Grid tie solar with a backup generator

I get questions about how a backup generator works with a grid tie solar system so I thought I would talk about that here today. The short answer is that they work fine together, but you want the inverter on the grid side of your transfer switch. I’ll explain why below.

When the grid goes down and you have a backup generator, your automatic transfer switch will disconnect you from the grid and turn on your generator to supply your house or business with power. Where it gets confusing/interesting is what does the solar system do at this point? If it’s a sunny day, the solar panels should be producing power which is a good thing when the power is out right? Not exactly.

Luckily, here in the Durango area the grid is fairly reliable so we don’t have grid outages that often, but when it does happen it can be very inconvenient which is where the backup generator comes in. If its a sunny day you would think that having the solar system and the generator producing power at the same time would be great right? More power for your hot tub and big screen TV. Here’s the theoretical situation where damage could occur to your generator. IF you have a high end generator that produces a clean enough sine wave to make your inverter think that the grid is back up AND the generator and solar system are producing more electricity than you’re consuming THEN you might back feed your generator and damage it. Not likely to happen, but it theoretically could. So that’s why you want your solar inverter on the grid side of your transfer switch; just to be safe.