Consumer Center
Solar Electric Stand Alone Systems

PV sales and installation people talk about off-grid installations. This simply means that the house has no connection to the electrical utility grid, i.e., all electricity used by the home is manufactured by the home electrical generation equipment. Such an installation is also called a stand alone system. The PV system may have some sort (propane, diesel, or gasoline driven) of motor-generator to be used as a backup to the PV generating system. A string of cloudy days or a PV system failure could leave a family in the dark and the generator would then be a welcome insurance policy.

On this page we will show you what a typical off-grid stand alone system might look like. Such a system might be quite small, i.e., only consisting of one or two 75 or 100 watt PV modules and two or four 6 volt batteries for electrical storage. On the other end, a larger residential system might contain 30 or more 120 watt PV modules and 40 or more 6 volt batteries for electrical storage. But the basics of hooking up the systems are the same. One of the beauties of PV systems is that you can start small and expand as you can afford it. Keep in mind, buying a PV system is not cheap and is often hard to justify unless the electric utility wants to charge you an exorbitant

fee to bring the power lines to your house. It is not unusual for such charges to range from $10,000 to over $40,000 if the utility has to extend their lines for up to a mile.

One of the simplest systems is a 12 volt Direct Current (DC) system. Such a system collects DC from the PV module(s) and stores it in a 12 volt battery. The electrical load(s) will be driven directly from the battery, i.e., 12 volt loads. The power to the load(s) may also be routed back through the controller for better control of the load voltage. There is nothing wrong with this type of system, however, it is somewhat limited in its versatility and ability for future large scale expansion. Note the liberal use of circuit breakers and/or fuses. All circuit breakers and/or fuses must be rated for DC usage and UL approved. This is required in order to meet the National Electrical Code (NEC). If you are installing your own system, it is recommended you get professional help or ask your local electrical inspector since understanding the NEC is not an easy task and mistakes could easily result in a serious fire.

One step up from the above system is to add an inverter, i.e., a piece of electrical equipment that converts DC to alternating current (AC). Addition of the inverter now allows AC loads to be used in the house. Such loads are more easily obtained and typically less costly. The major question when buying an inverter for a small system, besides size, is what type of output waveform is appropriate for your application. The two primary types available today are the true sine wave and the modified or quasi-sine wave. The modified or quasi-sine wave is less expensive than the true sine wave but cannot drive certain solid state loads. Some solid state clocks on microwave ovens have been found to run twice as fast and some computer printers and solar thermal controllers have problems with these inverters. Additionally, some motors will tend to run warmer and

certain light bulbs will produce an annoying hum. Note that the above system still retains the capability to drive DC loads. This capability may or may not be needed once the inverter has been added.

The next step up in complexity is to expand the system to incorporate more PV modules and batteries. Although this system could retain DC loads as in the above system, usually as the system becomes larger, only AC loads are used. Additionally, as the system becomes larger (usually greater than 1200 - 1500 watts of PVs), the system voltage, i.e., the battery voltage, usually increases to 24 volts and for yet larger systems (2500 - 5000 watts and above), to 48 volts. As more PV modules are added, the number of batteries to store the added energy needs to be increased. Larger systems will generate larger currents and hence

larger diameter wires will be needed. The NEC addresses wire sizes and failure to adequately size these wires can result in over heating and possible fire.

Finally, the last system to be addressed is the stand alone PV hybrid system. Such a system is simply the above system with a secondary back-up fuel (propane, natural gas, gasoline, or diesel) powered generator. Such a system uses the back-up generator in case of a PV system failure or in the event the sun does not shine for an extended period, i.e., longer than the design period of the battery system. Sometimes a secondary generator such as a wind turbine or small hydro system may be used to augment the PV system.

Although there are some PV systems that convert the DC gathered by the modules to AC at the module, these systems are not widely used and so will not be covered here. Also, PV cells are available as a material resembling roof shingles and can be attached directly to the roof. You will need to look into whatever system appeals and is right for you.

There are also PV mounting systems that will allow the modules to track the sun as it crosses the sky. Such trackers can increase the amount of yearly energy collected by a considerable amount. It may or may not provide an economical advantage for your chosen system but should, at least, be considered.

It should be noted that the NEC requires a device called a ground-fault protection device be installed on any roof mounted PV system located on a dwelling whereas if the modules are ground mounted, the ground-fault protection device is not required.

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This page last updated on 10-3-05