This is a very common assumption made by grid-tie PV homeowners: PV systems will continue providing power during a power outage as long as the sun is shining. 

However, commonly used grid-tie inverters safely disconnect from the utility grid if the grid goes down and won’t deliver power even though the sun is up and shining.  This is, in part, a safety precaution required to protect utility workers repairing the wires and prevent fires.

Independent Operation Function: Power During Outages 

In Japan, standard grid-tie inverters disconnect themselves from the grid, but can draw a small amount of power from the PV system even during daytime power outages. Production of emergency energy is enabled by a function of the inverter known as “independent-operation.”

According to a report published by the Japan Electrical Manufacturers’ Association (JEMA), during the fiscal year 2013 (April 2013 through March 2014), over 630,000 inverters were sold for the residential PV market in Japan. This number is equivalent to about 3 GW in capacity. Out of 630,000, 99.9 percent of the residential PV inverters sold were equipped with an independent-operation function.

“We started selling PV inverters with the independent-operation function after the Kobe Earthquake,” said Ichiro Ikeda, marketing manager at Kyocera. The Kobe Earthquake knocked out power to over 2.5 million homes and businesses in the southern part of Hyogo Prefecture in January 1995 and it took about 150 hours before all the power lines were fully restored and recovered.

“[The earthquake] was a trigger to develop and market inverters with the independent-operation function,” continued Ikeda.

The independent-operation function can provide emergency power of up to 1500 watts through a dedicated, 5-amp, 100-volt outlet. This dedicated outlet can be found on the side of most of the indoor inverters or can be wired and installed as a wall-mounted receptacle at a distance from the inverter.

Photo: Residential Solar Inverter with Independent-Operation Function (Credit: Kyocera)

PV owners will not get this emergency power automatically during a daytime power outage. They must manually turn on the independent-operation function. When the power goes down from the grid while the sun is up, they first turn off a circuit breaker, turn off a PV breaker (an inverter AC disconnect), and then turn on the independent operation switch to activate.

The power available during the independent operation is limited to 1500 watts, whether you have a 3-kW system or 10-kW system on your roof. The amount of power is also limited by the availability of the sun – it can fluctuate considerably depending on the weather.

Solar Emergency Power Keeps Communication Devices Charged During Disasters

Even with the limited power, this function had an extremely important role during the Fukushima Disasters in March 2011, when 4 million homes and businesses lost power for many days and weeks.

During the disasters, the emergency power supply was able to keep cellphones charged and radios or TVs on during the sunny days so that victims could communicate and get the latest and most accurate information. Some PV owners used the power for a rice cooker during the sunny daytime to make rice balls for neighbors and rescuers.

A PV homeowner in Fukushima during the disasters wished that her PV system could generate 100 percent of the power and feed-back the power to the grid so that her neighbors could use electricity. Nevertheless, she was grateful for the amount (up to 1500 watts) of energy she could get from her PV system, with which she could still help her neighbors by charging their cell phones. 

Sekisui Chemical, a leading solar home builder in Japan, surveyed their PV homeowners about their usage of the independent-operation function during the disasters. According to the survey, 80 percent of the homeowners knew how to use the function.

During the disasters, 67 percent of those who lived in the disaster-affected areas used the function, while 33 percent of those who lived outside of the disaster-affected areas with black-outs used the function.

In the disaster-affected areas, 85 percent of the generated power was used for cell phones, 51 percent for rice cookers, and 40 percent for TV.  Outside of the disaster-affected areas with power outages, the power was used for TVs (26 percent), cell phones (24 zpercent), and refrigerators (22 percent).

After the Fukushima disasters, Mitsubishi Electric, a maker of both solar PV modules and solar inverters, received an overwhelming number of requests from its customers to increase the amount of emergency power. Since 1997, every solar inverter the company sold in Japan has been equipped with the independent-operation function. 

This July, the company announced the release of four residential solar inverters (PV-PN30K, PV-PN40K, PV-PN-44KX, and PV-PN55K), with the independent-operation function with two electric power outlets, which can almost double the emergency energy supply to 2700 watts (20-27A, 100 V). 

The two outlets (one is on the inverter and other can be wired as a wall-mounted outlet) will allow PV homeowners during emergencies not only to power up lights, TVs, and refrigerators, but also power up a microwave or high-energy demanding appliances.

Image: Comparison of Available Power by Residential Solar Inverter with an Independent-Operation Function (Credit: Mitsubishi Electric)

Inverters with Secure Power Supply Available in the US

In the U.S., inverters with this function are not yet as prevalent as in Japan, but they are moving into the mainstream.

SMA, the world largest inverter maker, is the only maker to offer inverters with the Secure Power Supply (SPS) function, which is equivalent to the independent-operation function. Similar to what happened in Japan after the Kobe earthquake, a disaster triggered the design and sale of this function. SMA started marketing these inverters under the brand name “TL series” in the U.S. after Hurricane Sandy, which swept through the Caribbean and up the East Coast of the US in late October 2012. 

Sullivan Solar Power, the largest residential solar installer for San Diego Gas Electric (SDGE) in California, has started installing Sunny Boy TL series with the SPS function since SMA released these inverters in the fourth quarter of 2013 in the U.S. 

“Many people ask ‘what happens during a blackout?’ We have had some clients tell us that the SPS function on the inverters we offered was one of the reasons they went solar with us,” said Tara Kelly, Deputy Director of Community Development of Sullivan Solar Power. 

“About 80 percent of our clients are using the SMA TL inverters with the SPS function. Those who are not, it’s because SMA doesn’t make inverters with that function for our larger systems.  SMA recently came out with the 7000TL and 7700TL inverters so more customers will be able to have the SPS function now. If all of SMA’s Sunny Boy inverters had the SPS function, 100 percent of our clients would have had the SPS function,” Kelly continued. 

“We expect this residential solution will continue to increase in popularity because it enhances homeowner security, comfort, and peace of mind,” commented, Tiffany Scalone, Public Relations Specialist of SMA America.