Introduction to Solar Electricity

Electrical Characteristics

PV panels produce Direct Current (DC) electricity. This is the same type of electricity that is produced by your car battery or other batteries. The appliances in our homes use a different type of electricity called Alternating Current (AC). DC electricity flows in one direction only, while AC electricity changes direction rapidly, offering certain advantages in transmission (greater distances through smaller wires). In order to use solar electricity to run normal household devices, you’ll need an inverter, which converts DC to AC. A small system to charge batteries or power small electrical devices (like cell phones and personal music players) doesn’t require an inverter, but be sure to use the correct adapters and, if needed, voltage converters or limiters.

PV modules’ power output is rated in Watts. A Watt is an instantaneous measurement of electrical power. Power is a rate, much like miles-per-hour on your speedometer shows the rate at which your car is traveling. Power is not the same as Energy, which is power drawn over a period of time. Electrical energy is expressed in Watt-hours (Wh) as a quantity relating electrical power to time. In the car analogy, the odometer counts the quantity of miles traveled. Your electric utility bill shows the number of kilowatt-hours (kWh, or 1,000 Watt-hours) you use per month. So how does the wattage rating of a solar panel affect the amount of energy the panel produces? Simple-- with all else being constant, a higher-wattage solar panel will produce more energy over time than a lower-wattage panel. A 100-Watt module will produce twice as much energy as a 50-Watt module at the same location during the same period of time. It’s worth mentioning also that solar modules’ wattage output rating is based on what the modules produce under laboratory-controlled conditions, called Standard Test Conditions (STC). STC allows solar panels to be compared to each other using the same metric. However, because these rated wattages represent ideal laboratory conditions, it's likely that the module will produce lower wattage in actual use.

PV modules also have voltage and current ratings. The rated wattage of a panel is equal to its operating voltage multiplied by its operating current: Watts = Voltage x Amps. The amount of energy in Watt-hours that a panel will produce is a product of the wattage of the panel and the number of hours of full-intensity sunlight, or insolation, that it receives. For example, a solar panel that outputs 100 Watts for two hours will produce 200 Watt-hours of energy. Insolation values are also tied to STC and are based on location. They can be found in data tables for most locations in the U.S. and the world. The actual number of Watt-hours a panel produces will very likely be less than this theoretical value due to many factors that affect the efficiencies of the system components. There are standard factors used to correct for real-world energy losses, but our present focus is the basic electrical principles.