Do solar panels work when the power goes out?

One common misconception about home solar panels is that they’ll keep your house powered even if the electric grid goes down. They can, but only if you have the right equipment to go with it.

First of all, let’s understand one thing that might seem obvious: solar panels don’t work in the dark. For the majority of homes that have solar panels, this means having a grid connection that supplies power when your panels don’t generate any electricity.

But what if there’s plenty of sunlight while the grid is suffering a blackout? What happens then?

It turns out that most solar photovoltaic systems will shut down when there’s a blackout. Your inverter does this automatically to prevent a phenomenon known as islanding, which can be dangerous to utility workers who don’t expect wires to be live.

If your system includes batteries, your lights will stay on when the neighbors are in the dark, but only a small percentage of solar homes have batteries installed. We’ll also tell you about one inverter system that can supply electricity during a blackout even if you dont’t have batteries.

But before getting into how solar photovoltaic systems behave when the power goes out, here’s some background into why blackouts happen and why they may be on the increase.

How do blackouts occur?

The very first electric power plant in the United States was the Pearl Street Station in New York. It first generated electricity on September 4, 1882, but burned down just eight years later.

While technology has come a long way since then, especially with advanced control systems and the introduction of renewable energy, much of the basic infrastructure of the modern electric grid looks remarkably similar to the grid of the earliest days of electricity. Wires, transformers, and substations carry electricity over vast distances and bring it into your home, just like in the days of Thomas Edison.

That infrastructure, made of up millions of miles of dangling wires, is vulnerable to acts of nature like hurricanes and thunderstorms, but also more mundane threats - such as rodents. In fact, squirrels and other rodents are the second most common cause of power outages, after equipment failures.

There are other emerging threats too: because of climate change, extreme weather events are becoming common occurrences. Wildfires have caused massive destruction to California, and some of those have been caused by power lines. With the threat increasing, California utility companies are going to start preemptively turning off the power to help avoid future fires.

And we can’t talk about blackouts without talking about the year-long blackouts that devastated Puerto Rico after Hurricane Maria hit the island in 2017. The hurricane tore through the island’s main high-voltage transmission lines and damaged 80 percent of the grid.

Maria won’t be the last hurricane, and major wildfires are becoming a yearly occurrence. If you’re a homeowner, especially in a vulnerable area, planning your home to be resilient is a smart idea.

How often do blackouts happen?

California residents and those in hurricane prone areas should expect to see more blackouts in the future. But exactly how common are outages now?

The US Energy Information Administration tracks this data and reports on it annually. In the United States in 2016, the average customer experienced 1.3 blackouts and went without electricity for about 4 hours every year.

These statistics vary quite a bit by state. South Carolina experienced more than 20 hours of outages (due in large part to Hurricane Matthew) in 2016 while Nebraska residents went only 27 minutes without power.

Why don’t my solar panels generate power during a blackout?

One of the key parts of a solar energy system is the inverter - or inverters, if you have microinverters or a large system with multiple strings. In a grid-tied system (which are most of the residential systems that are installed), the inverter must synchronize the timing of its alternating current (AC) frequency to the frequency of the electric grid AC power.

It does this by “listening” to the signal from the electric grid and adjusting its frequency to match. If an inverter can’t detect a signal from the grid, it will shut down automatically. If it didn’t, it would send power into the grid and potentially electrocute utility workers who are repairing the grid and expect the electricity to be off.

Because of this feature, when grid power goes down, your power shuts down, even if there’s plenty of sunlight for your solar panels.

How can I keep my house powered during a blackout?

The first option is to use a generator. Depending on the frequency and duration of blackouts in your area, the amount of electricity you want to be able to use, and your ability to keep your generator fueled throughout an outage, a generator could be your best and most cost-effective solution. This is especially true if you have infrequent blackouts and only want to keep a few key appliances running, such as your refrigerator.

A small generator that provides enough power for lights, some low power appliances, and one or two refrigerators can cost only a few hundred dollars.

But if you’re a solar homeowner and want to use solar electricity rather than fossil fuels to keep your house powered during a blackout, you have two options.

Option 1: Solar battery system

A solar battery bank consists of lead acid or lithium batteries that are used to store your excess solar electricity. The system includes a charge controller that monitors your batteries and charges them at the correct rate, preventing them from overcharging.

If your batteries are full, the excess electricity will be sent into the grid (unless you have an off-grid system).

For a long time, lead acid batteries were the preferred choice of solar homeowners because the technology of lead acid is mature and the products are widely available, which helps to bring down the cost. It’s used in many applications, such as car batteries, deep cycle marine and RV batteries, and golf carts.

However, the price of lithium batteries has plummeted in the past decade. Lithium ion batteries are used in laptops, cell phones, electric cars, and are now being deployed in massive battery banks by electric utilities. These utility-scale batteries are cost-effective enough that they are now competitive with natural gas, and are starting to displace gas turbine peaker plants.

Because of these market forces, lithium ion is now often lower cost (in the long term) and the preferred technology for home solar batteries. Still, lead acid batteries remain popular, especially among the DIY crowd who often seek out cheap deals for golf cart batteries and other types of deep-cycle batteries.

One reason that lead acid is still attractive is because the upfront cost is lower than lithium-ion. However, lead acid has a number of disadvantages:

• Limited discharge. Lead acid batteries are unable to handle deep discharges. In fact, a single discharge cycle that takes the charge below 50% can cause lasting damage to the battery and permanently reduce its capacity. This means that if you buy 1,000 amp-hours of lead acid batteries, in practice you’ll only be able to use half that capacity - and that would be in an emergency situation. In regular use, a lead acid battery should be recharged once it drops to 80% charge. This means spending more to get the practical capacity that you want. Lithium-ion, in comparison, can routinely be discharged to 20% full or even less without damage.
• More maintenance. There are different types of lead acid batteries: flooded batteries are the cheapest, but require periodic maintenance by checking the fluid levels. In addition, flooded batteries can discharge hydrogen gas, so they should only be stored in vented areas. So-called maintenance-free absorbed glass mat and gel lead acid batteries don’t have these disadvantages, but also cost more. Lithium ion, on the other hand, requires no maintenance.
• Shorter lifespan. No matter how well you maintain and gently treat your lead acid batteries, they simply won’t last as long as lithium ion. A deep-cycle lead acid battery might tolerate around 1,000 discharge cycles (at <50% discharge) before they need to be replaced. In contrast, the Tesla PowerWall (a popular lithium ion battery for home solar) is warrantied for 10 years and an unlimited number of discharge cycles.

If it’s important to you that your home continues to work as normal as possible in a blackout, then a battery system with high capacity is your best bet. The battery will supply power to your home, and automatically switch over in the event of an outage.

Keep in mind, however, that unless you have a massive battery system at very high cost, your batteries won’t have the capacity to power high draw appliances such as central air conditioners, heat pumps, or electric clothes dryers. They simply require too much power.

However, if you heat your home with a gas furnace, a battery system can supply enough power to run the furnace fan, which may draw a few hundred watts - low enough that your batteries should have enough reserve capacity to keep your furnance running through the night until the sun starts shining again the next morning.

Option 2: SMA Sunny Boy inverters with Secure Power Supply

SMA is an inverter manufacturer that is headquartered in Germany. Their Sunny Boy home inverter products include a unique feature called Secure Power Supply.

In the event of a power outage, the Sunny Boy inverter will disconnect from the grid, just any other inverter. However, the Sunny Boy includes an outlet that will deliver up to 2,000 watts of power as long as the sun is shining. This will let you power a few critical appliances, like a refrigerator and some lights.

The SMA Sunny Boy inverters don’t use a battery to provide this feature. The electricity is supplied directly from your solar panels. This reduces the cost, but it also means that the flow of electricity could fluctuate dramatically as clouds pass over. According to the product’s user manual, “Secure power supply operation must not be used for loads that require a stable electricity supply.”

This means that you shouldn’t plug in sensitive electronics with large power demands, like your desktop computer system. However, a cell phone charger which draws only a few watts should be fine.

The amount of power you get will depend on what your solar panels are producing, so you might only have the maximum of 2,000 watts during the middle of a sunny day. That’ll depend on the size of your system, its orientation, and the weather conditions. However, this might be enough power to keep your freezer from thawing out, or even keep you cool by running a window air conditioner, which draws less that 2,000 watts.

In spite of these disadvantages, Secure Power Supply can be a pretty useful feature, and a lower cost alternative for people who only need to deal with infrequent blackouts.

Batteries can help you save money if you’re on a time-of-use plan

If you have net billing instead of net metering, or if you are on a time-of-use plan with your electric utility, having a solar battery can help you save money by shifting your use of grid power to times when electricity costs less.

For example, the Tesla Powerwall has a mobile app that lets you set up times of day when you draw power from your battery instead of the grid. This time-based control allows you to use solar power from your battery when rates are high, and then switch over to grid power during off-peak periods when grid power is often much cheaper.

Batteries are also very useful if you have net billing instead of net metering. Under net billing, time-of-use is also very important because you don’t get full credit for the electricity that you send back into the grid. But with a battery system, you can avoid this issue by sending your excess electricity to your battery so that you can use it later when the sun isn’t shining.

A few other caveats about solar battery systems

If you’re thinking about getting a battery with your photovoltaic system, you should be aware of a few more things before you make your decision:

• You will probably only be able to power critical loads. One of the installation steps with most solar battery system is the addition of a smaller electrical panel next to your main panel. This is called the critical loads panel. During a blackout, your inverter disconnects from the grid and your battery will supply power only to the critical loads panel. This is necessary with many installations because the maximum electrical load of the house is greater than the maximum amperage that the battery can supply. However, you might be able to power your entire house if your maximum load is small enough: for example, if you don’t have a central air conditioner or electric stove. Your installer will make this assessment for you.
• Plan for your solar panels to last longer than your battery will. Your solar panels probably have a 25 year power warranty, but your battery will have a 10 year warranty at the most. It’s possible for lithium ion batteries to have a longer useful life than its warranty period, but you should factor in a periodic replacement cost when calculating the financial impact of adding batteries to your system. This replacement will be more frequent if you opt for lead acid batteries.

Financial incentives include battery systems

If you’re seriously considering a battery system, you’ll be happy to know that the 30% federal tax credit also applies to your battery system. This means that the substantial upfront cost of a solar battery is reduced by at least the federal tax credit, and possibly other state and local incentives as well.

Future costs of batteries

The cost of lithium ion batteries has been dropping dramatically, just like it has with solar equipment. This means that if have a battery system, the inevitable replacement cost in 5 to 15 years should be substantially lower than the cost that you pay now.

Just keep in mind that the current 30% federal tax credit will be dropping in 2020, and drop to zero in 2022, unless Congress renews the credit again. This means that you might not have the benefit of incentives that are available now.

Work with a qualified installer

Whether you are thinking of a battery system or an SMA inverter to help you get through the next blackout, there is extra complexity with the installation and electrical work. Be sure to work with a qualified contractor to help you with the installation. You can use The Solar Nerd to help you get quotes from qualified installers to make sure that the work is done right.