People often ask why do Grid Tie Solar systems stop producing electricity when the utility power is out? The primary reason is safety --to prevent electrical energy from back feeding into the grid, and injuring a utility worker.
The next question is usually why can't a switch be installed to isolate the PV system from the grid so it can power my home when it's sunny? This can be done, but only if the PV system has batteries.
The next question often asked is why is it necessary to have batteries in order to have power when the utility is out? This is the real question. The answer is that appliances need a consistent amount of power to operate properly. They are designed to withstand some deviation, usually around +/- 10%, but deviations larger than this could potentially damage your appliances. Power is voltage multiplied by amperage. Your refrigerator, for example, which is 120 volts, may consume 10 amps, which translates to 1,200 watts of power. It is designed to operate efficiently on this amount of power. Obviously without any electricity the refrigerator will not operate. If the appropriate amount of power is not available, your appliances must make certain comprimises. These electrical comprimises result in heat, which is the ultimate enemy of all electronics.
This is true of all the appliances in your home. They require a specific amount of power to operate correctly. Without a specific voltage or amperage available to the appliance it will not work properly, or worse, become damaged.
Solar modules produce DC (Direct Current) voltage. Most grid tie solar modules are configured into strings to produce fairly high DC voltages and moderate amperages.This is advantageous because it helps reduce losses. These strings are fed into an inverter that converts this electricity into AC (Alternating Current) so the appliances in your home can operate on the power generated by the PV system.
PV systems are very dynamic and respond quickly to changes in the sun's available energy. If a cloud blocks the sun to your PV your system, it will reduce the power output substantially. Your appliances will not notice because the utility grid will instantly make up the difference and your appliances will never see a change in the power that is supplied to them. This is the beauty of grid tie systems. They seamlessly integrate solar power and utility power.
So for example, if your PV system was producing 1 KW of power and your appliance needed 1.5 KW of power then 0.5 KW (500 watts) would come from the utility.The appliance does not care about the source of its power supply as long as it gets what it needs to operate.
So lets assume that we have a PV system producing 1.5 KW of power and your appliance is happy consuming 1.5 KW of power. A cloud goes by, now the PV system is only generating 750 watts, half of what your appliances requires. Where will the other half come from?
This is where the batteries come in. They supply the other half of the required energy for your appliance. Remember batteries store electricity. They are rated in amp hours, but it is a quantifiable amount of energy in the battery. When that energy is used up, the battery must be recharged with more electricity.
ISI Solar 22 Third Street New City, NY 10956 - 845-708-0800 Fax 845-708-0802