Off-Grid and Grid-Interconnected Solar Systems

There are two types of solar electric systems: off-grid and grid-interconnected. As the name implies, off-grid systems have no connection to the electric utility grid and use batteries to power your home when the sun is not shining. Grid-interconnected systems connect the solar electric system to the utility grid. When there is not sufficient sunlight to power your home, electricity is drawn from the grid. When a surplus of solar energy is produced, the power is fed into the utility grid to be used by other customers.

Grid-interconnected systems are less complex, easier to install and less expensive because they do not require batteries. Off-grid systems are often used where there is not existing electrical service and by individuals who value energy independence.

Because of the inefficiencies of charging and discharging batteries, off-grid systems tend to be less efficient than grid-interconnected systems. Grid-interconnected systems also have a higher potential to generate electricity because they are not restricted by the charge capacity of a battery bank. When there is a surplus of power, it flows freely into the utility grid, spinning your electric meter backwards. When off-grid systems have a surplus of power, it is used to charge the battery bank until the batteries are full, and then the surplus can no longer be utilized. You can view schematics of an off-grid and grid-interconnected solar electric system.

What Components make up a Solar Electric System?

First, you need a sun. As luck would have it, our solar system has one! (The sun is 93,000,000 miles away, and that's as close to a nuclear reactor as you want to be.) Once you have sunlight the next thing you need is a photovoltaic (PV) module. They come in a number of shapes and sizes and produce direct current (DC) electricity when sun shines on the module. PV modules are comprised of many smaller PV cells. Most everyone has seen a PV cell. They are commonly found in calculators, watches and some portable radios. PV cells have been around since the development of the transistor, and through the years they have become much more efficient. They have the ability to turn sunlight directly into electricity using silicon-based semiconductor materials.

Pictured above is a Sharp 208 watt module that produces 28 Volts at its maximum power output. The 208 watt maximum power is based on a standard of 1000 watts/square meter of available solar energy striking the module perpendicular to its surface. For each additional module connected together to form an array, your power output would increase by 208 watts. A number of these modules can be tied together to produce enough electricity to adequately power your home.

Now that the PV modules are producing power, you need something to control the power. Charge controllers are used in off-grid PV systems to protect the batteries from overcharge and excessive discharge. The minimum function of the controller is to keep the batteries fully charged without damage. There are much more sophisticated charge controllers such as the OutBack Power Systems controller shown at right. It monitors the amount of incoming solar energy to optimize the power available from the system, increasing its overall efficiency.

Speaking of batteries, if you are off the grid or want some form of backup power, batteries are necessary to operate electrical loads when the PV array cannot supply enough power. They are needed at night, on overcast days or during severe weather. Batteries are not required for a grid interconnected system; however, a hybrid of batteries and grid interconnection can be used to supply modest reserve power for emergency backup. Most off-grid systems use deep-cycle lead-acid batteries that can withstand the abuse of frequent charge and discharge cycles.

Most electrical devices are made to connect to the 120 Volt AC electrical grid. Inverters convert DC power from a PV array into AC power for your household appliances. Inverters also monitor your power 24 hours a day. When your PV array is not producing enough power, the system draws power from its bank of batteries. With grid-connected systems, the inverter is used to connect with the utility grid. It regulates surplus power flowing back into the grid and shuts down automatically if there is a utility power failure. This protects utility crews from any unexpected energy as they service the distribution lines.

What are the Advantages of Solar Electric Systems?

Clean Energy: Compared to non-renewable energy such as coal, gas, oil and nuclear, solar electricity generation is totally nonpolluting. Currently the manufacture, distribution and installation of solar electric systems does depend on non-renewable energy sources; however, over the useful life of a solar electric system, the amount of clean energy produced far exceeds the energy required to manufacture and install the system.
Robust Design: Photovoltaic systems have no moving parts and the modules and electrical components are rugged and made to last for decades. Depending on their usage, the batteries in an off-grid home will need replacing approximately every ten years. This is the only component of an off-grid PV system that gets significant wear.
Low Maintenance: Modern PV modules are virtually indestructable and need only an occasional rinsing. The electrical components are equally robust and maintenance free. The battery bank of an off-grid system does need routine maintenance to ensure peak performance and full life. Grid-interconnected systems do not require backup batteries and are essentially maintenance free systems.

What are the Disadvantages of Solar Electric Systems?

Cloudy Days: The sun doesn't always shine. Climates vary globally, and no matter where you live, solar is only part of the answer to a renewable energy solution.
System Cost: In most cases, solar electric is still more expensive than nonrenewable energy; however, the gap is narrowing, and energy shortages are becoming more commonplace. A number of significant incentives such as state grants and federal tax credits are also making solar electric systems much more affordable to install.

We offer a wide range of quality solar electric products. Whether you are interested in a complete PV system or a few components, we have what you're looking for. Please contact us for a quotation on a custom solar electric system.