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Generators & Standby UPS Systems: Two Systems Of “Backup” Electrical Power

We saw with the January 1998 ice storm,  Hurricane Floyd in September 1999 and major storms since then that nature can overwhelm electric systems. Hundreds of thousands of Vermont residents were without power for days – in some cases, weeks.

These storms caused unusually severe outages. But even shorter outages can cause serious problems, such as loss of heat and consequent freezing of water pipes, not to mention inconvenience.

For homeowners who want to provide themselves with “electric insurance,” there are two main approaches to backup power: You can buy either a generator or a Uninterruptible Power Supply (UPS) battery-based system. Choosing between these alternatives depends in part on which systems in your home are most important to keep running in the event of a power failure. Your choice may also be affected by cost and the differences between the technologies that run these backup power devices.

Generators

A generator can be a portable unit (though larger units are typically mounted in place), driven by an internal combustion engine that runs on gasoline, diesel fuel or propane. The generator is connected to the home’s electric service panel by some type of transfer switch. (Readers might also be familiar with smaller generators with plugs and sockets, that can be used to run power tools and other equipment. This article focuses on generators that connect to the main electrical panel.) The size (maximum output) of your generator, and therefore the costs you might incur in purchasing, installing and operating it, will be determined largely by the electric “loads” – for example, heating systems, water pumps and refrigerators – you plan to use it for.

Sizing a generator properly is critical for reliability. Our research and experience have lead to several conclusions about generators. First, it must be understood that maintaining a generator can become a year-round obligation, even though the generator most likely will be used only for short periods. In Vermont, electric companies experience an average of less than 10 hours of outage per customer per year.

Compare that to what a generator manufacturer recommends for the maintenance of a stand-by residential-size unit: run the generator for 2 hours each month, with an electric load of approximately half of its capacity. So just for maintenance (and warranty considerations), the units must be run 24 hours a year, or more than twice the actual need for backup.

For gasoline and diesel generators, the fuel must be kept fresh, so fuel stabilizers are recommended for extended periods of inactivity, and oil must occasionally be changed. Also, generators produce exhaust, which must in all circumstances be vented outside of the building. An additional factor to be considered – and this varies with the size and type of generator – is noise. Siting the unit to minimize the effects of noise on the owners is an important decision affecting satisfactory use.

Finally, consider the engine’s starting mechanism. The options are: 1) manual start with a pull cable (like starting a lawn mower); 2) electric start (which is done by pressing a start button connected to a battery); and 3) an automatic, or “smart,” transfer switch that can tell when power has been lost and turn the generator on. The transfer switch is the point where the emergency power source is connected to the home’s electric system and control panel. Both a generator and a UPS require a transfer switch, but with a UPS the swtich is always automatic.

With a generator,there is a choice between a manual unit or an automatic switch. The automatic feature requires connection between the generator and a battery to initiate the automatic start. When the power system is operating normally, the battery is kept charged by the electricity from the utility’s system. An automatic system could add up to $1,000 to the cost of a generator.

Uninterruptible Power Supply

The UPS is a unit that includes a battery (or batteries), a charging device, and an inverter, which changes the stored 12-volt direct current (DC) electricity to the 120-volt alternating current (AC) electricity that households use. The UPS most commonly has been used for computers or other electronic equipment that must not lose power at all. A UPS “senses” when power supplied by the electric utility is being lost, and then provides the energy to keep the equipment running.

A new breed of UPS now on the market performs this same function for larger home systems such as refrigerators, water pumps, furnaces and lighting. As with generators, properly sizing a UPS is critical to its reliability. This whole-house system is controlled by computer circuits that monitor the status of power provided by the utility. If the power fails, the controller waits briefly (you set the time) before automatically switching over to battery storage. The delay is to avoid making the jump to backup power when the grid is just experiencing a momentary loss of power, not a full-scale interruption.

The battery system remains active until power from the grid is fully returned. In the case of an unusually prolonged outage, battery power can probably sustain a house for a couple of days, depending on the electric load it is being asked to supply. If more storage is required, additional batteries can be easily added. When power returns, the device then transfers automatically back to grid power, and the charger replenishes the batteries.

There is very little maintenance required with properly installed UPS. When fully charged, the batteries are ready to work when the need arises, whether the homeowner is present or not. Owners of vacation homes could install a system to let them know when the power has gone off and that the UPS is doing its job.

Safety

With all backup power devices, the most important factor of all is protecting your utility’s line workers. Generators can pose a hazard for line crews unless there is a properly installed switch that enables you to see, when the generator is in operation, that there is no electrical connection between circuits connected to the generator and the power company’s electric lines. A UPS system has an automatic transfer switch.

Like water, electricity follows the path of least resistance. That means that under some circumstances the power produced by a generator or UPS could feed back into the utility’s system. A worker repairing a damaged power line could be electrocuted by a charge carried over wires otherwise believed to be de-energized. Your Co-op needs to know if you own and use a generator so be sure to let us know if you install one.