The Power of Inverters

Should you decide to use your Solar Array to power your electronics and appliances, you are going to need an inverter.  An inverter simply takes the DC (Direct Current) power produced by your solar panels or battery bank and inverts it into AC (Alternating Current) power.  While this seems easy enough, there are items you need to be aware of when selecting an inverter.  Here is a short list for you:

1)      Inverters come in different output types.

The two most common types of inverters are Pure Sine Wave and Modified Sine Wave.  Pure Sine Wave inverters best emulate your existing Utility Grid.  In most cases, the power from a Pure Sine Wave inverter is cleaner than the Grid.  This type of inverter is best suited when sensitive electronics are going to be used.  A Modified Sine Wave Inverter does not produce as clean of power as a Pure Sine Wave inverter.  However, a Modified Sine Wave can be more economical when power is your sole focus.

2)      Inverters are rated by Power Output

You need to select an inverter with at least the same output wattage as your maximum wattage needs.  The inverter Wattage Output is the maximum power you can get from that inverter.  For example, a 2000W inverter will output up to 2,000 watts worth of power continuously.  If you need more power than the output wattage rating, you need to choose a bigger inverter.

3)      Input DC Voltage

 The Input DC Voltage of the inverter must match the output voltage of your battery bank.  If this is not the case, the inverter will never run properly and cause damage to the inverter or the battery bank.  If your battery bank is 24 volts DC, than your inverter must have a 24 volt DC input rating.

4)      Inverter verses Inverter/Charger

Even though your goal is to solely charge the battery bank using your solar array, you may need an inverter/charger.  This type of inverter will allow you to utilize Grid power to charge your battery bank when the solar array isn’t producing energy.

Planning your project will allow you to choose the right products to meet your needs.  

Ground Fault Protection

What is ground fault protection and do you really need it?  A ground fault is an abnormal condition in which current is traveling along the ground wire, which is normally a non-current carrying conductor.  This means that something in the Solar Array is shorting out, causing the current to flow into the ground wire.  If the condition is left to remain, the point where the short is happening will eventually fail causing a fire.  This can be seen in two recent devastating fires caused by Solar Array Ground faults. (Bakersfield, CA and Delanco, New Jersey)

Now is where the question arises, doesn't a circuit breaker of fuse remove the ground fault?  In a ground fault, a circuit breaker of fuse may not open clearing the ground fault.  The reason behind this is that in order for a circuit breaker or fuse to trip, the current flowing through that device must be higher than the devices rating.  For example, a 20 ampere circuit breaker needs over 20 amperes to trip.  In a ground fault there may not be enough current flow to trip the breaker.

The problem with ground faults is that not all of the current is flowing into the ground wire.  There is always a certain amount of resistance that happens in a ground fault that reduces the amount of possible current.  This means that even though there is a fault, it is not high enough to trip the circuit breaker of fuse that is normally in the system.  Therefore, ground fault protection is added to the circuit.  In general, a ground fault protective device is set to trip as low as 1 ampere.

A ground fault protection device will disconnect any loads on the Solar Array.  By disconnecting the loads, there is no longer a flow of current stopping the short circuit from persisting.  Now the Solar Array is brought to a safer condition until the short can be removed from the system.

Now for the million dollar question, do you need one?  If you are planning on putting solar panels on your dwelling, the answer is yes.  A ground fault protective device will help prevent your house from catching fire due to a ground fault.  Remember, ground faults generally do not trip your normal circuit breakers or fuses.  On the other hand, if your solar panels are going to be ground mounted, this isn’t a requirement, but still a good idea.

Having a ground fault protection device installed in your Solar Array is like having insurance.

To MPPT or to not MPPT, that is the Question

Do I need a MPPT charge controller for my system?  This is a question that comes up for any off-grid solar array.  And it’s an important question not just for your wallet, but your system as well. 

Remember, the whole goal of the array is to charge your battery bank.  Therefore, we want to harvest as much amperage from the array to fully charge our battery bank.  It doesn’t matter if you do this with or without an MPPT charge controller.  What matters is that your batteries always come back to a full charge as well as never dip below 70% depth of discharge.

So how does an MPPT help in the above goal is the real question?  First of all, what is an MPPT charge controller?  A maximum power point tracking (MPPT) charge controller is a device that can change its internal resistance in order to output the maximum possible power from your solar array.  It does this through algorithms used by a DC to DC converter.  In simple terms, this means that the charge controller can take any excess voltage your array produces and convert that into a higher amperage output.  And again, our goal is to harvest as much amperage from the array to fill our battery banks amp-hours.

In any off-grid design, the array voltage is always higher than the battery bank nominal voltage.  This allows for the charge controller to provide enough voltage for the different charging cycles.  For example, a 24 volt battery bank will require around 28 volts for the absorption charging cycle.  Also, the voltage of the array has to be increased to account for voltage drop and temperature change as well.  With this in mind, there is always excess voltage to be used for the MPPT charge controller to carry out its function.

Why wouldn’t I choose an MPPT charge controller?  Right now they sound awesome.  And they are awesome.  In reality, any off-grid design can be done without them though.  Every design simply has to pair the modules with the battery bank.  In areas or at times with less sunlight hours, the array size and battery bank size just has to be increased.  However, there will be a point where the costs of increasing the array size and battery bank exceed the cost of an MPPT charge controller.  In general, if you are in an area or time of year that has limited direct sunlight hours (2-3 hours/day), an MPPT charge controller can really benefit you.

Take your time planning your off-grid array.  In the end you will save money and have a great performing array that last a long time.