DC-to-AC Inverters for Your Solar Equipment
5 of 5 in Series: The Essentials of Photovoltaic Solar Power for Your Home
In your solar power system, you need inverters to take the low-voltage, high-current signals from the PV panels and convert them into 120VAC or 240VAC, which is directly compatible with grid power. Inverters cost around $0.70 per watt, or around $2,600 for a typical application. From a reliability standpoint, they are generally the weak link in any PV system, so quality is a must.
Most installations use only one inverter, but for big systems, having several inverters is common. You can install an inverter that's larger than the power output of the array you're installing, and then at a later time install more panels.
There are two basic types of inverters for residential applications: string inverters and microinverters.
String inverters: String inverters are large boxes mounted near your power meter or main fuse box. In the vast majority of applications, only one inverter is used. It's of critical importance to design the "strings" of PV panels that feed into an inverter, a job that's almost always better left to the experts.
String inverters are the least costly option for the amount of energy a system can output, and the technology has been proven over years of widespread use. But keep in mind that shading becomes a problem because arrays are arranged into series and parallel "strings." If a single panel is shaded, the entire string suffers, and the total energy output of the inverter suffers as well.
Microinverters: Microinverters are the new fad. Each PV panel is outfitted with its own inverter, and the output of each is AC (which matches grid power). The number of microinverters matches the number of PV panels. In the future, it's likely that the vast majority of systems will use microinverters, but for now they're relatively new and the reliability risk is high.
Pay attention to these specifications:
CEC-Rated Power Output: This tells you the maximum output watts from the inverter and varies over operating temperature.
Maximum recommended PV input power: This must never be exceeded by the power output from the combined panels or the output becomes clipped, which means that your inverter runs inefficiently. In other words, you must match the inverter to the panel array properly. Use PV Watts (enter "PV Watts" into your search engine) to find out whether your inverter is large enough for the array you're considering.
Maximum open circuit voltage: The PV array's maximum open circuit voltage must always be less than the inverter's limit or damage may occur. Calculating this number for a given array is a complex engineering problem.
PV Start Voltage: This tells you when the inverter will begin to operate. In the morning, when the sun comes up, the PV panels begin to output power, but inverters require a minimum amount before they start outputting their own power into the grid. This is an important specification because it relates to the overall efficiency of a system. You want your system to run as long as possible over the course of a day.
Maximum Power Point Tracking (MPPT) Range: MPPT circuits enable the inverter to harvest the most amount of energy over the course of a day.