How to Select an Inverter for Your Needs

On-Grid, without Backup

In places where "net metering" is in effect, the inverter feeds whatever solar electricity your home isn't using onto the electricity grid, and the excess production is credited to your account with the utility. When you system isn't producing electricity, for example at night, your home's energy needs are supplied by the utility. Net metering allows you to reduce your electric bill each month, while supplying clean energy to the nation's electrical grid.

Net metering laws have been passed in most states within the USA. However, some small electric co-ops and municipal utility districts may not be required to participate, even where states have passed legislation enabling the practice. So check your state's regulations (www.dsireusa.org) and then check with your utility for their requirements before you decide on this type of system. Many states have incentives such as tax credits, and may offer rebates for qualifying systems, in addition to the federal tax credits currently available. (Some utilities also have special incentive programs.) You can find out what's available to you, what the requirements are that you must meet, and how to claim your refund through the Web site listed above or by contacting your state's department of energy and your local utility company.

Choosing the correct size inverter for this type of system is easy because it's a matter of how much production you will have running through the inverter. First, you need to know the amount of total watts in your solar array, according to the nameplate. The rated watts are based on a measurement of the output under Standard Test Conditions (STC). The testing uses an ambient temperature of 77 degrees Fahrenheit and perfectly sunny conditions. Of course, your solar panels will not always have ideal conditions, so you will need to take that into account for your calculations. Colder temperatures boost the panels' performance above the STC ratings, while warmer temperatures de-rate the production figures. Most manufacturers provide online calculators or specification sheets that take into account your location's specific variables to help you figure out the correct inverter for your size system. To find the climatology information for your location, you can use this Web site, but replace the X's at the end with your five-digit Zip code: http://www.weather.com/weather/climatology/monthly/xxxxx

The biggest factors to consider when choosing between inverter brands for an on-grid application without battery backup are inverter efficiency and reliability. Most inverters on the market today can boast an efficiency rating above 90%. The largest downside of this type of system is that if there is a power outage, your system will go offline, since the inverter must sense the utility's presence and match its frequency perfectly in order to work. If you are worried about power outages, consider using a system with battery backup power.