Designing an Off Grid Solar Electric System, Step 1, Planning Considerations

Before you run any calculations you need to have a starting point and this is it. There are equipment limitations that you need to be aware of as you start to design an off grid solar electric system.

Charge Controller Limitations

The charge controller, controls the output of the solar array and prevents the batteries from being overcharged. There are two basic types available a PWM and a MPPT.

A PWM controller, is a pulse width modulation controller and with this style controller you need a solar module that is designed to charge batteries producing the correct output voltage.  For small systems with solar arrays under 100 watts, a PWM will give you the best design for the lowest cost.

Any larger systems should use a more expensive MPPT controller that will generate more power from the solar module. An MPPT controller is a max power point tracking controller that uses a DC to DC voltage converter to find the best voltage for the solar array to operate and produce the most power. This voltage is typically very different then the battery voltage so a second DC to DC voltage converter is used to change the solar voltage to a voltage needed to properly charge the battery. An MPPT controller can extract 20% to 30% more power from the same solar module just because of its more sophisticated design.

MPPT controllers offer other advantages with larger solar arrays from the standpoint that the solar array can be wired for a high voltage output to charge a low voltage battery bank. For example, with the correct controller, the array can be wired for a max of 600Vdc and charge a 48 volt battery bank. This will let you run longer DC conductors from the solar array to the battery bank with a lot less voltage loss. This also eliminates the need for a lot of fusing and high current DC disconnects resulting in a less expensive installation.

Solar Module Limitations

Quick background on solar modules, in general each solar cell produces approximately 0.5 volts so the more cells a solar module has the higher its output voltage will be.

In the 1990’s and earlier most solar modules had 36 solar cells and produced approximately 18 volts which was good for changing  a 12V battery. Once you accounted for heat and wiring losses the 18 volt output would roughly match the battery voltage which ranges from 10.5 volts (dead battery) to 14.3 volts (full battery while charging). Since the voltage roughly matched, solar modules were directly connected to batteries with nothing more than a voltage controlled switch (charge controller) between them that disconnected the battery at a certain voltage to prevent over charge. As the industry moved to higher volume grid connected system in the 2000’s solar modules were becoming larger as larger and more solar cells were used in their construction. 54 solar cells was normal with 60 cell and now 72 cell modules quickly following suit. In order to use these lower cost standard designed 60 and 72 cell solar modules you must use an MPPT controller to properly charge a battery bank.