How Solar-Electric Systems Work


Array Mounting Rack

Photovoltaic panels need to be secured to a stable platform.  A mounting rack provides this function and also keeps the array oriented properly. Mounting racks may incorporate other features, such as seasonal adjustablility.  Adjustable mounting racks allow for the angle of the PV panels to be set seasonally, since the sun is higher in the sky during summer and lower in winter.

Racks can be mounted on roofs,  the ground, on poles, or on the unit itself.

Array DC Disconnect/Breaker

The PV array electricity flow can be safely interrupted by DC disconnection.  During those infrequent occasions when maintenance is performed on the system, this is an essential component.  A DC disconnect houses an electrical switch (breaker) rated for DC circuits. 

Often, the array DC disconnect is wired into the main DC disconnect (usually mounting on the side) as an additional breaker controlled by the main DC disconnect.  This allows the PV array and the remainder of the DC circuitry to be powered off with one switch on the main DC disconnect panel.

Some systems may use a fuse instead of a disconnect, particularly portable systems, like those manufactured by North Star Energy Services, as there is almost no need to disconnect from the array at this point in the circuitry.  The fuse protects the system like the breaker in a DC disconnect, but when a fuse trips (burning out in the process) it will need to be replaced.

Charge Controller

A charge controller is an essential component in an off-grid system as its primary function is to protect the battery bank from overcharging.   The controller monitors the battery bank and when the batteries are fully charged, it interrupts the flow of electricity from the PV array.

Battery Bank

Since the solar array will produce electricity whenever the sun shines on it, you need a place to store that energy so you have a supply when the sun is not shining.  That is the purpose of the battery bank.  A battery bank is a group of batteries wired together, and they are wired in such a way as to promote even charge and discharge of all batteries at once.

Though it may be similar to ordinary automotive batteries, they are called deep cycle batteries. They are specialized batteries that are used in solar electric systems in charging and discharging cycles they need to endure.

The second alternative are sealed absorbent glass mat (AGM) batteries. They are maintenance free and less hazardous than flooded lead-acid for two reasons: (1) because the acid is absorbed into the glass mat, they will not leak acid, and (2) they do not require as much ventilation as lead-acid batteries as the charging process dissipates lower levels of hydrogen.

A third option, gel-cell batteries, should be reserved for applications where the batteries will be in an unheated and cold space for a long period of time.  It is almost impossible to freeze this type of battery and they will work more consistently at lower temperatures than the other types of lead-acid batteries.

How is a Deep Cycle Battery Different than Your Car Battery?

Automotive and deep cycle batteries are generally lead-acid - they use the same chemistry for their operation.

A car battery is designed to give a very large amount of current for a short period of time.  This is needed to turn the engine over when starting.  Once the car starts, the alternator takes over supplying all the electrical power that is needed.  So these batteries often go through their entire life without ever being discharged more than 20% of total capacity.

A deep cycle battery is designed to provide a steady current over a long period of time.  A deep cycle battery can provide a surge whenever needed, but not like an automotive battery.  And they can be discharged (and recharged) over and over again, typically to over 80% discharge, something that would ruin a car battery very quickly.

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