According to many renewable energy experts, a standalone “hybrid” system that combines generation sources, such as wind and PV, offers several advantages over a single generation system.
In much of the United States, wind speeds are low in the summer when the sun shines brightest and longest. The wind is strong in the winter when there is less sunlight available. Because the peak operating times for wind and PV occur at different times of the day and year, hybrid systems are more likely to produce power when you need it.
For the times when neither the wind generator nor the PV modules are producing electricity (for example, at night when the wind is not blowing), most standalone systems provide power through batteries and/or an engine-generator powered by fossil fuels.
If the batteries run low, the engine-generator can be run at full power until the batteries are charged. Adding a fossil fuel-powered generator makes the system more complex, but modern electronic controllers can operate these complex systems automatically.
Adding an engine-generator can also reduce the number of PV modules and batteries in the system. Keep in mind that the storage capability must be large enough to supply electrical needs during non-charging periods. Battery banks are typically sized for one to three days of operation.
A general rule is to design the renewable energy system to provide 80% of the energy and use fossil fuels for the remaining 20%.