatt-hours (kWh). If you keep all the lights on for one hour, you have just used 1 kilowatt-hour of energy. In our hose analogy, electric energy would be compared to the amount of water you sprayed on your garden in total, over the course of time.
The utility charges you for energy—how many kilowatt-hours you used this month. Total kWh will show up as a line item on your bill, multiplied by your electric rate. If you have a business and turn on all of your machinery at once the utility might also charge you for demand in kilowatts, because it costs them money to meet that demand when you suddenly use a lot of power. Demand charges will be billed by the number of kW used in a 15-minute window.
The electric meter that measures your electric usage in kilowatt hours (kWh) is similar to the water meter that measures your water usage in gallons per hour. So when you pay your electric bill, you are paying for kilowatt-hours of energy. And when the sun is out and your solar array is producing power, you are selling it to the utility in the form of kilowatt-hours of energy.
A single 3-foot by 5-foot SunPower module can produce 360 watts of power when there is sun shining on it. If the sun shines on it for 1 hour, that panel produces 360 watt-hours of energy. If the sun shines on it for 4 hours, the panel produces 1,440 watt-hours, the same as saying 1.44 kilowatt-hours.
Power is in kilowatts (kW)
Power (kW) x Time(h) = Energy (kilowatt-hours kWh)
A 5kW solar array produces approximately 6,250 kWh in energy over the course of a year**, and now you know the difference.
*Note that this example uses old inefficient incandescent light bulbs, if you have any left, you should replace them with LED light bulbs that are as bright as a 100w and almost ten times more efficient.
**This can be better or worse depending on weather, shading, direction of the roof, a lot of factors can change production.
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