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There are three basic types of solar systems: ‘Off the Grid’, ‘Grid Tie’ and a ‘Hybrid’ of the two. Let’s explore each of these:

Off the Grid

An off the grid photovoltaic electric system is how most people envision solar power. On sunny days solar panels produce electricity, which is then stored in batteries for later use.

With an off the grid system your are completely on your own. You are not dependent on any ties to the electric company. This is an ideal situation to have when it’s not feasible to be tied to any outside wires.  If you have a house way back in the sticks, where it’s not possible to get a connection to the grid, or it’s not cost effective, this system is the one for you. Other applications for an off the grid system include; horse-drawn wagons, RV’s, and areas remote from normal sources of power.  An off the grid system is also the way to go if you’re looking for independence from the electric company.

There are two major disadvantages to an off the gird system: First, you have to purchase and maintain batteries, which are expensive, require maintenance, and have a limited useful lifetime.  The second drawback is that you are limited to how you can load your electric system. Large loads like air conditioners are not feasible.  With an off the grid system, you have to be frugal in the amount of electricity you use.

Solar Panels

When you’re laying out your electrical system, you first have to consider what voltage to operate your batteries at.  Most inexpensive inverters use 12, 24 or 48 volts DC to convert into AC power (with 12 or 24 volts being the most common). If you want to build a system with solar panels of less than 300 watts, I recommend a 12 volt system and panels designed to produce slightly more than 12 volts (30 cell solar panels are ideal.)  For a system that has 300 to 1000 watts of solar panels, I would operate at 24 volts. In this case, 60 cell panels are ideal.  If your photovoltaic system is larger than 1000 watts, you will probably be operating at a higher voltage (>24 volts DC) so you can purchase panels of 30 to 60 cells; then wire them to produce the desired voltage and current  (Reference the Primer on Electricity and Photovoltaics).

Panels should be installed in accord with the manufacturer’s directions and oriented to receive the maximum amount of sunlight.  Consult an Electrician on the size of wire to use.  The higher the current passing through the wire, the larger the wire will have to be. If your panels are not located too far from the charge controller, your wire size will range from 12 gauge to 4 gauge for currents levels up to 50 amps, (Note this is just a general guideline as much depends on the type of wire and the length it has to run). 

Charge Controller

The primary purpose of this device is to limit the amount of current that can flow to the batteries, preventing them from being overcharged. A good charge controller will also act to prevent current from reversing at night and leaving the batteries for the solar panels.  If you are using solar panels and batteries, this device is essential.  If the batteries become overcharged, the electrical current will essentially boil the water out of the batteries via electrolysis.

Most of the time, charge controllers will operate with a “Float” voltage of slightly more than 13 volts DC (for a 12 volt battery) to keep the batteries fully charged.  After the batteries have been heavily discharged or ‘watered’, then can be placed in an “Equalize” mode for several hours to ensure the sulfuric acid and water are properly mixed in the battery cells.

Batteries

Batteries are the most expensive and heavily maintained feature of your Off the Grid system.  Always use “Deep Cycle” batteries.  These are designed with heavier metal plates inside to allow repetitive charge and discharge cycles. Normal car batteries are designed with thinner plates, which provide for a lot of current surge to start a car, but are not designed to be discharged below 50%. “Deep Cycle Marine” batteries are a hybrid between car batteries and deep cycles.  These are also not suitable to a solar system, where batteries may often be nearly fully discharged before being recharged.

Like solar panels, batteries can be wired in series, parallel, or series - parallel to provide for additive voltage or current.

Safety - Batteries can be hazardous for three primary reasons:

1) Nearly unlimited amount of current is released when they are shorted.

2) They contain Sulfuric Acid, which can cause sever burns,

3) They produce hydrogen gas when they are charged.  Always ensure a battery space is vented to the outside.

Maintenance- At least once per month, check the water level in each of the battery cells. Ensure the cells are full. Unless you want to buy a new battery, don’t let the metal plates become uncovered. Only use distilled water to fill the batteries.

If properly maintained, lead-acid batteries can last anywhere from 4 to 10 years.

Inverters

An inverter is a device that converts the direct current (DC) from a battery or solar panel to an alternating current (AC) which is what most lights and appliances use. Most inverters produce 120 Volts AC, which is what is in all but the largest appliances in a house.  Most well pumps, electric stoves and dryers use 240 Volts AC.

Inverters for ‘off the grid’ applications come in two primary types: ‘True Sine Wave” and “Modified Sine Wave” (also called ‘modified square wave’ or ‘pseudo-sine wave.)  I haven’t had a problem using the Modified Sine Wave inverters for powering just about everything, including a computer and refrigerator, Since they are cheaper, I recommend them for most applications.

When selecting an inverter for your off grid use, there are four basic criteria that you have to look at:

1) True or Modified Sine Wave?

2) Input DC Voltage - Typically this will be either 12 or 24 Volts DC for inverters rated for less than 2000 watts.  For larger inverters, the input DC voltage can often be in a large range up to 500 volts.

3) Power Rating - this will be listed as a continuos rating and as a peak rating.  Calculate the power you need from the individual loads and ensure it is under the continuos rating.

4) Output Voltage - Most will be 120 Volts AC. If you have a 240 Volt motor for your well pump, you may need a separate inverter for that.  To determine what you have, look in your electrical breaker box. If the well pump breaker has two poles, it is probably 240 Volts.

If you are using a solar system to power a house or RV that is off the grid, choices have to be made to limit the loads that are placed on it.  Instead of using electric stoves or clothes dryers, gas can be used.  To replace an air conditioner, use a swamp cooler.  A gas stove can be used to toast bread, heat up food or percolate coffee.   Also, the number of lights an electrical appliances can be cut back without dramatically affecting you lifestyle.

I have a friend, Harry, who lives off the grid.  Being frugal with his use of electricity, he gets by with a 450 watt system.  Other ‘off the grid’ friends run their households and live a normal life with a 2000 watt system.

Because of the high cost of batteries, your typical off-the-grid electrical system will probably end up costing as much as paying for a utility hookup.  The reasons for this type of an electrical system vary, but if you are installing it only to save money, you’ll probably be disappointed.  But there are advantages as well!

Grid Tie System

Nearly all utilities are required by federal law to buy back power produced by an independent power producer.  An exception to this is Rural Electric Cooperatives. 

Some States and Utilities allow a single meter that spins in both directions: forward, which charges you when you are using more juice than you are producing, and reverse, which credits you when you are producing more electricity than you are using. With this type of system, you are both buying and selling power at the retail rate. 

Other States and Utilities require separate meters: a retail rate meter for the power you are using from the utility, and a wholesale rate meter for the excess power you are selling back to the utility. Check with your utility to see which applies for you.

Tying your photovoltaic system into utility lines also requires that you meet other requirements.  First, the AC power that your inverter puts out has to be of the ‘True Sine Wave’ type. It also has to be of the correct voltage and frequency.  A synchronizing circuit ensures that your voltage waveforms perfectly matches the waveform that is present on the grid.  Most utilities also require “Island Protection”.  Island Protection acts to disconnect your system from the electric grid when there is a power outage on your electric lines. If the Island Protections was not present, your inverter could cause the electric lines to remain energized and possibly kill a Lineman sent to repair electric lines. Most ‘Grid Tie’ inverters have all of these features.

This is an illustration of a “Sunny Boy” Grid Tie Inverter.  Most grid tie inverters can be used in either a single or dual meter application.  They can also be installed to supply house loads before sending excess power to the utility. The example I reviewed sold for $1900 and could handle loads up to 3000 watts.  Other inverters that I reviewed cost up to $5000 and could handle up to 8000 watts of power.

An electrician should be employed to install a grid tie inverter system. The National Electric Code has specific requirements on how the installation should be made. In addition, there may be other State, Local or Utility requirements.

Depending on where you live, a system like this could be more cost effective than buying power from a utility. In the future, I firmly believe installations of this type will be common place.

The two primary advantages to having a grid tie system are that there are no batteries, and you aren’t limited on the size of your electrical loads.

I have some friends, living in the La Plata Mountains of Colorado that have a 2000 watt grid tie system. Their net electric bill is Zero!

Hybrid System

A Hybrid system would use photovoltaic panels to power some dedicated loads, while the remaining loads are either powered directly by the utility or with a grid tie system.  An advantage to using this type of system is that you would still have some electricity to things like refrigerators and some lights during an outage.  Also, your overall electric bill will be less.  You would still have some batteries, but their number and cost would be reduced.

I have an Amish friend that has a small 600 watt photovoltaic system that he uses to power one refrigerator and two freezers. The remainder of the house is powered with LP gas lighting and appliances.

The battery bank is relatively small, so I asked him, “What happens if you get two cloudy days in a row?”

He smiled and replied, “Here in the San Luis Valley of Colorado - it has never happened!”