Saturday, August 15, 2009


The following information may help you with your off-grid system design...

Off-grid systems are electric systems where you are your own electric supplier. In the past, such systems relied on the use of noisy generators to supply the electric. Today however, we are fortunate to have other options such as Solar Panels, Wind Turbines, and Micro Hydroelectric Turbines. This means that you can have a good source of electrical energy even if there is no utility to get it from. So now you can produce electric with the help of our planets natural resources. All three of these methods are producers of DC electric and therefore require solar storage batteries to store the electric you produce. Even though Micro Hydroelectric runs 24 hours a day, you will still have to have batteries plus a load diversion in your design. All of these energy generators are relying on energy sources whose energy output varies. For example the Sun Rays hitting your Solar Panels will increase and decrease because of clouds and the time of day, Wind speeds are also variable causing the electric output to vary as well. So the batteries not only store the electric but also provide a more consistent supply.

Should you wire your electrical system for AC, DC or Both?

To design your system output you need to decide which type of voltage you will use, ie. AC, DC, or a combination of both. Most off grid homes, and some RV's and boats use a combination of both.

I will use our Off-Grid Solar Ranch as an example of a working off-grid independent system.

We have a 15 acre parcel of land in northern New Mexico, which contains our home, office and studio, and our motor home (a converted bus that resembles a boat from inside).

Our motor home and house both use AC and DC, while the woodcarving studio runs strictly on 120 volts AC. The house operates primarily on 120 volts AC with the exception of our hot water Circulating Pump and Booster Pump (12VDC) that provides us with water from our cistern. These two pumps run from a separate self contained system which is made up of our 12vdc panels and batteries which I gave a second life to when I up-graded our solar system from 12vdc to 48vdc. Our well (which is 480 feet deep) uses a 48 Volt DC Lorantz ETA Pump and runs directly off our 48 Volt battery bank. And this Blog and my websites are all generated on computers that are powered by sun and wind power which is stored in Solar Storage Batteries. This stored energy is than transformed into 120vac by an inverter. We started with a 12 Volt input system but once we installed our well, we had to convert to a 48 Volt system. So from my example, you can see how a system can evolve over the decades. I hope this may help you in figuring out your own system.

What about PV System Voltage Input?

Before I go any further on your system's output, I need to mention a little something about your Systems DC input voltage. Today there are three main voltages you can choose from, 12, 24, and 48vdc. 12vdc had been the standard in the US for years before solar became popular. The main reason for 12vdc is that is what vehicles and boats used. As solar use in homes increased the manufactures started looking into more efficient options thus the creation of 24 and48vdc systems. Early on PV Panels were only available in 12vdc but this has now changed.

So which one of these input voltages should you choose?

  • The 48vdc systems are the most efficient and also using smaller connection wires. However, you need to take into consideration that you will need to purchase four times as many batteries as a 12vdc system. With this system you can take advantage of the larger wattage of the 24vdc PV Panels. So if you can afford the extra cost of the batteries I would recommend the 48vdc system. Also you might take this into consideration, if there is a possibility that some day in the future you may have access to selling your extra energy to the grid, with the 48vdc system you can just swap out your Off-Grid Inverter for a Grid-Interactive Inverter.

  • The 24vdc systems are the most common as it uses half the number of batteries as that of the 48vdc system and half the wire size of a 12vdc system. With the 24vdc system you can also use the larger 24vdc PV Panels as with the 48vdc systems.

  • The 12vdc systems are being used less these days mainly because of the limitation in 12vdc PV Panel wattage output (basically from 5 to 140 watts per panel while the 24vdc panels put out 160 to over 200 watts). Also your wiring sizes on these systems is much larger and should be taken into consideration. If your system is going to be strictly DC, a system in say an RV or boat I would recommend the 12vdc system

Back to your systems voltage output

If you have a small system or if cost is a consideration, then you might consider a DC only. DC is simpler and is a less expensive way to go because you will not require having an inverter. I see DC systems used a lot in remote cabins, on small boats, and in under developed countries. However, if you plan to run appliances that you already own (Stereo, TV, Power Tools, Toaster, etc.) you will need an Inverter (an inverter is an electronic piece of equipment which turns DC electric power into AC electric power). Since AC products are more readily available and generally less expensive you may want to run AC. Remember, that if you go 100% AC, you are relying on an inverter. If the inverter goes down there goes your electric. The good news is that today's inverters are very sturdy and long lasting. At the Solar Ranch we run redundant inverters with a backup generator just to be on the safe side. In the near 30 years that I have been living with solar I have only had two minor problems, one was a relay on my original Trace square wave inverter and the other was a faulty temperature sensor.

Wiring your home for both AC and DC is another option. By doing this, you can run both AC appliances, some DC appliance and lights. When you are running your wires to your outlets use 12 gauge wire and if you think you may want to have some DC outlets, run a set of wires for each outlet you want. You can run 10 or 12 gauge wire for your lighting. Run all your AC outlet wires to your main AC Load Center, all your DC outlets to a DC Load Center, and all your lighting wires to a third Load Center. By doing this, you can use either AC or DC lighting depending on which of the other two Load Centers you connect to. This also allows you to change your mind at a later date.

Terry R. Wolff