​Generators For Domestic & Commercial Backup and Renewable Energy Systems


Generators are typically used a backup energy source to charge your battery bank or supply electricity to your house when your renewable energy sources are not keeping up with your electricity consumption or when the grid goes down (power failure, rolling blackouts). They are a necessary resource for most independent, off-grid power producers or as a low cost backup for grid-tied systems during power failures. Additionally, generators can be a relatively inexpensive way to limit the size and cost of your main power producing source (solar, wind, hydro, battery storage size) during the inevitable periods of cloudy, windless days or drought periods). Generators are typically powered by gasoline, diesel, or natural gas/propane depending on size and customer requirements. Generator systems can be fully automated or fully manual to energize and provide power during low-solar or grid failure conditions.

What Type of Generator is Best?

Residential and small commercial generators can be portable or stationary. They run on a variety of fuels, such as gasoline, diesel, natural gas (NG), and liquefied petroleum gas (LPG). Diesel is the least expensive in terms of fuel consumed to electricity produced. Propane, or LP, is convenient and is cleaner than other fuels, but it is usually the most expensive. Slow speed (1800 rpm) generators generally last much longer and are worth the extra cost over most 3600 rpm generators. Portable gas models are generally less expensive, however, such devices have a short run time and require fuel refills several times a day for continuous operation at rated load. In addition, they may not be suitable as a long-term power backup since gas pumps may not work during a widespread blackout. For a long term emergency consider stationary or standby generators. They can provide continuous power because they are hooked up to an external fuel source, such as a natural gas line. Some portable devices can also be fueled from an external source and can therefore provide extended run time as well. The main differences between portable and stationary generators are in their connection and activation. A portable device has to be rolled out from storage, filled with fuel or hooked up to a fuel line, manually started, and connected to your loads. A permanent standby generator by contrast can start immediately either by a push of a button or automatically because it is already connected to the house wiring and to the fuel source. Automatic systems have an auto transfer switch that can sense a power outage or low battery charge condition, isolate your electrical wiring or designated emergency circuits from the grid, and start the generator. When external power is restored, automatic systems will reconnect back to the grid and power off. You don’t have to be at home to activate the generator.

The typical transfer time of an automatic system is 10-30 seconds, so if you run important computer applications, you may still need an Uninterruptable Power Supply (UPS) or battery bank to prevent data loss during the transfer time. A UPS will also be useful because even though permanently installed natural gas powered generators can provide practically unlimited run time, with some generators you will need to shut your system down as often as every 50-100 hours of operation for maintenance.

Another important issue often overlooked when selecting and installing generators is their power ratings and how that power output is distributed at 115 / 230 volts. The first point to consider is surge power. Generators are typically sold at their surge rating in watts. If you buy a 4,000 watt generator it will put out 4Kw at most, and has no surge above that. Its continuous power rating will actually be 500 watts lower, or, 3.5Kw. Compare this to most inverters which are sold at their continuous power ratings and will surge more than twice their rating to start motors. Secondly most residential electrical systems are set up to use only one leg of 115 volt generator output and most generators do not put out full power on only one leg (they will put out full power using two separate legs of 115 volt and full power line to line at 230 volts). So what does this mean to you? Depending on how your electrical system is designed, you may have only half the generator power available.

Finally, there is the issue of voltage regulation. Inexpensive generators have poor, or no, voltage regulation which is fine if you are just running the normal tools used to build your house. But modern electronics are quite a bit more picky when it comes to bad voltage. Invest in a generator with full sine wave capability and good voltage regulation. When choosing your residential or small commercial backup system, aside from wattage, application, and cost, consider the duration of time you may need emergency power, fuel availability, safety and convenience of use.

How Do I Get Started?

RES-TEC can optimally size generator requirements for our customers reducing total system cost while providing peace of mind and security, knowing you will have electricity under most conditions. Simply give us a call or contact us by email. We will schedule an appointment to visit your site in order to evaluate your potential solar resource and backup needs. RES-TEC will provide you with a detailed explanation of the various backup systems available, generate performance estimates, and answer any questions you may have. We will provide a preliminary proposal showing you the estimated system cost and performance data. We will apply for all rebates and incentives available, federal and state. We look forward to hearing from you and working together with you to bring more “Green Energy to the Green Mountains!”


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