How Does Solar Work When The Power Goes Out?

Solar power systems typically connect to the electric grid, allowing homes and businesses to draw power from the grid when needed and send excess solar power back to the grid. This is known as a grid-tied solar system. During a power outage when the grid goes down, grid-tied systems shut off automatically for safety reasons. However, solar panels can be configured to provide backup power by adding batteries that store solar energy for use when the grid is unavailable. With proper equipment like inverters and disconnect switches, solar systems can provide electricity to keep critical loads running even when the power goes out.

Recent data from the Lawrence Berkeley National Laboratory uncovered encouraging statistics on solar adoption in lower-income groups. Nearly 50% of solar installations in California in 2020 were for households with incomes below $100,000. As solar and battery storage become more affordable, they allow more homeowners to maintain power during outages. (https://electriqpower.com/2023/10/energy-storage-can-make-community-solar-more-financially-resilient-in-wake-of-bank-failures/)

How Grid-Tied Solar Systems Work

Grid-tied solar systems are designed to operate while connected to the utility grid [1]. Under normal conditions, the solar panels produce DC electricity during daylight hours, which gets converted to AC power by an inverter and feeds into the home to power lights, appliances, and other loads [2]. If the solar system produces more electricity than the home is using, the excess power gets fed back to the utility grid.

The grid essentially acts as a battery, absorbing excess solar production during the daytime and supplying additional electricity at night when solar production goes offline. This allows grid-tied systems to offset 100% of a home’s electricity needs over the course of a year, while avoiding the added cost of battery storage [3]. The utility net meter tracks both the electricity consumed from and fed back to the grid, and the homeowner is billed or credited accordingly.

Adding a Battery for Backup Power

A key component of using solar power when the grid is down is adding a battery backup system. Batteries are able to store excess solar energy that is generated during the daytime when the sun is shining. This stored energy in the batteries can then be used to provide electricity at night or during a power outage when the grid is unavailable.

There are several options for batteries when putting together a solar + storage system. Popular choices include lead-acid batteries, lithium-ion batteries, and flow batteries. Lead-acid tend to be cheaper but have shorter lifespans. Lithium-ion cost more upfront but last longer. Flow batteries offer flexible storage capacity. When selecting a battery, factors like upfront cost, lifespan, depth of discharge, and maintenance requirements should be considered (Altestore).

The size of the battery bank will depend on factors like the home’s energy usage, the size of the solar array, and how long backup power is needed during an outage. Typically, larger battery banks are required to sustain longer periods of backup power. Properly sizing the solar array and battery bank is key for effective off-grid usage (Solarips).

By adding a battery bank to a solar system, homeowners can gain energy independence and ensure critical loads remain powered even when the main grid goes down and electricity supply is interrupted.

Islanding and Anti-Islanding

Islanding refers to a situation where a solar system continues to power a location even though the main electrical grid has gone down. This occurs when the solar panels and inverter isolate a location into an “island” and provide power independently from the grid.

While this may seem beneficial, islanding can be extremely dangerous for utility workers. If the grid is down for maintenance or repairs, workers assume the lines are deenergized. However, an islanded solar system may backfeed electricity to the grid, putting them at risk of electrocution.[1]

For this reason, grid-tied solar inverters have anti-islanding protection to detect grid failures and automatically shut down. This prevents them from creating unsafe islanding scenarios.[2]

There are several techniques inverters use for anti-islanding, such as voltage and frequency monitoring. When the inverter senses the grid is down, it will disconnect within seconds.

So while solar panels may seem to work during an outage, the inverter’s anti-islanding system quickly disables that capability for safety reasons. Backup batteries are required to use solar energy when the grid is down.

[1] https://palmetto.com/learning-center/blog/what-is-solar-islanding-and-anti-islanding
[2] https://www.linkedin.com/pulse/everything-you-must-know-solar-islanding-anti-islanding-engineerinc

Disconnect Switches

Disconnect switches are an important component required by the National Electrical Code (NEC) for all grid-tied solar PV systems [1]. The NEC requires a visible load break disconnect switch that can manually isolate the solar system from the grid [2]. This is critical for when the grid goes down and the home switches to backup power, as it prevents electricity from the solar system backfeeding to the grid which could injure workers trying to restore power [3].

The NEC allows up to six disconnect switches to isolate the solar system. Having multiple switches allows different components like inverters and subarrays to be independently disconnected. The disconnect switch must be able to manually isolate solar system conductors carrying current, including the grounded conductor. It is typically an external lockable switch located near the main service panel.

Sizing a Solar + Storage System

It’s important to properly size a solar and storage system to meet your home’s critical load needs. Undersizing the system means you may not have enough backup power when needed. Oversizing can lead to wasted investment in excess capacity.

When sizing the system, calculate the critical loads you want to power during an outage. This includes essentials like refrigeration, lighting, medical devices and other necessities. Experts recommend planning for 3-5 days of autonomy as a reasonable backup timeframe for outages. For a typical home, this equates to 10-15kWh of usable battery storage capacity.

Work with a qualified solar installer to conduct a full home load analysis. They will evaluate your energy consumption and recommend an appropriately sized solar array and battery backup based on your needs and budget. Be sure to follow local building codes and get necessary permits.

According to one source, “Good Quality 345w 350W 355W Poly Solar Panel” are commonly used in residential solar+storage systems for adequate critical load coverage. Proper sizing and installation will provide you with peace of mind that you’ll have power even when the grid is down.

Installing Critical Loads Subpanel

When a power outage occurs, most grid-tied solar systems will shut down for safety reasons. This prevents them from sending electricity back into the grid while it’s down. To keep critical circuits powered during an outage, a backup power source like batteries is required. Along with batteries, a critical loads subpanel needs to be installed.

A critical loads subpanel is a secondary electrical panel that separates critical circuits from non-critical ones. Critical circuits may include refrigeration, lighting, well pump, computer, and outlets for medical devices. These get wired to the critical loads subpanel, which connects to the batteries. That way, when the main electrical panel loses grid power, the critical loads panel can still receive backup power from the solar batteries. According to EnergySage, critical load panels are a key part of resilient solar+storage systems.

Installing a critical loads subpanel involves running new wire from the main panel to the subpanel location. An electrician will move the selected critical circuit breakers and wiring over to the new subpanel. It gets connected to the solar batteries through an automatic transfer switch that engages during a grid failure. This allows seamless backup power to critical loads day and night. With proper sizing, a critical loads panel helps maintain essential circuits no matter what happens to the grid.

Maintenance and Safety

Proper maintenance and safety practices are crucial for solar backup systems to operate effectively and avoid hazards. It’s recommended to keep lead-acid batteries at least 80% charged to prevent sulfation, a buildup of lead sulfate crystals that can permanently damage the battery (https://solarlighting.com/solar-lighting-blog/solar-backup-safety-lighting/). Check the battery bank regularly and charge it to full if the state of charge falls below 50%. Lead-acid batteries also need to be equalized every few months by charging to a higher voltage to prevent sulfation and balance the cells.

Inspect all system components periodically, including PV panels, mounts, wiring, and the battery bank. Look for any damage, loose connections, corrosion, or leaks that need attention. It’s advisable to have a certified solar professional do a thorough inspection once a year. They can use tools to check electric currents, voltages, and system performance. This will identify any problems to address before emergency power is needed (https://www.etchellsandyoung.co.za/2-bedroom-apartment-rental-monthly-in-ferndale-randburg-2211059).

Safety gear like insulated tools and voltage meters should be used when working on solar systems. Turn off all power when servicing. Batteries can deliver dangerous electric shocks and contain hazardous materials, so take precautions. Ensure the system installation meets all codes and standards. With regular maintenance and safe handling, a home solar backup system can provide reliable emergency power for years.

Costs

The cost of installing a solar system with battery backup has declined substantially in recent years. According to S&P Global, in 2018 the cost of solar paired with lithium-ion storage batteries dropped to $135 per MWh in some parts of the Southwest United States. This is less than half the cost of natural gas peaker plants.

The overall cost of a home solar plus storage system depends on many factors, including location, system size, battery capacity, and local incentives. As a rough estimate, a typical 5 kW solar array paired with a 10 kWh Tesla Powerwall battery costs $15,000-$20,000 for parts and installation. Larger battery capacity and additional solar panels can quickly increase costs to $25,000 or more. Homeowners may recoup these costs over time through energy bill savings and tax credits.

Conclusion

In summary, installing a solar and battery storage system can provide homeowners with backup power during grid outages. The key components are the solar panels, inverter, battery storage, and critical loads subpanel. During normal operation, the solar energy supplements grid power to reduce electricity bills. But when the grid goes down, the system instantly switches to “island mode” to power critical loads only. This provides resilience and avoids generator fuel costs and noise. The battery capacity must be properly sized to support essential loads for the desired backup time. With proper installation and maintenance, a solar + storage system provides clean, silent backup power to keep your lights on when the grid power is out.