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

Sizing Your Off-Grid Solar Electric System

The average US home consumes about 940kwh of electricity each month. For many, electricity use could be cut in half with a serious conservation effort. But if you had to rely on a small photovoltaic (PV) system could you get by on 120kwh per month?

To get by on less you’ll first need to make sure you’re using electricity as efficiently as possible. Replacing incandescent light bulbs with compact fluorescent (CFL’s) is a good start. You’ll also benefit by eliminating phantom loads and replacing inefficient appliances. I’ve listed many more things you can do in previous posts, so I won’t repeat them here. Check this blog’s archives for that information.

Living (comfortably) off-grid on less than 120kwh of electricity per month (about 4kwh per day) may sound impossible, but you just might be able to do it. Here’s how:

Of the 20 or so 13 watt CFL lights in your home, you might use each (on the average) 1 hour per day. So, 13 times 20 times 1 = 260 watt/hours. Shown below is the total for lights, and a list of other ways you might use this limited supply of electricity.

Lights: 13 watts X 20 hours = 260 watt/hours
Refrigerator: 50 watts (average) X 24 hours = 1200 watt/hours
TV and Cable box: 125 watts X 3 hours = 375 watt/hours
Radio: 5 watts X 6 hours = 30 watt/hours
Fans: 35 watts X 16 hours = 560 watt/hours
Computer and monitor: 120 watts X 2 hours = 240 watt/hours
Microwave oven: 1000 watts X 0.5 hours = 500 watt/hours
Toaster: 900 watts X 0.1 hours = 90 watt/hours
Vacuum Cleaner: 750 watts X .2 hours = 150 watt/hours
Cell Phone Battery Charger: 25 watts X 2 hours = 50 watt/hours
Washing Machine: 500 watts X .25 hours = 125 watt/hours
Iron: 1000 watts X .25 hours = 250 watt/hours

Total: 3930 watt/hours per day

How you use the available electricity will not exactly match my list of course. This is simply an example to show how you might get by on much less electricity than you’re currently using. Off-grid doesn’t have to mean living like a caveman. A small PV system can meet most of your electrical needs, including a limited amount of cooking and climate control. As long as you have other systems in place for heating, cooling, and other high-energy devices, you could live quite comfortably on much less than you currently use.

Why is this important?

Most of us purchase electricity from our local utility company for less than 2% of our household income. Because grid-supplied electricity is inexpensive and convenient, few people have any interest in alternatives at this time. But just as gasoline prices have skyrocketed in the last two years, we’ll soon see the cost of electricity increase dramatically. Most consumers will deal with this by cutting back, but some will choose to disconnect from the grid. A PV system large enough to meet your current electricity requirements may cost 25 to 35 thousand dollars. For most, reducing usage and installing a smaller PV system will be easier and less costly than installing a system big enough to meet current demands for electricity.

What would this smaller PV system cost?

First of all it’s important to understand that a system capable of providing 4kwh of electricity a day will not provide 4kwh on a cloudy/rainy day. Typically, a lack of sunshine prompts the user to either cut back on electricity use that day, or to use another source of electricity during those times, typically a generator. It is also important to understand that we’re discussing an off-grid system, not a grid-tied system. An off-grid system includes the extra expense of batteries, and is not as efficient as a grid-tied system. Your system design might include a battery bank large enough to compensate for a day or two of cloudy conditions.

PV panels produce electricity when the sun strikes them, but are most productive during hours of peak-sunlight, or stated another way, when the sun is almost directly overhead. We’ll do our calculations based on an average of 4 hours of sunlight each day. A 100 watt solar panel can produce 400 watts/hours (100 watts times 4 hours) of power each day. It follows then that to get 4000 watt/hours (4kwh)from the panels each day, you’ll need 1000 watts of PV panels. To make up for system inefficiencies, you should shoot for at least 1200 watts of PV. That would be 12 one hundred watt panels for example. If you shop around, you’ll find solar panels for less than $4.50 per watt, so you’ll spend about $5400.00 for PV panels alone. You’ll also need a charge controller, batteries, an inverter, panel mounting hardware, wire, and safety components. These items can be bought for $2600.00 if you shop around. If you’re not able to do the installation yourself, you might spend another $3000.00 for that, making your total cost about $11,000.00. If this sounds expensive, don’t forget that it eliminates your electric bill. The system could pay for itself in 5 years, or less as electricity prices increase. And since solar panels can be expected to last in excess of 20 years, you’ll be getting many years of low-cost electricity after that.

Using your system:

Your small system may not always keep up with your needs, but you’ll learn techniques to maximize efficiency. Using energy from the sun as it’s generated (instead of storing it in batteries for later use), increases system efficiency greatly. By using the vacuum cleaner, washing machine, and other appliances during peak-sunlight hours you eliminate losses associated with converting, storing, and retrieving energy. Your goal should be to use electricity wisely, ensuring a surplus. That surplus will come in handy when it’s cloudy.

Conclusion:

Having your own power plant means that you’ll not be affected by outages and brown-outs that grid-connected customers often experience. News reports about rate increases will no longer concern you. You’ll feel good knowing that by disconnecting from the grid you’re not contributing to the environmental problems associated with mining and burning coal to produce electricity. By installing your own PV system you’ll be taking an important step toward personal electric transportation, or as a politician might say eliminating your “addiction to oil”. Declining oil supplies will soon usher in the age of electric cars, and it’s not unreasonable to think that someday you’ll be able to drive on free energy from the sun. That’s something to get excited about!

John

Hollywood Green vs. Real Green

Although alternative technologies are often mentioned in the media, the details remain esoteric. Hollywood gives us a vague overview with programs that attempt to show what green living is all about, but they’re obviously trying harder to entertain than to teach. I’m somewhat impressed by “Living With Ed”, a program that attempts to teach and entertain at the same time, but the trend seems to be to move away from substance. Les (Survivorman) Stroud’s journal of his off-grid living project was interesting, but woefully short on details. Les admits that photovoltaic (PV) technology is not his strong point, and as a casual viewer I spotted several mistakes.

The worst I’ve seen so far is the Tommy Lee vs. Ludicris competition called Battleground Earth on the Science Channel. Here we have two people who don’t have a clue, competing with each other to see who can be more green. What a joke! The contestants and their teams compete to solve riddles and to assemble pre-fabricated projects. It’s like watching someone put together a small jigsaw puzzle, and with the same educational value. This so-called “reality” program is laughable, and the Science Channel should be embarrassed for showing it. The producers believe that these two “big stars” will inspire others to go green. I doubt it. The goal of the first challenge was to see who could be the first to use solar power to illuminate a large sign with their name on it. Someone should remind the Science Channel that solar power can also help reduce our dependence on oil, while helping to clean up the environment. There are so many things wrong with this program that I won’t even begin to list them here.

Programs like those listed above may have some entertainment value, but you’ll need details if you want to accomplish anything. I suggest that you:

Subscribe to Home Power Magazine: www.homepower.com

Visit discussion sites, like: www.wind-sun.com/ForumVB

And keep reading this blog.

John