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Grid Tie Solar Power System w/ Battery Backup Drawing

Earlier this week we featured a drawing of a simple grid tie system. A common misconception about grid tie solar electric systems is that you will have a power source when the power goes out, although unless you have a battery backup system your system will go down when the grid goes down. Some people who need backup power and don’t mind the maintenance and replacements every few years of a battery bank consider a grid tie system with battery backup. Today’s drawing also comes from the Guide to Photovoltaic System Design and Installation by the California Energy Commission and features a battery backup system.

As you can see compared to the simple grid tie system, the battery backup system is a little more complex. It includes a battery system, which is typically a group of lead acid batteries, a charge controller to keep the system from cooking the batteries from an overcharge and a critical load sub-panel. The point of a battery backup system is not to power you entire house through a blackout but to provide backup power to your most important electrical loads in your home or business. The downsides of a battery backup system are the increased cost of the overall system, the maintenance of the batteries and the installation time of the system with batteries jumps up 40% compared to a system without batteries.

So it really depends on where you live, how reliable your utility company is and if you have loads in your house that need to be redundant. What do you think? Is adding batteries to your system worthwhile?

Author: Deep Patel

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5 Comments

  1. I could be wrong here, but it seems that even with the grid down, as long as the sun was out and your panels were generating electricity you would be able to have power.

    It would be a poorly designed system if this were not the case. Also, a back-up generator would suffice for your power needs unless the grid outage were unusually long.

    S Chambers
    Solar Power Enthusiast

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  2. S Chambers,

    thanks for your comment. As long as the electricity has somewhere to get stored, the grid or a battery you will have power. Simple grid tied systems without batteries go down when the power goes out even during the day when the sun is out. That is why there is a difference between a grid tie system and a grid tie system with battery backup. In a simple grid tie system without batteries, the inverter shuts down when the power goes out.

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  3. I just learned this about grid-tie systems yesterdasy during the solar home tour. Without power from the grid, the inverter shuts down. There is a fail-safe that keeps the olar panels from feeding power back into the grid, otherwise, down powerlines might continue to be a hazard etc., even if the power company had cut their power to the lines.

    But do you have to have a battery bank to feed power into the house? Couldn’t you do it like a generator power source with an on-off-on switch to function as a grid lock-out. You take the grid off and turn on the UPS. The UPS then powers the inverter long enough with its small battery back-up to get the solar panels running again. Once the inverter and solar panels are running again, the solar panels should power the UPS until the sun goes down. At night you switch the system off and save the UPS battery power to start up the inverter the next day.

    You’d have a temporary shut-down and switch over, but if grid power was out for multiple days, at least you’d have power during the daylight hours to keep your frozen foods from thawing, etc.

    Why couldn’t you use UPS battery power to get the inverter back up and running long enough to have the solar panels feed non-grid tied electricity back into your home? What am I missing here?

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  4. I’ve been googling for an answer and think I may have found one… TerraWatt Power has come up with an inverter that can dynamically disconnect from the utility grid during a power outage as mandated by the power company, and automatically switch to providing power directly to the home without needing a battery back-up. They also claim a few other design advantages that sound interesting… I’m thinking I want enough night-time/cloudy battery back-up to keep the most critical systems running (alarm system, freezer, etc., but no ovens, dryers, HVAC, etc.) for about 8 hours. With a longer power outage I can hook-up a generator. With a PV system that works when the sun is out, my life won’t be too terribly disrupted even for extended periods, as long as my freezer is powered long enough to avoid expensive losses.

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  5. GWhittleAL,

    thanks for the comment, your right the only grid tied system without batteries that can provide you backup power during daylight is the TerraWatt power, I did a short review on the product on the blog when I saw them exhibiting at Intersolar US in July. Although their inverter is still work in progress, last I heard from TerraWatt power is that the product is not ready to be sold.

    I understand you do not want to deal with batteries, although why not just buy a grid tied system with battery backup? Just keep the battery bank small so its just enough juice to power your most critical loads.

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