All grid-tied solar photovoltaic systems have at least one inverter. The inverter’s role is to convert the direct current (DC) output from the solar panels into an alternating current synchronised with the mains supply, so that it can be used on-site or exported to the grid, and in turn to neighbouring properties.
Whichever method is used to invert the DC into AC, there are three power stages in every grid tied photovoltaic system:
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Maximum Power Point Tracker (MPPT) stage
The more current is taken from a solar panel, the lower their voltage becomes. All solar panel manufacturers specify an optimum voltage and current combination at which the panel will produce its maximum power.
A Maximum Power Point Tracker (MPPT) is a system which calculates this optimal voltage and current, allowing the PV system to get the best from the photovoltaic panel(s) that feed it. -
DC-DC converter stage
The DC-DC converter adjusts the DC voltage coming from the MPPT(s) to 380 volts. This is the ideal voltage for the next stage of the process.
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DC-AC inverter stage
The inverter stage does the conversion of the DC from the DC-DC converter into the AC that we need to connect to the mains electricity grid. It is important to know that this stage uses electrolytic capacitors. These electronic components contain a liquid electrolyte which slowly dries out during their operation. This means that these components have a finite life expectancy of around 12 years.
String inverters
The string inverter is the most common variant of inverter to be found in grid tied photovoltaic systems. It combines the MPPT(s), DC-DC converter and DC-AC inverter into one unit.
Microinverters
A microinverter is a small device that is installed behind a solar panel. It too combines the three power stages into one unit, but in this case each panel has its own microinverter. These offer the advantages that each panel is treated independently
Power optimised systems
Power optimiser systems such as SolarEdge combine the MPPT and DC-DC converter stages into an optimiser that is either installed behind each solar panel, or embedded into the panel during manufacture. There is a separate inverter that contains the DC-AC inverter stage.
Pros and cons of each type of system
| Type of inverter system | Advantages | Disadvantages |
|---|---|---|
| String inverter | Low initial cost | Short warranty (usually 5 or 10 years) |
| Ease of access for servicing or repairs | Monitoring limited to whole system, not individual panels | |
| Microinverters | Uses one MPPT for each panel | Expensive to access for servicing or repairs (roof mounted systems) |
| Allows panels to be monitored individually | 10 year warranty | |
| More expensive than any other system | ||
| Optimised systems | Uses one MPPT for each panel | Initial purchase price higher than string inverters |
| Allows panels to be monitored individually | ||
| Inverter easy to access | ||
| Electrolytic capacitors are retained in the inverter, not on the roof | ||
| 25 year optimiser warranty | ||
| 12 year inverter warranty, cheap to extend to 20 years |
Summary
Inverter manufacturers and solar photovoltaic system installers will often push their products as the best solution for a particular installation. The 12-year life expectancy of electrolytic capacitors ought to be a fundamental factor in choosing your inverter solution.
Prices of string inverters do vary and the most expensive devices can be twice the price of budget models. They all contain electrolytic capacitors, so you need to decide if it’s worth spending the extra on the premium brands, or would you be better buying a budget brand with good UK support and extending the warranty to ten years? As an experienced installer there are certain brands that I particularly like for reliability and UK support, and others that I simply won’t use.
Microinverters have the major disadvantage that they contain electrolytic capacitors in a location that is not easily accessible (on the roof). If you do choose microinverters then you need to factor in the future cost of replacing faulty devices. You will find that your warranty doesn’t include the cost of erecting scaffolding.
SolarEdge optimised systems keep the electrolytic capacitors in the inverter unit, which is mounted in an easily accessible place. The optimisers have a 25 year warranty and will in all probability last that long. The inverters have a 12 year warranty which (at the time of writing this page) for just £165 can be extended to 20 years. Like all inverters, it’s not likely to last beyond 12 years but at least it will be under warranty when it does fail.