- Introduction
- A Standard Grid-Tied Solar System
- When the grid goes down, your solar shuts down too.
- Adding Battery Backup
- What Happens to Your Solar Panels During an Outage?
- What If the Solar Panels Are Producing More Power Than the House Needs?
- How Long Will the Battery Last?
- What About the Rest of the House?
- Conclusion
Introduction
One of the questions I get quite often is, “If I have solar and the power goes out, won’t my solar panels keep my house running?”
The answer is not necessarily.
This is one of the most misunderstood parts of grid-tied solar. Having solar panels on your roof does not automatically mean you have backup power when the utility goes down. In fact, a standard grid-tied solar system will normally shut down when the grid fails.
That might seem strange. After all, the sun is still shining and your panels are still producing power. So why can't you use that power?
The reason comes down to how a grid-tied system is designed to work and, more importantly, how it keeps utility workers safe.
When we add battery backup and the right inverter equipment to a grid-tied solar system, however, things change. The system can separate your home from the utility grid and create its own stable electrical system for your house.
So let's take a look at what actually happens when the grid goes down.
A Standard Grid-Tied Solar System
A basic grid-tied solar system is designed to work with the electrical grid.
Your solar panels produce DC electricity, and your solar inverter converts that electricity into AC power that your home can use.
If your solar system is producing more electricity than your house needs, the excess can be sent back to the grid. When your house needs more electricity than your solar system is producing, you simply draw the difference from the grid.
It's a pretty simple arrangement.
But there's an important safety feature built into these systems.
When the grid goes down, your solar shuts down too.
This is called anti-islanding protection.
If the utility grid is down because of an outage, the solar inverter cannot simply continue pushing electricity onto the utility lines. Utility crews may be working on those lines, and the system needs to make sure it isn't energizing a line that they believe is dead.
So even though your panels may be sitting in full sunlight, a normal grid-tied solar system will shut down during a power outage.
That's where battery backup comes into the picture.
Adding Battery Backup
A properly designed battery-backup system changes the way your home interacts with the grid.
When the utility power fails, the system detects the outage and disconnects your home from the utility grid.
This is sometimes referred to as islanding your home.
Once your home is safely separated from the utility, the battery inverter can create its own electrical grid for the loads that are backed up.
Now your house has a source of electricity even though the utility is offline.
The battery supplies power to the inverter, the inverter supplies AC power to your home, and your solar system can potentially continue producing energy.
This is where things get interesting.
What Happens to Your Solar Panels During an Outage?
This is one of the areas where battery-backup systems can get a little more complicated.
Your solar inverter normally expects to see the utility grid. When the grid disappears, it needs another stable source to synchronize with.
A properly designed battery-backup system can provide that reference.
The battery inverter essentially creates a small, controlled electrical grid for your home. Your solar inverter can then synchronize with that system and continue producing power, assuming the equipment is designed and configured to work together.
So instead of:
Solar → Grid → House
you can have something more like:
Solar → Inverter → House
while the battery acts as the energy buffer that helps keep everything balanced.
What If the Solar Panels Are Producing More Power Than the House Needs?
This is where the battery becomes especially important.
Imagine your house is using 2,000 watts, but your solar array is producing 5,000 watts.
Normally, the extra 3,000 watts could be sent to the grid.
But during an outage, there is no grid available to accept that excess power.
The system has to do something with it.
Depending on the equipment and system design, the battery may absorb the excess energy. If the battery is full and the house isn't using enough power, the system may need to reduce or shut down solar production temporarily.
This is one reason that designing a battery-backup system isn't simply a matter of saying, “Let's add a battery.”
The inverter, batteries, solar array, loads, and control systems all have to work together.
How Long Will the Battery Last?
That's another question I hear all the time.
The honest answer is:
It depends.
A battery doesn't have a fixed number of hours of backup.
It depends on how much energy is stored in the battery and how much power your home is using.
For example, if you have a 15 kWh battery and your backed-up loads are using an average of 1 kW, you could theoretically have many hours of runtime.
But if you're running a well pump, electric heat, hot water, cooking appliances, refrigerators, and other large loads, that same battery can be depleted much faster.
This is why I like to look at the loads first, rather than simply asking how big a battery someone wants.
The goal isn't necessarily to power everything in your house indefinitely.
Sometimes the smarter approach is to identify the things that are most important during an outage and build the system around those loads.
What About the Rest of the House?
This is another important consideration.
Not every battery-backup system is designed to power every circuit in the house.
Some systems back up an entire electrical panel.
Others use a critical-loads panel that includes only the circuits you consider essential.
For example, you might decide that during an outage you want:
- Refrigerator and freezer
- Lights
- Internet
- Well pump
- Furnace or heating controls
- Some kitchen outlets
- Security systems
- Garage door
- Other essential loads
But you may decide that electric vehicle charging, electric baseboard heaters, hot tubs or other large loads don't need to operate during an outage.
This can make a huge difference in the size and cost of the system.
Conclusion
When the power goes out, having solar panels on your roof doesn't automatically mean your house will stay powered.
That's probably the biggest misconception I see with grid-tied solar.
A grid-tied solar system and a grid-tied solar system with battery backup are two very different things.
A basic grid-tied system is primarily designed to reduce your electricity bill and provide energy while the utility grid is available.
A battery-backup system adds another layer of functionality.
It can provide backup power, allow solar production to continue during an outage when properly designed, store excess solar energy, and give you more control over how your energy is used.
But there is no one-size-fits-all system.
The right solution depends on your home, your electrical loads, your solar production, your battery capacity, your inverter, your utility connection and, most importantly, what you actually want the system to do.
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Every solar system is different. If you'd like personalized advice on your RV, cabin, home, or off-grid property, book an Energy Strategy Session and we'll design a solution that fits your needs.
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