Friday, February 09, 2007

Off-grid Solar Photovoltaic System Design Simplified

If you've ever thought about building your own off-grid photovoltaic (PV) system, don't let a lack of electronics knowledge hold you back. The technology is not complicated, and I've condensed everything you need to know into six steps.

Step 1. Determine your daily needs.

In order to keep energy needs low, look for ways to reduce power consumption. Replacing incandescent light bulbs with compact fluorescent bulbs is one way to do that. Electric clocks, doorbells, and certain other devices can be replaced by mechanical ones that do the same job. Once these things are done, you're ready to calculate your electricity needs.


A 13-watt compact fluorescent bulb for 5 hours = 13 times 5, or 65 watt hours.
A 25-watt laptop computer for 2 hours = 25 times 2, or 50 watt hours.
A 60-watt TV for 3 hours = 60 times 3, or 180 watt hours.

Total energy needs are found by adding up all of the loads, or in this case, 295 watt hours.

Step 2. Install enough solar panels to meet your energy needs.


If you get at least 3.5 hours of sunlight where you live, an 85-watt solar panel will give you 85 times 3.5, or 297.5 watts each sunny day. This exceeds your energy needs, but due to clouds and system inefficiencies, you're cutting it a little too close. You would be better off with a 100-watt panel or larger in this example.

For more information concerning the amount of sunlight you'll receive at your location, click here.

Step 3. Determine the size and type of batteries you'll need.

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 295 watt hours that you need divided by 12 equals 25 (rounded off), amp hours. Since we don't want to discharge the battery below 50% of it's capacity, we need a battery rated at no less than 50 amp hours. Because power needs are modest in this example, a department store Marine Deep-Cycle battery will do nicely. For a bigger system, I would recommend high-performance batteries instead.

Step 4. Select an inverter.

An inverter converts the 12-volts DC from your battery 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 of the loads in step 1 was just under 300 watts. Choose an inverter that is able to provide that amount of power continuously. For best results, choose a true sine wave inverter, not a modified sine wave inverter.

You can eliminate the inverter if you only want to provide power to 12-volt DC loads.

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 single-panel system like the one described here, you need not get an expensive charge controller. A single panel can produce no more than 5 to 10 amperes of current, and just about any charge controller will be able to handle that. If you plan to add more panels to your system later, it would be wise to get a bigger charge controller up front, rather than replacing a small one later. You may choose to get a charge controller with additional features, such as a built-in meter, but expect to pay more.

The best charge controller will be the one that most efficiently charges your batteries, while using very little current for its operation.

Step 6. Additional considerations.

For system wiring, be sure to use wire that is large enough to handle the maximum current that will flow through it. Battery to inverter wires need to be heavy, since the current flow there can be very high. Solar panel to charge controller wire must be able to handle the 5 to 10 amperes of current that the panel produces in full sun, and therefore is not as thick as battery-to-inverter wire. Panels, as well as batteries, are connected in parallel for a 12-volt system. Keep in mind the fact that parallel devices double the current, but the voltage stays the same.

You'll also need at least one fuse or circuit breaker, and a lightning protection device. Make drawings, including a wiring diagram showing the size of each wire, before you begin.

By following these steps you can design a complete, off-grid, photovoltaic system. Take the time to explore battery options, as well as other system design details before starting. You'll find different brands of panels and equipment, and a wide range of prices as you shop. For additional details, check my previous blog posts and the websites listed below.

I'll be posting some how-to articles in the future, including instructions for building a simple solar panel mounting system using angle aluminum. So bookmark this blog and check back often.

As always, I look forward to your comments.

Solar John


Anonymous said...

I'm trying to determine my energy needs as you suggest, but I'm not sure what to do about intermittent devices, such as my refrigerator and freezer. Since the compressor only runs for a few minutes out of each hour, how do I determine my daily requirements for those devices? Thanks in advance for your response.

Solar John said...


A Kill-A-Watt meter can be used to determine the electrical requirements of a device over time. Simply plug the device into the meter, and the meter into an AC outlet. Check the KWH reading after 24 hours have passed. For example; you might find aht your refirgerator measures 1.260KWH over a 24 hour period. That is the same as 1260 Watt/hours. Add that to the load list as you determine your daily requirements.

It is also important to observe the maximum watts reading. You need to know that to determine the size of the inverter needed.

My original post lists three on-line sources for equipment. I haven't checked, but most likely they all sell the Kill-A-Watt meter. It will set you back about $35.00, but is well worth it.