Design and Build an Off-Grid Solar Electric System

Anyone with a basic understanding of electricity and good mechanical skills can design and build a solar photovoltaic (PV) system. Here, condensed into a few easy steps, is what you need to know.

Overview

There are two basic types of solar PV systems, off-grid and grid-tied. An off-grid system uses batteries, while batteries are optional in a grid-tied system. In this article we’ll be discussing off-grid systems. An off-grid system uses solar photovoltaic (PV) panels to turn sunlight into electricity, and stores that electricity in batteries for later use. Battery charging must be done in a controlled manner to protect them from damage, and for efficiency and safety. The stored energy must be converted to AC voltage in order to power ordinary household appliances.

Building a system large enough to meet your daily needs for electricity can be an expensive project. For most people, reducing the load by improving energy efficiency will be more cost effective than building a system big enough to handle a heavy load. Replacing incandescent lights with compact fluorescent (CFL’s), and upgrading to energy-efficient appliances are a couple of things you can do that will pay off in the long run. Having done that, you’re ready to start the design phase.

Step 1. Determine your daily needs.

List the electrical requirements of each device that you plan to power with the PV system. Example:

A 13-watt bulb in use for 5 hours each day (average) uses 13 watts times 5 hours, or 65 watt/hours per day.

Enter the information for each device into a chart as shown below:


Your total energy needs are the sum of the individual requirements of all devices, or 4985 watt/hours per day in this example. You may choose to build a system to meet all of your needs, or choose instead to build a system to meet a portion of your needs.

Tip: If you don’t know the electrical requirements of a particular appliance or device, an inexpensive Kill-A-Watt meter can help you find out. Click (HERE) for more information.

Step 2. Determine the amount of PV needed.

PV panels are rated in watts. One 100-watt panel produces the same amount of power as two 50-watt panels. If you get 4 hours of sunlight each day, a 100-watt panel is capable of producing 4 times 100, or 400-watt/hours of power daily. The example above lists your needs at almost 5000 watt/hours per day. Dividing 5000 by 400 shows that you’ll need twelve and a half 100-watt panels to meet your daily needs. To make up for system losses, and because you’ll probably want all panels to be the same size, you should go at least 20% bigger, opting for 15 panels. You might want even more panels to compensate for extended periods of cloud cover.

Step 3. Planning your battery bank.

Batteries are rated in amp/hours. Begin by converting watt/hours to amp/hours by dividing watt/hours by 12 (the battery voltage). In this example, the 4985 watt/hours that you need divided by 12 equals about 415 amp/hours. Since discharging batteries beyond 50% of their capacity will shorten their life, you’ll need a battery bank rated at no less than 830 amp/hours (in this example). Additionally, you’ll have to increase the size of your battery bank by about 20% to compensate for conversion losses. Having done that, you should have enough battery capacity to get you by for one full day. Ten 100 amp/hour batteries connected in parallel will do the job in this example, but if you want to compensate for extended periods of cloud cover you’ll need more. In addition to keeping your equipment running in the event of extended cloud cover, over-sizing the battery bank helps to extend the life of the batteries as a result of less-aggressive use.

As you shop for batteries, be sure to select those designed for deep cycle applications, not automotive batteries. Batteries designed for golf-carts, floor scrubbers, and forklifts are all good choices. The most expensive batteries tend to have the longest lifespan. Your bank of batteries will be wired to provide 12, 24, or 48 volts. More about that later.

Step 4. Select an inverter.

An inverter converts the low DC voltage from your battery bank to 120-volts AC. To determine the size of the inverter needed, add up the power requirements of all of the loads that you intend to run simultaneously. The total load in Step 1 was just under 5000-watts, but it’s unlikely that you’ll ever use all of those devices at the same time. You might, however, use the microwave oven and toaster at the same time, a total of 1900 watts. You might also have a few lights on at the same time. In this example, an inverter rated at 2000-watts would just meet your needs.

There are two basic types of inverters, modified sine wave and true sine wave. Modified sine wave inverters are much less expensive, but some equipment may not work well with modified sine waves. Motors may overheat and run at the wrong speed, and sensitive electronic equipment can be damaged. For best results, I highly recommend a true sine wave inverter.

The choice of an inverter will also influence another important design decision. Inverters typically accept an input voltage of 12, 24, or 48 volts. Generally speaking, a 12-volt inverter would be the best choice for a small system, while a 24 or 48 volt inverter would be better for a large system.

Step 5. Select a Charge Controller.

A charge controller efficiently controls the battery charging voltage and current, and keeps the batteries from overcharging. If you choose to build a small system, you need not get an expensive charge controller. A single PV panel can produce no more than 5 to 10 amps of current, and just about any charge controller will be able to handle that. A large PV system may require you to use more than one charge controller, splitting the PV panels into two or more sections. Your charge controller should include a battery temperature probe. The charge controller cannot efficiently charge batteries unless it has a way to compensate for battery temperature.

Unless you have a separate device for monitoring system parameters, you should opt for a charge controller with a digital meter. Most importantly, you’ll want to monitor battery voltage. The ability to monitor PV panel voltage and current is also helpful. Reduced output may alert you to the need to clean the panels, for instance.

The best available charge controllers (suitable for large systems), are able to convert voltage to lower or higher levels. Your PV array, for example, could be wired to provide 48 volts to the charge controller, which is converted to 24 volts in order to match the voltage requirements of the inverter. Operating at voltages greater than 12-volts can cut system losses due to the resistance of the wiring. By increasing voltage you can use thinner, less expensive wire, and cut costs.

Choose a charge controller that best matches the size of your system. For small systems, the charge controller should consume very little current for its operation. Typically, these are PWM (Pulse Width Modulation) controllers. PWM types provide pulses, instead of a steady DC voltage, to the batteries. For large systems the charge controller should have the ability to track PV panel output and adjust to provide the most efficient charging. This is called MPPT, or Maximum Power Point Tracking.

Step 6. Mounting the Solar Panels.

Keep in mind that cool panels operate much more efficiently than hot panels. Mount the panels in a way that allows good air circulation under them. Check my blog of 2/15/2007 for mounting ideas, and information you’ll need to determine the ideal panel orientation for your geographical location. If panels are to be mounted on a pole or roof, a lightning protection device is a good idea. Install that in accordance with the manufacturer’s instructions.

Step 7. Wiring and Safety Considerations.

Be sure to use wire that is large enough to handle the maximum current that will flow through it. Typically, a set of wires from each solar panel terminates in a combiner box or breaker box, and a thicker wire connects the solar panel array to the charge controller. Since the output of each solar panel is usually less than 10 amps, 10 gauge wires can be used from each panel to the combiner box. Battery interconnections and battery-to-inverter wires will need to be much thicker, since the current flow there can be very high. Fuses, breakers, and disconnect switches should be included in your design for safety.

Check with an electrician for the correct type and size of wiring if you’re not sure, and to make sure everything gets done according to code.

The drawing below is a typical wiring scheme for a small off-grid system. Be sure to include safety devices (not shown here):



Adding Functionality

Perhaps you’ve decided to build a system to lower your electric bills, or to serve as an emergency supply of electricity. The system described here will certainly do those things, but it also has its limitations. You may want your system to kick-in automatically in the event of a power failure, perhaps to prevent frozen food from spoiling when you’re not home. The addition of an “Automatic Transfer Switch” will provide that functionality.

You might want to automatically disconnect batteries from the load, perhaps switching to another source of power, when batteries reach a predetermined state of discharge. Check out my blog entry of 2/25/2008 for more information on that topic.

Be sure to consult with a licensed electrician before connecting to your house wiring.

Conclusion

Don’t let a lack of technical training or experience discourage you from building your own PV system. It’s not that complicated. You can build a safe, efficient system with off-the-shelf equipment from numerous sources. Learn as much as you can before you begin, to avoid altering your plans after you’ve purchased equipment. Be especially careful to take good care of your batteries, as they can be easily damaged by abuse. If you plan to start small and add to your system over time, develop a plan that will allow you to do that with as little waste as possible. Since this post has been primarily an overview, check other websites for in-depth information as needed.

Don’t let the cost of system components discourage you from building your own PV system. Start small if you must, but start. The world is changing, and we cannot continue to burn fossil fuels as we currently do. Future generations deserve more from us, and it seems that we cannot rely on politicians to do the right thing. In the words of Charles Darwin:

“It is not the strongest of the species that survives, not the most intelligent, but the one most responsive to change.”

John