We typically use generators as a supplement to photovoltaic power. There are some very large applications for which continuous use generators may prove more cost-effective than a PV system, but in almost all of the applications with which we work, the economics of generators are maximized by restricting them to providing backup power. Generators are used for backup in situations where seasonal variability of insolation is substantial as in cloudy climates, or for systems where occasional very large loads are powered, as intermittent use of large tools or a deep well pump in a residence. We typically design residential PV systems to provide 80 to 90 percent of the home's annual electrical power. In many cases we would double the cost of the system to provide this last 10 to 20 percent of annual power which can be more economically supplied by a generator.
It is much more cost effective to employ a backup source of power during t he least sunny time of the year. The cost per kilowatt hour of electricity produced by a generator used in conjunction with a battery bank and inverter is much cheaper for residential type load profiles than is power produced by a continuously running generator. This is because engine-driven generators perform poorly when under-loaded. Low load hours on the engine, especially diesel, can actually age it more than hours under full load. Fuel costs suffer too. A 6500 watt generator, for example, powering a 100 watt load will consume perhaps 50 percent as much fuel as it would consume perhaps 50 percent as much fuel as it would consume if operating at full capacity.
Therefore, work the generator near its capacity for shorter periods and then shut it down. Batteries can be charged while washing machines, pumps or other large loads are running. This maximizes efficiency while reducing generator run time, wear, and fuel costs.
Battery chargers take the 120 volt AC power from the generator and convert this power from the generator and convert this power to low voltage DC. They are typically the largest consumer of the generator's output.
We recommend a generator of at least 4 to 5 KW in size for full time remote homes.
Remember that these inverter/chargers also include an automatic transfer switch. This switch selects among the two sources of AC power to be delivered to the loads - inverter or generator power. The switch is biased to inverter power which is supplied to the loads whenever the generator is off. Once the generator is started, the switch senses the presence of generator voltage, waits a predetermined "warm up" period then switches over allowing generator power to flow to the loads.
The generator power is now running al loads in the house as well as powering the battery charger. Therefore the generator should be sized to not only run the battery charger at a high rate, but also any AC loads that may be running at the same time. If the generator is undersized for the loads being run, the battery charging rate will be reduced. This may mean the generator is run for a very long time to fully charge the batteries. So how do you avoid this problem that many have experienced? Simply select a good-sized battery charger and generator when designing your system. This is not the place to cut the budget, as generator fuel can cost you dearly.
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