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.

Pairing Modules with Batteries-A lost Art

Before charge controllers hit the market, you would need to pair your solar modules properly with your batteries.  System designs required finesse to make them work properly, not a piece of equipment.  Pairing seems to be an art that is slowly disappearing.  But the importance of proper pairing cannot be overstated.

What is pairing?  In simple terms, it is matching the solar production with the batteries capacity.  By properly pairing your solar modules with your batteries capacity, you are maximizing the efficiency.  Remember, there are only so many hours of sunlight per day.  Higher efficiency equates to more production, which in turn means you have more electrical energy at your disposal.

While there are many considerations in pairing, here are a few key points to follow:

1)      Understand the Solar Window

All solar production is based upon the solar window.  The solar window is considered the time when maximum energy harvest is possible.  In general, this is from the hours of 9am to 3pm.  With that in mind, you should design your array assuming that you will only have 6 hours of solar module production max.  You could get better results on some days; conversely, you may also get fewer hours on others.

2)      Solar Module Production needs to be greater than the Depth of Discharge

The best way to insure your batteries will receive a full charge is to match you solar module production to the depth of discharge you plan on using.  Remember, we use a DOD of 50% to 70% of the batteries rated capacity.  If your solar module production is slightly more than the DOD of the battery, the battery can be fully recharged and will last longer.  If your solar module production is slightly lower than the DOD, you will be deficit cycling your batteries effectively reducing its lifespan. We learned about deficit cycling in this article: http://www.solarunplugged.com/blog/2014/5/3/deficit-cycling-dead-batteries.

3)      Solar Module Voltage must be slightly higher than the Battery Bank

In order to charge a battery bank properly, you must provide a higher voltage than the battery bank nominal voltage.  Even in the last charging cycle (Float), the voltage going into the batteries will be higher than the battery bank.  This allows the batteries to remain topped off at all times.  More importantly is the equalization charging cycle.  During equalization, the voltage going into the battery is much higher than normal.  This allows for the electrolyte to boil and clean the battery plates.  Having a voltage that is too low will not allow for the batteries to be properly maintained. Again, reducing the longevity of your batteries.

Advanced charge controllers involved or not, there is one sure-fire way to get the most out of your solar array.  Take the time to understand the necessity in pairing solar modules with your battery bank and then further implementing them into your own system design to maximize the lifespan and production of your solar array.