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Third Choice. String Tie or Microinverter

So we’ve decided to install a grid tied system to take advantage of the utility and government subsidies and not have to deal with batteries.

The next choice is microinverters or strings.

String systems have a number of solar panels connected in series much like a flashlight might have two or three 1.5V batteries connected in series. In series connections the voltages add. So a flashlight with 3 1.5V batteries has a total of 4.5V to the bulb. In a solar installation a set of 13 solar panels might be connected in series. If they each put out about 30 Volts then the sum would be 390V.  Many solar panels have about 200W of power output under full sun so that might mean that the panels produce a current of about 7 Amps. (Assuming 210 Watt panels.)

If you need more power than 390 V at 7 Amps, (2,730 Watts), then you need to add additional strings. You can’t practically mix string voltages so this makes it a little difficult to design and build. In our example you have a choice of a 2,730 Watt system or a 5,460 Watt system or a 8,190 Watt system. Trust me, it starts to get a little complicated.

Also with a string system the whole string can be negatively impacted if one panel has a problem. For example if a shadow from a tree is over one panel that will limit the output from the whole string, even if the other panels aren’t shaded.

The alternative that has just been practical for the last few years is having an inverter connected directly to each panel. One inverter per panel. The output of the inverter is the 240VAC that can feed back into your breaker panel. These inverters that mount on the back of the solar panels are called microinverters. The most popular one is from Enphase.

Up to 15 of these can be grouped together on one circuit, and there is basically no limit to how many groups you can have.

This is a very flexible alternative because with microinverters you can start with just one panel and one inverter and build from there.

Again since I have a background in power supply design I found these systems fascinating. There were just two problems:

1. I am concerned about the long term reliability of that many power converters in that extreme environment. Roof temperatures in the summer can get quite high and obviously during the winters they can get quite low. And that temperature swing will be happening every 24 hours for the next twenty/thirty/forty years. (Solar panels can easily last 40 years, however their output will drop some over time.)

2. I wanted the option of doing something else with the power from the panels at some point in the future.  (This is a geeky engineer thing most people probably won’t care about.)

The microinverters do cost a bit more on a per Watt basis, but the installation is easier so they end up fairly close in total price.

In the end we decided on a string tie system with three strings of 14 panels.

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