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Generators as a Backup Power Source Richard Perez

©1996 Richard Perez Generators driven by engines

offer one big advantage—lots of

power available on demand. This advantage has made the engine- generator or alternator the most popular backup power source for RE systems. This article focuses on choosing the right generator and then how to effectively and efficiently employ it as a backup power source.

Analyze the Situation

Every situation is different. Some folks employ generators as backup to their regular utility power. Other folks use the generator (or genny) to recharge their batteries when the sun or wind sources have not kept up with their energy consumption. The amount of power required varies from application to application. The climate and physical accessibility varies from system to system. All of these factors need to be analyzed before deciding on a specific genny. Make the wrong decision at this early stage of the process and you are doomed to unsatisfactory gennny performance.

Let’s look at Home Power Central’s system here on Agate Flat. We are eight miles for the nearest electric utility service. Our office and home are sourced by a 2,000 Watt photovoltaic array and a 1,000 Watt wind generator. Our battery, while large enough to have its own room, only stores enough power for about three days of cloudy, windless weather. And usually this is more than enough. For the last three years we have only used our little 12 Volt backup generator (see HP#42, page 28) for about five or ten hours each winter. When we expanded our office, the additional computer equipment consumed more power. Over time, our energy consumption has grown to over 10 kiloWatt-hours daily. This fall we experienced the worst cloudy weather we have seen in 25 years of living on Agate Flat. We had about two full sun days in two, windless months. We were in big trouble. Home Power Magazine has solid deadlines. We need power for computers to keep this magazine on schedule. I went shopping for a generator and here is what I found.

Backing Up the Grid

Consider the situation of a household wishing to back up their utility power during blackouts. The most common reason given is the freezer full of food. A blackout of four or five days will certainly ruin all the refrigerated food. In fact, most utility power outages in the USA are less than a few hours in duration. If the utility did fail for durations of five days, the cost of buying and running the generator would be paid for by five to seven long-term outages. This situation doesn’t occur to many folks. In some grid-connected homes the heating system, even though fired by natural gas or oil, requires electricity to operate controls, pumps, and/or fans. Outages of as little as one day may make the home cold and uncomfortable. Power outages of several days may freeze the home’s plumbing. If your home could be damaged or severely inconvenienced by a power outage of longer than three days, then the genny is your logical choice for backup power. RE sources with battery backup are not cost effective if all you wish is power during infrequent, short-duration, utility power outages. If you are grid connected, then you will probably run the generator less than 50 hours yearly. This means that buying an expensive generator is not cost-effective. Even the cheapest generator will run about 500 hours and this means ten years of use.

Backing Up PV and Wind Systems

Since renewable energy systems are designed with days of energy storage in their batteries, only prolonged cloudy or windless periods are a problem. The best way to deal with energy shortages in RE systems is conservation. If the system is low on energy, then put off energy intensive jobs like doing the wash. Inevitably, most RE systems will experience infrequent periods of extreme power shortage. It is just too expensive to size the RE system for the absolute worst case scenario. If these low power periods cannot be met with conservation, then the genny is the most cost- effective backup power source. Of the 3869 PV systems listed in our subscribers’ database, 1941 or 50% use gennies for backup power. Of the 623 wind systems listed in our subscribers’ database, 372 or 60% use a generator for backup.

The energy deficit in RE systems is deeper than the grid connected system. We not only want to run the household, but also need to recharge the depleted battery. This means more power is demanded from the generator. Recharging large batteries is a long procedure requiring more generator operating time. Backup gennies in RE systems will typically run between 100 and 200 hours yearly.

Please note that I am talking about backup power here. The genny is not suitable as a prime power source for

66 Home Power #51 • February / March 1996

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