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Solar 101: Choosing the right Inverter for your solar power system
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SMA Inverter 5000 Watt
In a solar electric system the Inverter changes the DC power generated by the solar panels to AC power that is used in your home or business. In the morning when sun first strikes the solar panels, voltage begins to rise turning on the Inverter. The electronics inside the Inverter 'mix' the electricity from the solar panels with the energy from the grid. Solar panel energy is used first with grid power used as a supplement. If power from the grid is interrupted such as during a power failure, the Inverter shuts off thereby blocking the energy being produced from the solar panels. This surprises many people until they learn that this is done deliberately as required by the National Electrical Code (NEC) section 690. If the Inverter did not turn off, utility company workers would risk electrocution from back-fed electricity coming from the solar panels. Battery back up can be added to the solar system which can supply temporary power during a time when the grid is off line.
Selecting the right Inverter is quite simple. Almost every Inverter commonly sold is of good quality, very reliable, and carries a good warranty (usually 10 years or more). For residential applications, consider brands such as SMA, Fronius, and Solectria. For commercial applications, consider brands such as Satcon, PV Powered, and Solectria. For off-grid applications, Outback is a common choice. The Inverter business is very competitive making the pricing differences between these manufacturers minimal.
Inverter size and installation location are the most important considerations especially in hot climates. Most Inverters are rated for a maximum ambient temperature of 105 degrees F. In places such as the desert southwest, temperatures regularly exceed this. The manufacturers' system design guidelines do not address high temperature applications. But, experienced designers and installation companies with extensive local knowledge know that by slightly over-sizing the Inverter (10-20%), it will run cooler, last longer, and experience fewer problems. For example, a 4700 watt (DC) solar array can be connected to a 4000 watt inverter, according to the manufacturer's specifications. However, using a 5000 watt inverter instead, will ensure longer life, cooler operation, fewer breakdowns, and less 'clipping' on crisp winter mornings. A 5000 watt inverter may cost a few dollars more, but the lifetime benefits are easily justified. Even using a 6000 watt Inverter can be used if you anticipate expanding your solar power system in the future.
A lot of questions get asked about Inverter efficiency. Most Inverters made for residential and small/medium commercial applications boast efficiency ratings of 95% and above. The difference is usually seen in how efficiency is actually specified. For example, some Inverter manufacturers such as Fronius, use a multi-stage design enabling a very consistant efficiency across a wide operating rage. While other manufacturers specify thier efficiency only at peak operating conditions. I good solar design engineer can help you understand what is best for you.
The last, but certainly not least important thing to know about the Inverter is the placement. The Inverter should NEVER be located in direct sunlight. The Inverter will overheat and fail if not properly protected. Locating the Inverter in an alcove, behind a tree or some shrubbery, even in the garage are all good considerations. The most ideal location for the Inverter would be the north side of the building in a location that is protected from direct sun, and close to where the electrical service entrance is located.
Inverter selection is simple, once you know the basics. Following these guidelines will ensure you selected the right Inverter, that is the right size, and is mounted correctly.
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