By Jeff Harrop
An inverter is just a DC to AC converter. Why do we have them in solar applications? Solar PV (photovoltaic panels produce DC (direct current) electrical energy. In our homes and businesses we use electrical devices that take their energy from an AC (alternating current) source. Direct current sources are ones like batteries, where the voltage (electrical push) and current (electrical flow) combine to make power (Watts) as a product. If an electrical device has a “Wattage” rating it is derived from the product (multiplication) of the voltage and current (Amps). The electrical devices we use normally have a Wattage relating to AC. An example would be your toaster which is rated (see back of toaster). Mine is 120 Volts AC 60Hz 1600 Watts. Hz stands for Hertz which is the rate of which the voltage changes every second. Watts = volts times amps = 120 x 13.3 = 1600 Watts. Devices rated for AC power are not normally able to be used on DC sources. This means if we have solar PV panels producing power (DC) devices plugged into AC sources will not work. So how do we use our AC devices when we have solar power? We must use a device to convert DC power to AC power. An inverter is the device of choice. If we look at PV power it may not have the same voltage as our household receptacles (120 volts and 15 amp rated). Therefore we need to change it to a “standard” that our electrical devices will work with. This is the job of the inverter.
How do we know what inverter to buy?
Inverters come in different configurations and ratings. Inverters are based on two basic designs, grid connected and non-grid connected. Grid connected inverters are able to connect to the utilities power source (NSPI in NS) and non-grid connected or standalone inverters.
Non-grid inverters cannot sell PV power to the grid in NETMETERING situations. NETMETERING is where you sell your green power to NSPI and get credit for it; it is not the same as a feed in tariff contract.
Non-grid inverters have the greatest quality variance; they can be very good to terrible. They can take the PV power and supply it to a load, and they can use PV power to charge batteries. They can be connected to AC power, so that when the AC power goes down it supplies the energy from batteries (and possibly PV) to loads.
The grid interactive inverters can be designed to just sell the PV power to the grid only. They can also be capable to take the PV power and send it to batteries, and when the grid (NSPI) goes down can be used to supply PV and battery power to electrical loads in your home. These inverters are supplying a very high quality AC power to the grid and or loads. Not all inverters deliver this quality. Grid quality inverters supply energy that is most effectively transferred to the load.
The math and results of efficiency for this type of inverter is somewhat complicated but in short, your electrical device performance and life will be compromised with anything less than grid quality inverters. If you understand this and can match your application correctly, the matching of inverter to your application is calculated.
Solar PV connected quality inverters are of two types: string or micro inverters. The string inverter has been around the longest and is usually found in the utility room. It is heavy and rated from 2kW – 10kW for the residential applications. Common manufacturers are Xantrex (now Schneider); Outback; Fronius; and SMA. When you connect PV panels to an inverter or charge controller that connects to an inverter, it is put into strings. A string is a series of PV panels connected in “series” (connected one after the other so that the current flows from one to the other. The string provides a voltage (made up of each PV panel connected and adding their voltages to a higher value). This charge controller or inverter can take a maximum voltage input based on this string. If more than one string is used then each string maximum amperage is added together to provide a maximum amperage for the inverter or charge controller. These strings are combined into a “combiner box” which sums these strings together (adds).
A micro inverter is small and lightweight and mounts behind the PV panel to the racking. Output power can range from 190W to 380W. Common manufacturers are: Enphase; Enecsys; SMA; and Canadian Solar. This kind of inverter is connected to each panel or a pair of PV panels. It does not require a combiner box and has no DC wiring on the design. It has the advantage of the shading phenomenon (where string connected PV panels have their power reduced to the lowest producing PV panel due to shading) where only the shaded PV panel is reduced, and not the total string connected PV panels. These inverters are grid connectable only (they need a grid to connect to). The output leaving the installation is an AC feed to the panel or utility disconnect switch.
This is an overview of PV inverters; each inverter has options that will appear to the user. Careful attention to detail will help the user get the inverter required for the application. Please note it is illegal to connect any inverter the grid without a proper permit and all inverters must bear an approved Certification Marks (CSA ULC etc.)